can a moon sustain life in a solar system?

Wayne Throop wrote:

So the right comparison is the delta-v for transfer orbits.

Speaking of delta-v, another interesting scenario would be an earthlike
planet at the Lagrange point of a heavy inner-system planet.

I'm still curious about this question: given a star of the mass of the
Sun, a planet of N^3 lunar masses orbiting at a distance of N times the
lunar distance from an earth-mass planet in reasonably circular orbits,
what can be said about the long-term stablility of the resulting 3-body
system?

: "Gene Ward Smith"
: I'm still curious about this question: given a star of the mass of the
: Sun, a planet of N^3 lunar masses orbiting at a distance of N times
: the lunar distance from an earth-mass planet in reasonably circular
: orbits, what can be said about the long-term stablility of the
: resulting 3-body system?

( I assume you mean "an earth-mass planet orbiting a planet of N^3
lunar masses at N times lunar distance, the pair orbiting about 1au
from the star". Unless I'm not understanding the discussion upthread. )

IIRC a rule of thumb is that the earthlike planet would have to be
close enough to the gas giant (or far enough from the star, but that's
a fixed distance onaccounta requirement for insolation) so that the
stellar tides would be pulling the planets apart less strongly than
the gravitational acceleration of the GG on the ELP. The distance
would actually have to be quite a bit smaller than to make these equal
iirc, but it's a finger-to-the-wind initial estimate on an upper bound,
I guess. If I've done my arithmetic right, that point is about 1e10
meters from a jupiter-mass GG, which seems more than large enough to
fit the situation above. No, wait, we want a Uranus-sized GG... um,
that one doesn't seem to fit.

As always, my arithmetic is slapdash and to-be-skeptical-about,
but that's what I get.

Interestingly, though I haven't shown it analytically, I think one of
the crazy Velkovskian/Saturnian orbits that have been proposed would
actually have been stable. It doesn't behave as the Saturnians require,
so their scheme still doesn't hold water (or much of anything else),
but it'd be stable (naict by numerical integration), the ELP isn't at
the GG lagrange point, and has the distance between the GG and ELP too
large for the above rule of thumb.

See http://sheol.org/throopw/grubaugh-synch-retro.html

( You folks via sci.astro should excuse the crudity of the presentation. )
( Well, other folks should excuse it too. )

"The solar system consists of the sun, jupiter, and some debris."

--- attribution lost...


Wayne Throop http://sheol.org/throopw

Wayne Throop wrote:

( I assume you mean "an earth-mass planet orbiting a planet of N^3
lunar masses at N times lunar distance, the pair orbiting about 1au

from the star". Unless I'm not understanding the discussion
upthread. )

I wasn't making any assumptions about N, though of course we know it
can be at least as high as 1 and give stable orbits. By orbiting I
meant around the center of mass. How large can N get and still give us
stability?

:: ( I assume you mean "an earth-mass planet orbiting a planet of N^3
:: lunar masses at N times lunar distance, the pair orbiting about
:: 1au from the star". Unless I'm not understanding the discussion
:: upthread. )

: "Gene Ward Smith"
: I wasn't making any assumptions about N, though of course we know it
: can be at least as high as 1 and give stable orbits. By orbiting I
: meant around the center of mass. How large can N get and still give
: us stability?

Ah. Silly me, I was holding the mass constant at N=10.

Well. Fixing that, I get N can be something like 30, corresponding
to a mass about that of jupiter, but I don't have much confidence
I got it right. It becomes a near thing somewhere around N=8 (where
"near thing" is "if tides were 10 percent bigger, it'd fall apart"),
so it's skating on thin ice from there on out. Well probably even from
further in, I guess.

Thing is, since we're driving mass up proportial to the cube of the
distance, gravity of the body keeps up with growth of solar tides for
quite a while. In fact, since tides are growing at... hm.

OK, so I'm even *less* confident of my model. I'd be interested
to know if N is anywhere *near* being between 8 to 30.


Wayne Throop http://sheol.org/throopw

On Sat, 22 Jan 2005, in rec.arts.sf.written,
Wayne Throop said:

"The solar system consists of the sun, jupiter, and some debris."

--- attribution lost...

Also, the debris consists of the Outer Giants and some debris, and that
debris consists of Earth and some debris.

(I could be generous about the last and say "Earth, Venus and some
debris", but the rest of the debris, including Mars, comes to less than
the difference between Earth and Venus)

--
Del Cotter
Thanks to the recent increase in UBE, I will soon be ignoring email
sent to . Please send your email to del2 instead.

