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[sci.astro] Solar System (Astronomy Frequently Asked Questions) (5/9)

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Old October 6th 05, 02:36 AM
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Default [sci.astro] Solar System (Astronomy Frequently Asked Questions) (5/9)

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Archive-name: astronomy/faq/part5


Subject: Introduction

sci.astro is a newsgroup devoted to the discussion of the science of
astronomy. As such its content ranges from the Earth to the farthest
reaches of the Universe.

However, certain questions tend to appear fairly regularly. This
document attempts to summarize answers to these questions.

This document is posted on the first and third Wednesdays of each
month to the newsgroup sci.astro. It is available via anonymous ftp
from URL:ftp://rtfm.mit.edu/pub/usenet/news.answers/astronomy/faq/,
and it is on the World Wide Web at
URL:http://sciastro.astronomy.net/sci.astro.html and
URL:http://www.faqs.org/faqs/astronomy/faq/. A partial list of
worldwide mirrors (both ftp and Web) is maintained at
URL:http://sciastro.astronomy.net/mirrors.html. (As a general note,
many other FAQs are also available from

Questions/comments/flames should be directed to the FAQ maintainer,
Joseph Lazio ).


Subject: E.00 Sun, Moon, and Planets

[Dates in brackets are last edit.]

E.01 How did the solar system form? [2000-07-15]
E.02 Has anyone attempted to discern details of the star that went
supernova and formed our local group of stars? [2002-05-04]
E.03 What is the "Solar Neutrino Problem"? [1997-07-01]
E.04 Could the Sun be part of a binary (multiple) star system?
E.05 When will the Sun die? How? [1995-08-23]
E.06 What happens to the planets when the Sun dies? [2000-03-17]
E.07 Could the Sun explode? [1995-07-07]
E.08 How are solar system objects and features named? [1995-11-29]
E.09 Where can I find pictures and planetary data? (ref)
E.10 Could Jupiter become a star? [1995-07-07]
E.11 Is Pluto a planet? Is Ceres? Is Titan? [1995-08-18]
E.12 Additional planets:
12.1 What about a planet (Planet X) outside Pluto's orbit?
12.2 What about a planet inside Mercury's orbit? [1996-11-20]
E.13 Won't there be catastrophes when the planets align in the
year 2000? [2000-07-15]
E.14 Earth-Moon system:
14.1 Why doesn't the Moon rotate? [1997-10-01]
14.2 Why does the Moon always show the same face to the
Earth? [1997-10-01]
14.3 Is the Moon moving away from the Earth? (and why is Phobos
moving closer to Mars?) [1997-06-04]
14.4 What was the origin of the Moon? [1998-11-04]
E.15 What's the difference between a solar and lunar eclipse?
Where can I find more information about eclipses?
E.16 What's the Oort Cloud and Kuiper Belt? [1998-02-28]
E.17 Asteroid Impacts
17.1 What would be the effects of an asteroid impact on the
Earth? [1998-04-14]
17.2 What can we do about avoiding impacts? [2000-01-26]
17.3 I heard that an asteroid was going to hit the Earth?!
E.18 What's the difference between meteoroids, meteors, and
meteorites? [1998-04-14]
E.19 How do we know that meteorites are from the Mars? (or the
Moon?) [2002-05-04]


Subject: E.01 How did the solar system form?
Author: Joseph Lazio

Any theory of the formation of the solar system must explain at least
the following two observations: First, the planets, with the exception
of Pluto, orbit in almost the same plane (the "ecliptic"). Second,
the inner four planets are small and rocky, while the outer four
planets are large and gaseous. One theory that does a reasonably good
job of explaining these observations is the disk model.

The Sun is thought to have formed by the collapse of a large
interstellar gas cloud. The original cloud was probably thousands of
times larger than the present solar system. Initially the cloud had a
very slow rotation rate (it's essentially impossible for one of these
clouds to have a rotation rate of exactly zero). As it collapsed, it
began rotating faster (much like a skater will spin faster if she
pulls her arms to her sides---this principle is known as the
"conservation of angular momentum"). The collapse process is not 100%
efficient, though, so some of the material did not fall into the
proto-Sun. This rotating gas that was left behind settled into a

In addition to gas, interstellar clouds can also contain dust.
Therefore, the rotating disk consisted of dust grains and gas. In the
process of settling into a disk---and even after the disk had
formed---the dust grains began to collide and stick together.
Initially quite small, this process of colliding dust grains sticking
together (known as "accretion") began to build up larger dust grains.
The accretion process continued with large dust grains accreting to
form small pebbles, small pebbles accreting to form large pebbles,
pebbles forming rocks, rocks forming boulders, etc. Initially this
process is quite random: Two dust grains collide only if their paths
happen to cross. However, as particles became larger, they exert a
larger gravitational force and attract smaller particles to them.
Hence, once started, the accretion process can actually speed up.

The collapse process itself can generate considerable heat.
Furthermore, as the Sun's mass grew, it eventually reached the point
at which fusion reactions in its core could be sustained. The result
was that there was a heat source in the middle of the disk: the inner
parts of the disk were warmer than the outer parts.

In the inner part of the disk, only those materials which can remain
solid at high temperatures could form the planets. That is, the dust
grains were composed of materials such as silicon, iron, nickel, and
the like; as these materials accrete they form rocks. Farther from
the early Sun, where the disk was cooler, there were not only dust
grains but also snowflakes---primarily ice flakes of water, methane,
and ammonia. In the outer parts of the disk, not only could dust
grains accrete to form rocks, but these snowflakes could accrete to
form snowballs.

Water, methane, and ammonia are relatively abundant substances,
particularly compared to substances formed from silicon, iron, etc.
In the inner part of the solar system, where only rocks could remain
solid, we therefore expect small planets, whereas in the outer solar
system, where both rocks and ices could remain solid, we therefore
expect large planets. (Not only did the gaseous planets form from
more abundant substances, they also had more raw material from which
to form. Just compare the size of Earth's orbit to that of Jupiter's

The formation of the giant planets, particularly Jupiter and Saturn,
deserves an additional comment. It is currently thought that they
formed from a run-away accretion process. They started accreting
slowly and probably initially were quite rocky. However, once their
mass reached about 10--15 times that of Earth, their gravitational
force was so strong that they could attract not only other rocks and
snowballs around them, but also some of the gas in the disk that had
not frozen into an ice. As they attracted more material, their
gravitational force increased, thereby attracting even more material
and increasing their gravitational force even more. The result was
run-away accretion and large planets.

One of the problems with this scenario for the formation of Jupiter,
though, is that it seems to take longer than the disk may have
existed. The conventional scenario predicts that Jupiter might have
taken several million years to form. Alan Boss (2000, Astrophysical
Journal, vol. 536, p. L101) has suggested that the conventional model
for the formation of Jupiter is wrong. His work indicates that a
giant planet might also form from small, unstable clumps in the disk.
Rather than being "bottom-up," like the conventional model, his
"top-down" idea is that an entire region of the disk might become
unstable and collapse quite quickly, perhaps in only a few hundred

One of the results of finding planets around other stars is the
realization that this model does not require the planets to always
have been in the same orbits as they have today. Interactions between
the planets, particularly the giant planets, and the disk of material
could have resulted *migration*. The giant planets may moved inward
or outward from their current locations during their formation. If
planets can migrate during or shortly after their formation, it makes
it easier to explain the presence of Uranus and Neptune. A
straightforward application of the above model encounters a slightly
embarrassing problem: The time to form Uranus and Neptune is longer
than the age of the solar system. If, however, these planets formed
at a closer distance, then migrated outward, it may be easier to
understand why Uranus and Neptune are at their current distances from
the Sun. (See Science magazine, vol. 286, 1999 December 10 for more


Subject: E.02 Has anyone attempted to discern details of the star that went
supernova and formed our local group of stars?
Author: Joseph Lazio

There's one reason, and possibly two, why this cannot be done.

First, our local group of stars is not the group of stars near the Sun
when it formed. All stars have some small random motion, in addition
to their general revolution about the center of the Milky Way Galaxy.
This random motion is typically 10 km/s. Moreover, in the solar
neighborhood, stars tend to have roughly the same velocity (~ 200
km/s), but stars slightly closer to the Galactic center have a smaller
orbit than stars slightly farther away from the Galactic center. The
combination of these factors means that, over the roughly 20 Galactic
orbits that the Sun has completed since it first began fusing hydrogen
in some molecular cloud, its sister stars have dispersed all over the
Galaxy. They are all probably at roughly the same distance from the
Galactic center as the Sun, but some might be on the other side of the
Galaxy by now.

Second, when referring to a supernova and the formation of the Sun,
most people have in mind the hypothesis that the solar system's
formation began as the result of a supernova shock wave impinging on a
molecular cloud. This hypothesis was proposed to account for the
presence of very short-lived isotopes in meteorites. For instance,
the decay products of Aluminum-26 have been found in meteorites. The
half-life of Al-26 is less than 1 million years. Thus, the hypothesis
asserts that, in order for any substantial amount of Al-26 to have
been incorporated into solar system meteorites, there must have been a
supernova (within which Al-26 can be made) quite close to the nascent
solar system.

This hypothesis is being challenged. Recent Chandra X-ray Observatory
observations have shown that young stars may be much more energetic
than the Sun is currently,
If so, then it is possible that some of the X-ray flares produced by
the young Sun might have been enough to explain some or all of the
unusual isotopes found in meteorites. Thus, no supernova might be
required to explain the presence of the solar system.


Subject: E.03 What is the "Solar Neutrino Problem?"
Author: Bruce Scott TOK ,
Joseph Lazio

A middle-aged main-sequence star like the Sun is in a slowly-evolving
equilibrium, in which pressure exerted by the hot gas balances the
self-gravity of the gas mass. Slow evolution results from the star
radiating energy away in the form of light, fusion reactions occurring
in the core heating the gas and replacing the energy lost by
radiation, and slow structural adjustment to compensate the changes in
entropy and composition.

We cannot directly observe the center, because the mean-free path of a
photon against absorption or scattering is very short, so short that
the radiation-diffusion time scale is of order 10 million years. In
other words, the energy produced in the Sun's center and carried by
photons takes about 10 million years to make its way to the Sun's
surface. But the main proton-proton reaction (PP1) in the Sun
involves emission of a neutrino:

PP1: p + p -- D + positron + neutrino(0.26 MeV),

which is directly observable since the cross-section for interaction
with ordinary matter is so small (0.26 MeV is the average energy
carried away by the neutrino). Essentially all the neutrinos escape
the Sun. Of course, this property also makes it difficult to detect
the neutrinos. The first experiments by Davis and collaborators,
involving large tanks of chloride fluid placed underground, could only
detect higher-energy neutrinos from small side-chains in the solar

PP2: Be(7) + electron -- Li(7) + neutrino(0.80 MeV),
PP3: B(8) -- Be(8) + positron + neutrino(7.2 MeV).

Recently, however, the GALLEX experiment, using a gallium-solution
detector system, has observed the PP1 neutrinos to provide the first
unambiguous confirmation of proton-proton fusion in the Sun.

There are some discrepancies, however.

1. The first, and most well-known, "solar neutrino problem" is that
every experiment has measured a shortfall of neutrinos. About one- to
two-thirds of the neutrinos expected are observed, depending on
experimental error. In the case of GALLEX, the data read 80 units
where 120 are expected, and the discrepancy is about two standard

2. The second solar neutrino problem arises when one compares the
number of neutrinos detected at various detectors. The Kamiokande
experiment detects neutrinos by their interaction with water while the
experiment by Davis uses chlorine. One can use the Kamiokande
experiment to predict how many neutrinos can be detected in Davis'
experiment. The observed number is only 80% that of the predicted

3. The third problem arises when one compares how many neutrinos are
expected from the various processes shown above. The observed number
of neutrinos in the gallium experiments can be compared with the
number expected from the PP1 process and from the PP3 process, after
accounting for the fact that the gallium experiments only see a
fraction of the PP3 process neutrinos. The observed number agrees
with the expected number. But that means that the PP2 process cannot
contribute any neutrinos.

To explain these various shortfall, one of two things must be the
case: (1) the temperature in the Sun's core is slightly less than we
think it is, or (2) something happens to the neutrinos during their
flight over the 150-million-km journey to Earth. A third possibility
is that the Sun undergoes relaxation oscillations in central
temperature on a time scale shorter than 10 Myr, but since no one has
a credible mechanism this alternative is not seriously entertained.

(1) The fusion reaction rate is a very strong function of the
temperature, because particles much faster than the thermal average
account for most of it. Reducing the temperature of the standard solar
model by 6 per cent would entirely explain GALLEX; indeed, Bahcall has
ublished an article arguing that there may be no solar
neutrino problem at all. However, the community of solar
seismologists, who observe small oscillations in spectral line
strengths due to pressure waves traversing through the Sun, argue that
such a change is not permitted by their results.

(2) A mechanism (called MSW, after its authors) has been proposed, by
which the neutrinos self-interact to periodically change flavor
between electron, muon, and tau neutrino types. Here, we would only
expect to observe a fraction of the total, since only electron
neutrinos are detected in the experiments. (The fraction is not
exactly 1/3 due to the details of the theory.) Efforts continue to
verify this theory in the laboratory. The MSW phenomenon, also called
"neutrino oscillation", requires that the three neutrinos have finite
and differing mass, which is also still unverified.

To use explanation (1) with the Sun in thermal equilibrium generally
requires stretching several independent observations to the limits of
their errors, and in particular the earlier chloride results must be
explained away as unreliable (there was significant scatter in the
earliest ones, casting doubt in some minds on the reliability of the
others). Further data over longer times will yield better statistics
so that we will better know to what extent there is a
problem. Explanation (2) depends of course on a proposal whose
veracity has not been determined. Until the MSW phenomenon is observed
or ruled out in the laboratory, the matter will remain open.

In summary, fusion reactions in the Sun can only be observed through
their neutrino emission. Fewer neutrinos are observed than expected,
by two standard deviations in the best result to date. This can be
explained either by a slightly cooler center than expected or by a
particle-physics mechanism by which neutrinos oscillate between
flavors. The problem is not as severe as the earliest experiments
indicated, and further data with better statistics are needed to
settle the matter.


[0] The main-sequence Sun: D. D. Clayton, Principles of Stellar Evolution
and Nucleosynthesis, McGraw-Hill, 1968. Still the best text.
[0] Solar neutrino reviews: J. N. Bahcall and M. Pinsonneault, Reviews of
Modern Physics, vol 64, p 885, 1992; S. Turck-Chieze and I. Lopes,
Astrophysical Journal, vol 408, p 347, 1993. See also J. N. Bahcall,
Neutrino Astrophysics (Cambridge, 1989); J. N. Bahcall, "Solar
Neutrinos: Where We Are, Where We Are Going," 1996, Astrophysical
Journal, vol. 467, p. 475.
[1] Experiments by R. Davis et al: See October 1990 Physics Today, p 17.
[2] The GALLEX team: two articles in Physics Letters B, vol 285, p 376
and p 390. See August 1992 Physics Today, p 17. Note that 80 "units"
correspond to the production of 9 atoms of Ge(71) in a solution
containing 12 tons Ga(71), after three weeks of run time!
[3] Bahcall arguing for new physics: J. N. Bahcall and H. A. Bethe,
Physical Review D, vol 47, p 1298, 1993; against new physics: J. N.
Bahcall et al, "Has a Standard Model Solution to the Solar Neutrino
Problem Been Found?", preprint IASSNS-94/13 received at the National
Radio Astronomy Observatory, 1994.
[4] The MSW mechanism, after Mikheyev, Smirnov, and Wolfenstein: See the
second GALLEX paper.
[5] Solar seismology and standard solar models: J. Christensen-Dalsgaard
and W. Dappen, Astronomy and Astrophysics Reviews, vol 4, p 267, 1992;
K. G. Librecht and M. F. Woodard, Science, vol 253, p 152, 1992. See
also the second GALLEX paper.