An interesting side thought has to do with the viability of an earth
type planet in orbit of a red dwarf for the planet to recieve enough
energy for water to remain in a liquid state it would have to orbit so
close to its star as to be tidaly locked with one side to the star and
the other always in darkness this would be a poor place to hope to find
a viable biosphere However what if instead this world was in fact a
moon in orbit of a giant planet within the stars lifezone? this would
seem to solve the tidal lock problem. Someone tell me what I'm missing
here Im sure theres something wrong with this scenerio

Wasn't it jcamjr who wrote:
An interesting side thought has to do with the viability of an earth
type planet in orbit of a red dwarf for the planet to recieve enough
energy for water to remain in a liquid state it would have to orbit so
close to its star as to be tidaly locked with one side to the star and
the other always in darkness this would be a poor place to hope to find
a viable biosphere However what if instead this world was in fact a
moon in orbit of a giant planet within the stars lifezone? this would
seem to solve the tidal lock problem. Someone tell me what I'm missing
here Im sure theres something wrong with this scenerio

I don't see any problem with life on a world tidally locked to a star.

The big problem with life around a red dwarf, however, is that the
habitable zone is very narrow. As the brightness of the star evolves,
the habitable zone moves closer to or further from the star and the
planet is left in an uninhabitable region. The habitable zone of our Sun
is wide enough that the Earth has remained inside the habitable zone
despite moderate changes in solar brightness.

--
Mike Williams
Gentleman of Leisure

Mike Williams wrote:
Wasn't it jcamjr who wrote:
An interesting side thought has to do with the viability of an earth
type planet in orbit of a red dwarf for the planet to recieve
enough
energy for water to remain in a liquid state it would have to orbit
so
close to its star as to be tidaly locked with one side to the star
and
the other always in darkness this would be a poor place to hope to
find
a viable biosphere However what if instead this world was in fact a
moon in orbit of a giant planet within the stars lifezone? this
would
seem to solve the tidal lock problem. Someone tell me what I'm
missing
here Im sure theres something wrong with this scenerio

I don't see any problem with life on a world tidally locked to a
star.

The big problem with life around a red dwarf, however, is that the
habitable zone is very narrow. As the brightness of the star evolves,
the habitable zone moves closer to or further from the star and the
planet is left in an uninhabitable region. The habitable zone of our
Sun
is wide enough that the Earth has remained inside the habitable zone
despite moderate changes in solar brightness.

True, but aren't the changes in the brightness of red dwarves both
small and very, very slow (even in geological timeframes)?

-- Wakboth

Wakboth (& others) wrote:

[tide-locked worlds] would be a poor place to hope to
find a viable biosphere.

I'd have to agree with others that tide-locked worlds don't
represent a critical problem for a biosphere (at the very least, not
over here on rasfs grin). Atmospheric circulation is more than
enough, even with thin atmospheres, to prevent the "freeze out"
problem. The hydrologic cycle might be... interesting.

However what if instead this world was in fact a
moon in orbit of a giant planet within the stars lifezone?

This, too, would seem a likely solution. A possibility is that the
bulk of habitable planets orbit M-class stars - simply due to those
stars being by far the most numerous.

The big problem with life around a red dwarf, however, is that the
habitable zone is very narrow.

True, but aren't the changes in the brightness of red dwarves both
small and very, very slow (even in geological timeframes)?

Yes & no. The migration of the habitable zone outward as the star
slowly brightens is unlikely to be much of a problem, for the very
reasons you mention (the biosphere of these planets would fail due to a
lack of internal heat killing plate tectonics long before the habitable
zone migrates too far outward). A more serious issue is flares - low
mass stars are often rather active in terms of flares and sunspots, so
variability is rather greater than would be convenient.
--
Brian Davis

In other words, how far away can a moon be from a planet, given the
three masses involved and the star-planet seperation? This is a rather
well-known problem studied extensively (due to the ugly nature of our
own Earth-Moon system).
As a semi-empirical rule of thumb, a satellite orbit is stable if it
orbits closer to its planet than about 1/2 to 1/3 of the Hill radius:
a_hill = a ( mu / 3 )^(1/3)
a_hill = Hill radius
a = seperation of the planet and the star (semi-major axis
for circular orbits)
mu = m / M = reduced mass of planet/sun system
m = planet mass
M = mass of star
This assumes the satellite mass is negligable, but that's usually true.
For Jupiter, for instance a_hill = 5.29e+7 km, and it's furthest
satellite (Sinope) is closer than 1/2 a_hill at 2.37e+7 km.
Just specify the mass of the star (and it's age) to calculate a
luminosity. From the luninosity, you can figure out how far away the
habitable zone is. Now you have two of the numbers you need (stellar
mass & star-planet seperation), just guestimate a gas giant mass and
estiamte your maximum limit.

--
Brian Davis