Subject: E.04 Could the Sun be part of a binary (multiple) star system?
Author: Bill Owen ,
Steve Willner

Very unlikely. In the 1980's there was proposed a small companion, nicknamed
Nemesis, in a 26-million-year highly eccentric orbit, to explain apparent
periodicities in the fossil extinction record. However, these periodicities
have turned out to be more imagined than real, so the driver for the existence
of Nemesis is gone.

Furthermore, such an object would be relatively close by, bright enough in the
infrared to have been detected easily by IRAS, and its high proper motion
should have been detected by astrometrists long ago.

One very slim possibility is that a very faint companion now located
near the aphelion of an eccentric orbit is not ruled out. Such an
object would be hard to detect because its proper motion would be
small. It's not clear, however, that an orbit consistent with the
lack of detection would be stable for the Sun's lifetime.

So the chances are that there exist no stellar companions to our Sun.


Subject: E.05 When will the Sun die? How?
Author: Erik Max Francis

The Sun is a yellow, G2 V main sequence dwarf. Yellow dwarfs live
about 10 billion years (from zero-age main sequence to white dwarf
formation), and our Sun is already about 5 billion years old.

Main sequence stars (like our Sun) are those that fuse hydrogen into
helium, though the exact reactions vary depending on the mass of the
star. The main sequence phase is by far the most stable and
long-lived portion of a star's lifetime; the remainder of a star's
evolution is almost an afterthought, even though the results of that
evolution are what are most visible in the night sky. As the Sun
ages, it will increase steadily in luminosity. In approximately 5
billion years, when the hydrogen in the Sun's core is mostly
exhausted, the core will collapse---and, consequently, its temperature
will rise---until the Sun begins fusion helium into carbon. Because
the helium fuel source will release more energy than hydrogen, the
Sun's outer layers will swell, as well as leaking away some of its
outer atmosphere to space. When the conversion to the new fuel source
is complete, the Sun will be slightly decreased in mass, as well as
extending out to the current orbit of Earth or Mars (both of which
will then be somewhat further out due to the Sun's slightly decreased
mass). Since the Sun's fuel source will not have increased in
proportion to its size, the blackbody power law indicates that the
surface of the Sun will be cooler than it is now, and will become a
cool, deep red. The Sun will have become a red giant.

A few tens or hundreds of millions of years after the Sun enters its
red giant phase (or "helium main sequence"; the traditional main
sequence is occasionally referred to as the hydrogen main sequence to
contrast the other main sequences that a massive star enters), the Sun
will begin to exhaust its fuel supply of helium. As before, when the
Sun left the (hydrogen) main sequence, the core will contract, which
will correspondingly lead to an increase in temperature in the core.

For very massive stars, this second core collapse would lead to a
carbon main sequence, where carbon would fuse into even heavier
elements, such as oxygen and nitrogen. However, the Sun is not
massive enough to support the fusion of carbon; instead of finding
newer fuel sources, the Sun's core will collapse until degenerate
electrons---electrons which are in such a compressed state that their
freedom of movement is quantum mechanically restricted---smashed
together in the incredible pressures of the gravitational collapse,
will halt the core's collapse. Due to the energy radiated away during
the process process of the formation of this electron-degenerate core,
the atmosphere of the Sun will be blown away into space, forming what
astronomers call a planetary nebula (named such because it resembles a
planetary disk in the telescope, not because it necessarily has
anything to do with planets). The resulting dense, degenerate core is
called a white dwarf, with a mass of something like the Sun compressed
into a volume about that of the Earth's.

White dwarfs are initially extremely hot. But since the white dwarf
is supported by degenerate electrons, and has no nuclear fuel to speak
of to create more heat, they have no alternative but to cool. Once
the white dwarf has cooled sufficiently---a process which will take
many billions of years---it is called an exhausted white dwarf, or a
black dwarf.


Subject: E.06 What happens to the planets when the Sun dies?
Author: Joseph Lazio

A couple of possibilities exist. Prior to forming a planetary nebula,
a low-mass star (i.e., one with a mass similar to that of the Sun)
forms a red giant. Planets close to the star are engulfed in the
expanding star, spiral inside it, and are destroyed. In our own solar
system, Mercury and Venus are doomed.

As the star expands to form a red giant, it also starts losing mass.
All stars lose mass. For instance, the Sun is losing mass. However,
at the rate at which the Sun is currently losing mass, it would take
over 1 trillion years (i.e., 100 times longer than the age of the
Universe) for the Sun to disappear.

When a star enters the red giant phase, the rate at which it loses
mass can accelerate. The mass of a star determines how far a planet
orbits from it. Thus, as the Sun loses mass, the orbits of the other
planets will expand. The orbit of Mars will almost certainly expand
faster than the Sun does, thus Mars will probably not suffer the same
fate as Mercury and Venus. It is currently an open question as to
whether the Earth will survive or be engulfed.

The orbits of planets farther out (Jupiter, Saturn, Uranus, Neptune,
and Pluto) will also expand. However, they will not expand by much
(less than double in size), so they will remain in orbit about the Sun
forever, even after it has collapsed to form a white dwarf.

(Any planets around a high-mass star would be less lucky. A high-mass
star loses a large fraction of its mass quickly in a massive explosion
known as a supernova. So much mass is lost that the planets are no
longer bound to the star, and they go flying off into space.)

As for the material in the planetary nebula, it will have little
impact on the planets themselves. The outer layers of a red giant are
extremely tenuous; by terrestrial standards they are a fairly decent


Subject: E.07 Could the Sun explode?
Author: Erik Max Francis

The short answer is no; the detailed answer depends entirely on what is
meant by "explode." The Sun doesn't have anything like enough mass to
form a Type 2 supernova (whose progenitors are supergiants), which
require more than about 8 solar masses; thus the Sun will not become a
supernova on its own.

"Novae" arise from an accumulation of gases on a collapsed object,
such as a white dwarf or a neutron star. The gas comes from a nearby
companion (usually a distended giant). Although nova explosions are
large by human standards, they are not nearly powerful enough to
destroy the star involved; indeed, most novae are thought to explode
repeatedly on time scales of years to millenia. Since the Sun is not
a collapsed object, nor does it have a companion---let alone a
collapsed one---the Sun cannot go (or even be involved in) a nova.

Under conditions not well understood, the accumulation of gases on a
collapsed object may produce a Type 1 supernova instead of an ordinary
nova. This is similar in principle to a nova explosion but much larger;
the star involved is thought to be completely destroyed. The Sun will
not be involved in this type of explosion for the same reasons it will
not become a nova.

When the Sun evolves from a red giant to a white dwarf, it will shed its
atmosphere and form a planetary nebula; but this emission could not
really be considered an explosion.


Subject: E.08 How are solar system objects and features named?
Author: Bill Owen ,
Gareth Williams

Comets are named for their discoverers, up to three names per comet.

Minor planets are named by the Small Bodies Names Committee of the
International Astronomical Union Commission 20. Discoverers of minor
planets may propose names to the SBNC and minor planets have been
named to honor all sorts of famous (and some not so famous) people and
animals in all walks of life.

Planetary satellites are named by the Working Group for Planetary
System Nomenclature of the IAU, in consultation with the SBNC (mainly
to avoid conflicts of names), and they *usually* defer to the
discoverer's wishes. Names of satellites are usually taken from Greek
mythology or classical literature.

Features on Solar System bodies are named by the same commission, generally
following a specific theme for each body. For instance, most features on Venus
are named in honor of famous women, and volcanos on Io are named for gods and
goddesses of fire.

For additional discussion, see

The IAU Planetary System Nomenclature Working Group's Web site,
URL:http://wwwflag.wr.usgs.gov/nomen/nomen.html, has an extensive
discussion, as well as lists of names.


Subject: E.09 Where can I find pictures and planetary data?

See Part 1 of this FAQ, and
and URL:http://wwwflag.wr.usgs.gov/USGSFlag/Space/.


Subject: E.10 Could Jupiter become a star?
Author: Erik Max Francis

A star is usually defined as a body whose core is hot enough and under
enough pressure to fuse light elements into heavier ones with a
significant release of energy. The most basic (and easiest, in terms of
the temperatures and pressures required) type of fusion involve the
fusion of four hydrogen nuclei into one helium-4 nucleus, with a
corresponding release of energy (in the form of high-frequency photons).
This reaction powers the most stable and long-lived class of stars, the
main sequence stars (like our Sun and nearly all of the stars in the
Sun's immediate vicinity).

Below certain threshold temperatures and pressures, the fusion reaction
is not self-sustaining and no longer provides a sufficient release of
energy to call said object a star. Theoretical calculations indicate
(and direct observations corroborate) that this limit lies somewhere
around 0.08 solar masses; a near-star below this limit is called a brown

By contrast, Jupiter, the largest planet in our solar system, is only
0.001 masses solar. This makes the smallest possible stars roughly 80
times more massive than Jupiter; that is, Jupiter would need something
like 80 times more mass to become even one of the smallest and feeblest
red dwarfs. Since there is nothing approaching 79 Jupiter masses of
hydrogen floating around anywhere in the solar system where it could be
added to Jupiter, there is no feasible way that Jupiter could become a


Subject: E.11 Is Pluto a planet? Is Ceres? Is Titan?
Author: Andy Rivkin

While on the face of it, this seems a reasonably easy question with a
simple answer, like the "When does the 21st Century begin?" question
there is no hard and fast rule, no committee of astronomers who decide
these things. The best rule of thumb is that if people think
something's a planet, it is. Common criteria include orbiting the Sun
rather than another body (although sticklers find this troublesome)
and being "large". Some have suggested using "world" as a neutral
term for an interesting solar system body. The word "planet"
originally meant "wanderer", so using a strict definition, everything
in the solar system is a planet!

When Pluto was discovered in 1930, there was no question as to whether
it was a planet. The predictions made at the time imagined it to be
at least the size of the Earth. As better data became available with
the discovery of Pluto's moon Charon allowing the determination of a
mass for Pluto, and with Pluto and Charon eclipsing each other in the
late 1980's--early 1990's, it was found that Pluto is much smaller
than the Earth, with a diameter of roughly 2300 km (or about 1400
mi.). In the last several years, a number of small bodies at about
the same distance from the Sun as Pluto have been discovered,
prompting some to call Pluto the "King of the Kuiper Belt" (the Kuiper
Belt is a postulated population of comets beyond Neptune's orbit) and
rally for its demotion from bona-fide planet to overgrown comet.

Is Pluto a planet? It depends on what one thinks is necessary to
bestow planetary status. Pluto has an atmosphere and a satellite. Of
course, Titan has a much larger atmosphere, and the tiny asteroid Ida
has a satellite. Most astronomers would probably consider stripping
Pluto of its status akin to stripping [the U.S. states of] Connecticut
or Vermont of statehood because Texas and Alaska later joined.

Is Ceres a planet? Like Pluto, when it was first discovered there was
no doubt that it was. Within a few years, however, Pallas, Vesta and
Juno were discovered. While Ceres is the largest asteroid, the
second, third and fourth largest asteroids are roughly half its size,
compared to Pluto, which is about ten times larger than the Kuiper
Belt objects found so far. Ceres is also not thought to have
undergone large-scale geological processes such as vulcanism, although
Vesta has. The consensus is probably that neither Ceres nor any other
asteroid is a "planet", though they are interesting bodies in their
own right.

Is Titan a planet? In the 1940's a methane atmosphere was discovered
around Titan, making it the only satellite with a substantial
atmosphere. This atmosphere has long prevented observations of the
surface, frustrating the attempts of Voyager 1 and 2 and leading
theorists to suggest a Titan-wide global ocean of carbon compounds.
Recent observations have been able to penetrate to the surface of
Titan, showing tantalizing glimpses of what may be continents on the
surface. The atmosphere combined with Titan's large size have led
some to consider Titan a "planet", but what about Ganymede, which is
larger, or Mercury which is smaller and has no atmosphere? Again, the
general consensus is that satellites are not planets.


Subject: E.12 Additional planets:

In addition to the questions answered here, addition info is at


Subject: E.12.1 What about a planet (Planet X) outside Pluto's orbit?
Author: Ron Baalke ,
contributions by Bill Owen ,
edited by Steve Willner

Pluto was discovered from discrepancies in the orbits of Uranus and
Neptune. The search was for a large body to explain the
discrepancies, but Pluto was discovered instead (by accident, if you
will, though Clyde Tombaugh's search was systematic and thorough).
Pluto's mass is too small to cause the apparent discrepancies, so the
obvious hypothesis was that there is another planet waiting to be

The orbit discrepancies go away when you use the extremely accurate
measurements of the masses of Uranus and Neptune made by Voyager 2
when it flew by those planets in 1986 and 1989. Uranus is now known
to be 0.15% less massive and Neptune 0.51% less massive, than was
previously believed.

[N.B. These numbers come from comparing the post-Voyager masses to those in
the 1976 IAU standard.]

When the new values for these masses is factored into the equations,
the outer planets are shown to be moving as expected, going all the
way back to the early 1800's.

The positional measurements do not bode too well for the existence of
Planet X. They do not entirely rule out the existence of a Planet X,
but they do indicate that it will not be a large body.

Standish, E. M., Jr. 1993, "Planet X: No Dynamical Evidence in the
Optical Observations," Astronomical Journal, vol. 105, p. 2000--2006


Subject: E.12.2 What about a planet inside Mercury's orbit?
Author: Paul Schlyter

The French mathematician Urbain Le Verrier, co-predictor with
J.C. Adams of the position of Neptune before it was seen, in an 1860
lecture announced that the problem of observed deviations of the
motion of Mercury could be solved by assuming a planet or a second
asteroid belt inside Mercury's orbit. The only ways to observe this
planet (or asteroids) was if/when it transited the Sun or during total
solar eclipses.

In 1859, Le Verrier had received a letter from the amateur astronomer
Lescarbault, who reported having seen a round black spot on the Sun on
1859 March 26, looking like a planet transiting the Sun. From
Lescarbault's observations, Le Verrier estimated a mean distance from
the Sun of 0.1427 AU (period of 19.3 days). The diameter was
considerably smaller than Mercury's and its mass was estimated at 1/17
of Mercury. This was too small to account for the deviations of
Mercury's orbit, but perhaps this was the largest member of an
asteroid belt? Additional support for such objects was provided by
Prof. Wolf and others at the Zurich sunspot data center, who
identified a total of two dozen spots on the Sun which fit the pattern
of two intra-Mercurial orbits, one with a period of 26 days and the
other of 38 days.

Le Verrier fell in love with the planet and named it Vulcan. In 1860
Le Verrier mobilized all French and some other astronomers to find
Vulcan during a total solar eclipses---nobody did. Wolf's suspicious
"spots" revived Le Verrier's interest, and just before Le Verrier's
death in 1877, there were more "detections." On 1875 April 4, a
German astronomer, H. Weber, saw a round spot on the Sun. Le Verrier's
orbit indicated a possible transit on April 3 that year, and Wolf
noticed that his 38-day orbit also could have performed a transit at
about that time. That "round dot" was also photographed at Greenwich
and Madrid.

There was one more flurry of "detections" after the total solar
eclipse at 1878 July 29: Small illuminated disks which could only be
small planets inside Mercury's orbit. J.C. Watson (professor of
astronomy at the Univ. of Michigan) believed he'd found *two*
intra-Mercurial planets! Lewis Swift (co-discoverer of Comet
Swift-Tuttle, which returned 1992) also saw "Vulcan"---but at a
different position than either of Watson's two "intra-Mercurials." In
addition, neither Watson's nor Swift's Vulcans could be reconciled
with Le Verrier's or Lescarbault's Vulcan.

After this, nobody ever saw Vulcan again, in spite of several searches
at different total solar eclipses. In 1916, Albert Einstein published
his General Theory of Relativity, which explained the deviations in
the motions of Mercury without invoking an additional planet. In 1929
Erwin Freundlich photographed the total solar eclipse in Sumatra. A
comparison with plates taken six months later showed no unknown object
brighter than 9th magnitude near the Sun.

What did these people really see? Lescarbault had no reason to tell a
fairy tale, and even Le Verrier believed him. It is possible that
Lescarbault happened to see a small asteroid passing just inside
Earth's orbit. Such asteroids were unknown at that time. Swift and
Watson could, during the hurry to obtain observations during totality,
have misidentified some stars, believing they had seen Vulcan.

"Vulcan" was briefly revived around 1970-1971, when a few researchers
thought they had detected several faint objects close to the Sun
during a total solar eclipse. These objects might have been faint
comets, and comets have been observed to collide with the Sun.


Subject: E.13 Won't there be catastrophes when the planets align in
the year 2000?
Author: Laz Marhenke ,
Chris Marriott

Obviously there were no catastrophes in May (05-05-2000), nor were
there any in the year 1982.

For starters, the planets only "align" in a very rough fashion. They
don't orbit the Sun in the same plane, so it's impossible to get very
many of the planets in a straight line. Nevertheless, any time they
all get within about 90 degrees of each other, someone will claim
they're "aligned." The last time this happened was 1982 when dire
predictions were heard about how the "Jupiter effect" would lead to
world-wide disaster.

Second, even if they *were* all aligned, the effect on the Earth would
be miniscule. It's true that the other planets' gravity does affect
the orbit of the Earth, but the effect is small, and lining up all the
planets doesn't even come close to making it big enough for anyone to
notice. The effect on the Earth is dominated by Jupiter and Venus
anyway (Jupiter because it's massive, Venus because it's occasionally
very close to us). All the other planets put together only affect us
about 10% as much as those two, so the fact that they're all in the
same general direction as Jupiter and Venus doesn't make much

Third, even if all the planets could produce a strong gravitational
effect on the Earth (which they can't, unless they find a way to
increase their mass by a factor of 10--100), it wouldn't result in the
"crust spinning over the magma" or some other dire effect, since their
gravity would be pulling on every part of the Earth (almost) equally.

The "(almost)" is because the other planets do exert tidal forces on
the Earth, which means they pull on different parts of the Earth very
slightly differently. However, tidal forces decrease *rapidly* with
distance (as the third power), so these forces are very small: The
tidal force from Venus at its closest approach to Earth is only
1/17,000th as large as the Moon's, and we seem to survive the Moon's
tides well enough twice a day. If the Moon raises tides of 1 meter
(three feet) where you live, Venus at its closest will raise tides of
1/20th of a millimeter, or about the thickness of a hair. The other
planets have even smaller tidal effects on the Earth than Venus does.

Finally, it's worth remembering that the Earth is about 4.5 billion
years old. Whilst these "alignments" may be rare in terms of a human
lifetime (occurring once every few decades), they've occurred numerous
times during the time that life has existed on this planet, and many,
many times in the comparatively brief time that humans have been
around. Brian Monson found ten such "alignments" between AD 1000 and
AD 2000, URL:http://drumright.ossm.edu/astronomy/conjunctions.html.
Thus, over the history of this planet there have been about 45 million
such "alignments." The fact that we're still here to talk about it is
proof enough that nothing *too* terrible happens!


Subject: E.14 Earth-Moon system

Related questions include
B.11 Why does the Moon look so big when it's near the horizon?
B.12 Is it O.K. to look at the Sun or solar eclipses using exposed
film? CDs?
C.07 Easter
C.08 What is a "blue moon?"
C.11 How do I calculate the phase of the moon? and
C.13 Why are there two tides a day and not just one?


Subject: E.14.1 Why doesn't the Moon rotate?
Author: Laz Marhenke

In fact the Moon *does* rotate: It rotates exactly once for every
orbit it makes about the Earth. The fact that the Moon is rotating
may seem counterintuitive: If it's always facing towards us, how can
it be rotating at all? To see how this works, put two coins on a
table, a large one to represent the Earth, and a small one to
represent the Moon. Choose a particular place on the edge of the
"Moon" as a reference point. Now, move the Moon around the Earth in a
circle, but be careful to always keep the spot you picked pointed at
the Earth (this is analogous to the Moon always keeping the same face
pointed at the Earth). You should notice that as you do this, you
have to slowly rotate the Moon as it circles the Earth. By the time
the Moon coin goes once around the Earth coin, you should have had to
rotate the Moon exactly once.

This exact equality between the Moon's rotation period and orbital
period is sometimes seen as a fantastic coincidence, but, in fact,
there is a physical process which slowly changes the rotation period
until it matches the orbital period. See the next entry.


Subject: E.14.2 Why does the Moon always show the same face to the
Author: Laz Marhenke

When it first formed, the Moon probably did not always show the same
face to the Earth. However, the Earth's gravity distorts the Moon,
producing tides in it just as the Moon produces tides in the Earth.
As the Moon rotated, the slight elongation of its tidal bulge was
dragged a bit in the direction of its rotation, providing the Earth
with a "handle" to slow down the Moon's rotation. More specifically,
the tidal bulge near the Earth is attracted to the Earth more strongly
than the bulge away from the Earth. Unless the bulge points toward
the Earth, a torque is produced on the Moon.

If we imagine looking down on the Earth-Moon system from the north
pole, here's what we'd see with the Moon rotating at the same rate as
it goes around the Earth:

Earth Moon
/ \ ____ ^
| | / \ |
\__/ \____/ Orbiting
this way
Tidal bulge *greatly*

What if the Moon were rotating faster? Then the picture would look like:

Earth Moon
/ \ ___ ^
| | / ) |
\__/ (___/ Orbiting
this way
Tidal bulge *greatly*

If it isn't clear why the tidal bulge should move the way the picture
shows, think about it this way: Take the Moon in the top picture, with
its tidal bulges lined up with the Earth. Now, grab it and rotate it
counterclockwise 90 degrees. Its tidal bulge is now lined up the
"wrong" way. The Moon will eventually return to a shape with tidal
bulges lined up with the Earth, but it won't happen instantly; it will
take some time. If, instead of rotating the Moon 90 degrees, you did
something less drastic, like rotating it one degree, the tidal bulge
would still be slightly misaligned, and it would still take some time
to return to its proper place. If the Moon is rotating faster than
once per orbit, it's like a constant series of such little
adjustments. The tidal bulge is perpetually trying to regain its
correct position, but the Moon keeps rotating and pushing it a bit out
of the way.

Returning to the second picture above, the Earth's gravitational
forces on the Moon look like this:
F1 -----/ )
F2 -------(___/

F2 is larger than F1, because that part of the Moon (the "bottom" half
in the drawing, or the half that's "rearward" in the orbit) is a bit
closer to the Earth. As a result, the two forces together tend to
twist the Moon clockwise, slowing its spin. Over time, the result is
that the Moon ends up with one face always facing, or "locked," to the
Earth. If you drew this picture for the first case, (where the Moon
rotates at the same rate that it orbits, and the tidal bulges are in
line with the Earth), the forces would be acting along the same line,
and wouldn't produce any twist.

Another way to explain this is to say that the Moon's energy of
rotation is dissipated by internal friction as the Moon spins and its
tidal bulge doesn't, but I think the detailed force analysis above
makes things a little clearer.

This same effect occurs elsewhere in the solar system as well. The
vast majority of satellites whose rotation rates have been measured
are tidally locked (the jargon for having the same rotation and
orbital periods). The few exceptions are satellites whose orbits are
very distant from their primaries, so that the tidal forces on them
are very small. (There could be, in principle, other exceptions among
some of the close-in satellites whose rotation rates haven't been
measured, but this is unlikely as tidal forces grow stronger the
closer to the planet the satellite is.)

Pluto's satellite Charon is so massive (compared to Pluto) that it has
locked Pluto, as well as Pluto locking Charon. This will happen to
the Earth eventually too, assuming we survive the late stages of the
Sun's evolution intact. :')


Subject: E.14.3 Is the Moon moving away from the Earth? (and why is
Phobos moving closer to Mars?)
Author: Richard A. Schumacher ,
Michael Dworetsky ,
Joseph Lazio

Yes, at a rate of about 3--4 cm/yr.

The tidal bulges on the Earth (largely in the oceans), raised by the
Moon, are rotated forward (ahead of) the Earth-Moon line by the
Earth's rotation since it is faster than the Moon's orbital motion.

Using a similar picture as from the previous question, we'd see
(looking down from the north pole):

Earth Moon
/ ) ___ ^
/ / / \ |
(____/ \___/ Moon's orbit &
Earth's rotation
(Ocean) Tidal bulge this way
*greatly* exaggerated.

The gravity from these leading and trailing bulges impels the Moon
mostly forward along the direction of its motion in orbit (the Moon's
orbit is not exactly in the plane of the Earth's equator). This force
transfers momentum from the rotating Earth to the revolving Moon,
simultaneously dragging the Earth and accelerating the Moon.

In addition to causing the Moon to recede from the Earth, this process
also causes the Earth's rotation to slow and days to become longer (at
a rate of about 0.002 seconds every century). Eventually the result
will be that the Earth will show only one face to the Moon (much like
the Moon only shows one face to the Earth). A lower limit to how long
it will take for the Earth and Moon to become tidally locked is 50
billion years, at which point the month and the Earth's "day" will
both be approximately 50 (of our current) days long. However, this
estimate is based on the assumption that liquid water seas would be
present on Earth's surface to provide the tidal interactions
necessary. But as the Sun evolves, the seas will evaporate and tidal
interactions will be much slower (solid planet distortions only). The
oceans will evaporate about 1--2 billion years from now, so the actual
time will probably be much longer.

Considerably more detail on the evolution of the Earth-Moon system can
be found in an article by J. Burns in the book _Planetary Satellites_
(ed. J. Burns [Tucson: University of Arizona]) and in Sir Harold
Jeffries' _The Earth_, 3rd ed (Cambridge Univ Press, 1952).

It is also interesting to consider what would happen if a satellite
orbits its planet *faster* than the planet rotates. This is not the
case for the Earth and Moon, but it is true for Mars and Phobos. In
this case, Phobos also raises (crustal) tides on Mars. But now,
Phobos is in front of the tidal bulge, so the gravitational action of
the tidal bulge slows Phobos and Phobos moves *inward*. Thus, at some
point in the future, Phobos will hit Mars. The most recent estimate
is that the impact will occur in 40 million years, by A. T. Sinclair
(1989, Astronomy & Astrophysics, vol. 220, p. 321).


Subject: E.14.4 What was the origin of the Moon?
Author: George Cummings
Joseph Lazio ,

The Moon presents a curious problem. Of the terrestrial planets
(Mercury, Venus, Earth, and Mars) only Earth and Mars have satellites.
Mars' satellites are much smaller than the Moon, both in absolute size
and in comparison to their primary. (The Moon is 3476 km in diameter
while Phobos is 23 km in diameter; the Moon's diameter is 27% that of
the Earth while Phobos' diameter is 0.34% that of Mars.)

Furthermore, the Moon's chemical composition is peculiar. In many
respects it is quite similar to the Earth's, except that the Moon
seems to have less iron (and similar elements like nickel) and
considerably less water (it's quite dry!).

Until recently there were three competing theories to explain the
Moon's origin. (1) The Moon formed elsewhere in the solar system and
was captured eventually by the Earth. (2) The Moon and Earth formed
together at the same time in essentially the same place. (3) The
early Earth was spinning so fast that a portion of it broke off and
formed the Moon (possibly leaving the Pacific Ocean basin as a
result). All theories had their difficulties, though.

If the Moon formed elsewhere in the solar system (like between the
orbits of Venus and Earth or between the orbits of Earth and Mars),
how did it get disturbed into the orbit that took it near the Earth?
Furthermore, it is actually quite difficult for an object that is not
initially orbiting the Earth to begin doing so. The incoming object
must lose energy. In the case of Mars, its small satellites could
have gotten close enough to skim the upper part of its atmosphere,
which would cause them to lose energy from air resistance. Because
the Moon is so big, it probably would have hit the Earth rather than
passing just close enough to lose just enough energy to be captured
into orbit.

If the Earth and Moon formed simultaneously at nearly the same
location in the solar system, then the differing chemical compositions
of the two are quite difficult to understand. Why are they similar
yet so different?

Finally, there isn't much evidence to suggest that the early Earth was
spinning anywhere near fast enough for it to break apart.

With the realization in the 1980s that impacts (of comets, asteroids,
etc.) have played a major role in the history of the solar system, a
new theory emerged:

The Moon was formed when a Mars-sized object collided with the Earth
when the Earth was very young, about 4.5 billion years ago. Much of
the Earth's crust and mantle, along with most of the colliding object,
disintegrated and was blown into orbit thousands of kilometers high.
About half of this debris fell back to Earth. The rest coalesced into
the Moon. (Loose material in orbit can coalesce if it is outside the
"Roche limit," otherwise it will be pulled apart by tidal forces. The
Roche limit for the Earth is approximately 3 Earth radii. The
material outside this limit formed the Moon, the material inside the
limit fell back to Earth.) Since the time of its original formation,
the Moon has slowly moved farther from the Earth to its present

This theory does a good job of explaining why only the Earth has a
large moon and why the Moon's chemical composition is similar yet
different. Impacts are random events, and there almost certainly were
not a lot of large objects left in the solar system as the planets
were nearly the end of their formation. The Earth just happened to be
the planet struck by this large, rogue planetoid. If we could start
over the formation of the solar system, it might be Venus or Mars that
would end up with a large moon. The chemical composition of the Earth
and Moon are clearly predicted to be similar in this model, since a
portion of the Earth went into forming the Moon and a portion of the
impactor remained in the Earth. The Moon would be deficient in iron
and similar metals if the impact occurred after those elements had
largely sunk to the center of the Earth (i.e., after the Earth
differentiated). The Moon should also be quite dry because the
material from which the Moon formed was heated to a high temperature
in the impact, thereby evaporating all of the water.

Computer models of this event indicate that the Moon coalesced in only
about a year. Also interesting is that a large percentage of
simulations result in the formation of two moons. Some of the more
recent simulations suggest that the colliding object might have had to
have been much larger, about three times the size of Mars.

More information on this theory of Moon formation can be found at


Subject: E.15 What's the difference between a solar and lunar
eclipse? Where can I find more information about eclipses?
Author: Joseph Lazio

A solar eclipse occurs when the Moon passes between the Earth and Sun
and the Moon's shadow crosses the Earth, viz. (not to scale!)

Sun Moon Earth

Solar eclipses can be total, partial, or annular. A total eclipse is
when the Moon obscures the Sun entirely. A partial eclipse is when
the Moon only covers a portion of the Sun. Because the Moon's orbit
about the Earth is not perfectly circular, sometimes it is slightly
farther away from the Earth. If a solar eclipse occurs when the Moon
is at the far point in its orbit, the Moon will not cover the Sun
entirely. A thin ring, or annulus, of sunlight will be visible around
the Moon. This kind of eclipse is called an annular eclipse.

**Solar eclipses can be damaging to one's eyesight, unless proper
precautions are taken!** See FAQ Question B.11 and the Eclipse Home
Page, URL:http://sunearth.gsfc.nasa.gov/eclipse/.

A lunar eclipse occurs when the Earth passes between the Moon and Sun,
viz. (again, not to scale)

Sun Earth Moon

Lunar eclipses are either total or partial, depending upon whether the
Moon moves completely into the Earth's shadow or not. Lunar eclipses
are always safe to view.

Eclipses do not happen once a month because the Earth's orbit about
the Sun and the Moon's orbit about the Earth are not in the same
plane. The above "pictures" are if one is looking "down" on the Earth
from the North Pole (or "up" on the South Pole). If we look at the
system from the side (looking at the Earth's equator), the typical
situation is

Sun Earth

(with the angle shown exaggerated greatly, the actual angle is about 5
degrees). Only when the three bodies are in the same plane can an
eclipse occur. The total number of eclipses, both lunar and solar,
never exceeds seven in a year. Because the Moon is so much smaller
than the Earth, and casts a smaller shadow, solar eclipses are more
infrequent than lunar eclipses; in a year, between 2 to 4 lunar
eclipses will occur and at least 2 solar eclipses will occur. *Total*
solar eclipses happen only every 1.5 years or so.

For additional information see the Eclipse Home Page,


Subject: E.16 What's the Oort Cloud and Kuiper Belt?
Author: Joseph Lazio

Comets have highly elliptical orbits. When at perihelion or closest
approach to the Sun, they are typically about the same distance from
the Sun as the Earth is. When at aphelion or farthest distance from
the Sun, they can be well outside the orbit of Pluto. If a comet is
observed for a sufficient period of time, its motion on the sky allows
us to estimate when it is at perihelion and how far away aphelion is
(more precisely, we can estimate the major axis of its orbit).

In 1950 Jan Oort was analyzing the comets whose orbits had been
determined. He discovered that many comets had their aphelia at
roughly the same distance from the Sun, about 50,000 AU. (For
reference, the Earth is at a distance of 1 AU from the Sun, Neptune is
at a distance of 40 AU, and the nearest star is at a distance of
270,000 AU.) So Oort proposed that the Sun was surrounded by a vast
swarm of comets, stretching nearly 1/5 of the distance to the nearest

At these large distances from the Sun, these comets are only loosely
gravitationally bound to the Sun. A slight gravitational nudge, from
a star passing within a couple of light years or so perhaps, is enough
to change their orbits dramatically. The gravitational tug can result
in a comet either (1) becoming gravitationally unbound from the Sun
and drifting into interstellar space never to return or (2) falling
into the inner solar system. This is the currently accepted
explanation for the origin of so-called "long-period" comets. These
comets orbit the Sun at great distances, until a slight gravitational
nudge changes their orbit and causes them to fall into the inner solar
system, where we see them. Because their aphelia remain at large
distances, it can take hundreds, thousands, or maybe even 1 million
years before they return to the inner solar system. Comet Hale-Bopp
is an example of such a comet.

Theorizing that comets originate from the Oort cloud doesn't explain
the properties of all comets, however. "Short-period" comets, those
with periods less than 200 years, have orbits in or near the
ecliptic---the plane in which the Earth and other planet orbit.
Long-period comets appear to come from all over the sky. Short-period
comets can be explained if there is a disk of material, probably left
over from the formation of the solar system, extending from the orbit
of Neptune out to 50 AU or more. Collisions between objects in such a
disk and gravitational tugs from the gas giants in our solar system
would be enough to cause some of the objects to fall into the inner
solar system occasionally where we would see them. Comet Halley is
probably an example of such a comet.

Direct detection of Kuiper Belt objects occurred in the early 1990s
with the detection of 1992/QB1, see
URL:http://www.ifa.hawaii.edu/faculty/jewitt/qb1.html. Additional
indirect evidence for a disk of material around the Sun comes from
images of nearby stars which have disks around them. These disks
around other stars are several times larger than the Kuiper Belt has
thus far been observed to extend, but they might be qualitatively
similar to the Kuiper Belt. See

Interestingly, current theories for the origin of the Oort Cloud and
Kuiper Belt indicate that the Kuiper Belt probably formed first. The
Kuiper Belt is the detritus from the formation of the solar system.
Objects from it that make it into the inner solar system can interact
gravitationally with the giant planets, particularly Jupiter. Some
objects would have had their orbits changed so that they impacted with
one of the planets (like Comet Shoemaker-Levy 9 did in 1994); some
objects would be ejected from the solar system entirely; and some
objects would be kicked into very large orbits and into the Oort


Subject: E.17 Asteroid Impacts

Much of the material in this section is drawn from the SpaceGuard
Survey report, URL:http://ccf.arc.nasa.gov/sst/spaceguard.html.

A crucial point about asteroid impacts is that they are random. Below
are various estimates of the frequency with which the Earth is struck
by objects of various sizes. These estimates are, roughly speaking,
averages over the Earth's history. For instance, the average time
between the impact of a 100 m diameter object is roughly 100--200 yr.
The actual time between the impacts of such objects could be shorter
than 10 yr or longer than 1000 yr.

For more information about Near-Earth Objects, those asteroids (or
minor planets) that have orbits similar to Earth's, see the following.
A list of "Potentially Hazardous Asteroids" (PHAs) is at
URL:http://cfa-www.harvard.edu/iau/lists/Dangerous.html. These have
a projected closest distance to Earth of less than 0.05 AU (7.5
million km, about 1000 Earth radii). A list of closest approaches to
the Earth by PHAs between 1999 and 2099 is available at
URL:http://cfa-www.harvard.edu/iau/lists/PHACloseApp.html. A list
of moderately close (to within 0.2 AU) approaches to the Earth by
asteroids and comets between 1999 and 2032 is available at
URL:http://cfa-www.harvard.edu/iau/lists/CloseApp.html. It is worth
emphasizing that, at the moment, *none* of the known objects presents
a serious risk of collision.


Subject: E.17.1 What would be the effects of an asteroid impact on
the Earth?
Author: Joseph Lazio

The Earth is constantly pelted by bits of cosmic debris. Most of this
simply burns up in the atmosphere (as one can attest by simply
watching meteors on a dark night). However, if an object is big
enough it can survive passage through the atmosphere. The damage done
by a meteorite (an object that strikes the Earth) depends upon its
initial size.

10--100 m: Objects in this size range can produce devastation similar
to that of an atomic blast (leading to them occasionally being called
"city-busters"). Effects include severe damage to or collapse of
standing buildings and the ignition of flammable materials leading to
widespread fires. The radius over which such effects occur would vary
depending upon the size and composition of the object, but could
easily exceed 10 km. The Tunguska event, in Siberia, of 1908 is
thought to have been from an object about 60 m in size; it led to
trees being flattened out to 20 km and trees 40 km away being damaged.

At the small end of this size range, objects about 10 m strike the
Earth about once a decade. Fortunately, only the densest objects,
those containing iron, survive to the surface; most of the objects of
this size explode sufficiently high in the atmosphere that there are
no effects (other than maybe a loud noise) on the ground. At the
larger end of this size range, it is estimated that the Earth is
struck several times a millennium or about 1 impact every 100--200 yr.

100 m--1 km: Objects in this size range are likely to cause severe
damage over a regional area, possibly as large as a continent (hence
the name "continent-busters"). If they strike land, they will almost
certainly produce a crater, while an ocean impact will generate large
tidal waves. A 150 m object might produce a crater 3 km in diameter,
an ejecta blanket 10 km in diameter, and a zone of destruction
extending much farther out. For a 1 km impactor the zone of
destruction might reasonably extend to cover countries. The death
toll could be in the tens to hundreds of millions. A 1 km impactor
could begin to have minor global consequences, including global
cooling caused by vast amounts of dust in the atmosphere.

Estimates from the geologic record suggest that craters are formed on
the Earth roughly once every 5000 yr.

1--10 km: Objects in this size range are likely to cause severe global
effects ("species-busters"). An impact 65 million years ago by an
object of 5--10 km in diameter is thought to have been partially or
fully responsible for the extinction of half the living species of
animals and plants at the time, including the dinosaurs. The crater
alone from such an impact will be 10--15 times larger than the object
itself. World-wide crop failures from dust injected into the
atmosphere could imperil civilization, and the largest-sized objects
could make the human species extinct.

The frequency with which the Earth is struck by such objects has to be
estimated from the geological and paleontological record. At the low
end of this size range, estimates are that such impacts occur roughly
every 300 000 yr; at the upper end of the size range, impacts occur
about every 10 million years.


Subject: E.17.2 What can we do about avoiding impacts?
Author: Joseph Lazio

A number of papers on the risks, potential damages from impacts, and
ways to mitigate the danger is at

Our ability to prevent impacts depends upon several things, the size
of the object, its orbit, and the amount of time until impact.
Generally speaking, the more time the better. It is perhaps
counter-intuitive, but we could mount the best defense against objects
in orbits similar to that of Earth. Such an object would pass close
to Earth several times, giving us many chances to discover it,
calculate an extremely accurate orbit, and launch one or more missions
to it. We might have decades or even centuries to plan. Conversely,
a comet on an impact course might be discovered only a month or so
away from impact, giving us little or no time to act.

The optimum approach to avoiding an impact is to discover an object
well before impact and gently nudge it. If discovered long enough
before impact, only small nudges are sufficient to change the object's
orbit so that it will no longer strike Earth. There are a number of
strategies to nudge an asteroid including landing a rocket engine on
the asteroid or vaporizing a small portion of it with a laser or
stand-off nuclear blast or reflected, concentrated sunlight.

Popular depictions of laser beams or nuclear weapons being used to
blast asteroids into pieces are usually unrealistic; moreover, if
actually used, such "solutions" would probably make the situation
worse. First, it is unlikely that the firepower exists to blow apart,
say, a 5 km asteroid. Second, even if we could blow apart an
asteroid, most of the pieces would stay on essentially the same orbit,
i.e., on target to hit the Earth. A rain of 1000 100-m--sized objects
could still cause considerable damage.


Subject: E.17.3 I heard that an asteroid was going to hit the Earth?!
Author: Louis Strous

These such questions typically occur after a news report of a future
close encounter between the Earth and an asteroid. To date, all such
reports have resulted from (1) Astronomers did not yet know well
enough the orbit of a newly-discovered asteroid to say with any
certainty that it would not hit the Earth; (2) Reporters not checking
their stories or misunderstanding what they were told; or (3) both.

Objects that can potentially come close to the Earth are referred to
as Near-Earth Objects (NEOs). The International Astronomical Union
maintains lists of such objects. About 100 asteroids are classified as
"Potentially Hazardous Asteroids" (PHAs), at
URL:http://cfa-www.harvard.edu/iau/lists/Dangerous.html; they all
have a projected closest distance to Earth of less than 0.05 AU (7.5
million km). A list of closest approaches to the Earth by PHAs
between 1999 and 2099 is available at
URL:http://cfa-www.harvard.edu/iau/lists/PHACloseApp.html. A list
of moderately close (to within 0.2 AU) approaches to the Earth by
asteroids and comets between 1999 and 2032 is available at
URL:http://cfa-www.harvard.edu/iau/lists/CloseApp.html. At the
moment, NONE of these encounters is thought to pose a serious risk.

The "potential hazard" of PHAs lies in their orbits and the
perturbations on those orbits from the planets and the Moon currently
not being known with sufficient accuracy to completely exclude the
possibility of a collision, but, generally, labeling these asteroids
as PHAs is erring on the side of extreme caution. It is not worth
losing any sleep over them.


Subject: E.18 What's the difference between meteoroids, meteors, and

Briefly, a meteoroid is piece of cosmic debris in the solar system.
It becomes a meteor when it enters Earth's atmosphere and begins to
glow brightly. It becomes a meteorite if it survives and hits the

Three FAQs on different aspects of meteors and meteorites are
maintained by the American Meteor Society at


Subject: E.19 How do we know that meteorites are from the Mars? (or
the Moon?)

[This question comes up most frequently with reference to ALH 84001,
the Martian meteorite that has been suggested as carrying evidence of
past Martian life.]

Most meteorites are thought to originate from collisions between
asteroids in the asteroid belt. However, a small number have
characteristics suggestive of a Martian or lunar origin. Why do we
think this?

The short explanation is that we can compare the composition of a
meteorite to what various space probes and missions have told us about
the composition of Mars (or the Moon). Moreover, in the case of a
candidate Martian meteorite, it may have small pockets of gas trapped
within it, which can be compared to the Viking measurements of the
Martian atmosphere. Finally, it is possible to simulate launching a
small piece of rock from Mars or the Moon (say, from an asteroid
impact) and determine its path through space. Because of
gravitational perturbations from other planets (notably Jupiter and
the Earth), such a small rock could find its way to Earth, on fairly
short time scales even (a few million years or so).

For more details, see "On the Question of the Mars Meteorite,"
and Michael Richmond's archive of postings by James Head (from the
Lunar and Planetary Institute) on this topic,

Finally, the meteorite Northwest Africa #11 (NWA011) has a composition
similar to that of many Martian and lunar meteorites, but some
important differences as well (notably in the amount of oxygen). This
has led some to speculate that NWA011 might be from Mercury(!).


Subject: Copyright

This document, as a collection, is Copyright 1995--2000 by T. Joseph
W. Lazio ). The individual articles are copyright
by the individual authors listed. All rights are reserved.
Permission to use, copy and distribute this unmodified document by any
means and for any purpose EXCEPT PROFIT PURPOSES is hereby granted,
provided that both the above Copyright notice and this permission
notice appear in all copies of the FAQ itself. Reproducing this FAQ
by any means, included, but not limited to, printing, copying existing
prints, publishing by electronic or other means, implies full
agreement to the above non-profit-use clause, unless upon prior
written permission of the authors.

This FAQ is provided by the authors "as is," with all its faults.
Any express or implied warranties, including, but not limited to, any
implied warranties of merchantability, accuracy, or fitness for any
particular purpose, are disclaimed. If you use the information in
this document, in any way, you do so at your own risk.

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