Astronomers Announce the Most Earth-Like Planet Yet Found Outside the Solar System

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NSF Press Release 05-097

Astronomers Announce the Most Earth-Like Planet Yet Found Outside the
Solar System

National Science Foundation
June 13, 2005

Attention Broadcasters: animations will be available on Betacam SP.
Contact Dena Headlee, (703) 292-7739,

Arlington, Va.--Taking a major step forward in the search for
Earth-like
planets beyond our own solar system, a team of astronomers has
announced
the discovery of the smallest extrasolar planet yet detected. About
seven-and-a-half times as massive as Earth, with about twice the
radius,
it may be the first rocky planet ever found orbiting a normal star not
much different from our Sun.

All of the nearly 150 other extrasolar planets discovered to date
around
normal stars have been larger than Uranus, an ice-giant planet in our
own solar system that is about 15 times the mass of the Earth.

"We keep pushing the limits of what we can detect, and we're getting
closer and closer to finding Earths," said team member Steven Vogt, a
professor of astronomy and astrophysics at the University of
California,
Santa Cruz.

The newly discovered "super-Earth" orbits the star Gliese 876, located
just 15 light years away in the direction of the constellation
Aquarius.
This star also possesses two larger, Jupiter-size planets. The new
planet whips around the star in a mere two days, and is so close to the
star's surface that its dayside temperature probably tops 400 to 750
degrees Fahrenheit (200 to 400 degrees Celsius)--oven-like temperatures
far too hot for life as we know it.

Nevertheless, the ability to detect the tiny wobble the planet induces
in the star gives astronomers confidence that they will be able to
detect even smaller rocky planets in orbits more hospitable to life.

"This is the smallest extrasolar planet yet detected and the first of a
new class of rocky terrestrial planets," said team member Paul Butler
of
the Carnegie Institution of Washington. "It's like Earth's bigger
cousin."

The team measures a minimum mass for the planet of 5.9 Earth masses,
orbiting Gliese 876 with a period of 1.94 days at a distance of 0.021
astronomical units (AU), or 2 million miles.

Though the team has no direct proof the planet is rocky, they believe
its low mass precludes it from retaining gas like Jupiter. Three other
purported rocky planets have been reported outside the solar system,
but
they orbit a pulsar, the flashing corpse of an exploded star.

"This planet answers an ancient question," said team leader Geoffrey
Marcy, professor of astronomy at the University of California,
Berkeley.
"Over 2,000 years ago, the Greek philosophers Aristotle and Epicurus
argued about whether there were other Earth-like planets. Now, for the
first time, we have evidence for a rocky planet around a normal star."

"Today's results are an important step toward answering one of the most
profound questions that mankind can ask: Are we alone in the universe?"
said Michael Turner, head of the Mathematical and Physical Sciences
directorate at the National Science Foundation (NSF), which provided
partial funding for the research.

The team's work, conducted at the Keck Observatory in Hawaii, was also
supported by the National Aeronautics and Space Administration (NASA),
the University of California and the Carnegie Institution of
Washington.

Marcy, Butler, theoretical astronomer Jack Lissauer of NASA's Ames
Research Center, and post-doctoral researcher Eugenio J. Rivera of the
University of California Observatories/Lick Observatory at UC Santa
Cruz
presented their findings today (Monday, June 13) during a press
conference at NSF in Arlington, Va.

A paper detailing their results has been submitted to The Astrophysical
Journal. Coauthors on the paper are Steven Vogt and Gregory Laughlin of
the Lick Observatory at the University of California, Santa Cruz; Debra
Fischer of San Francisco State University; and Timothy M. Brown of
NSF's
National Center for Atmospheric Research in Boulder, Colorado.

Gliese 876 is a small, red star known as an M dwarf--the most common
type of star in the galaxy. It is located in the constellation
Aquarius,
and, at about one-third the mass of the sun, is the smallest star
around
which planets have been discovered. Butler and Marcy detected the first
planet there in 1998; it was a gas giant about twice the mass of
Jupiter. Then, in 2001, they reported a second planet, another gas
giant
about half the mass of Jupiter. The two are in resonant orbits, the
outer planet taking 60 days to orbit the star, twice the period of the
inner giant planet.

Lissauer and Rivera have been analyzing Keck data on the Gliese 876
system in order to model the unusual motions of the two known planets,
and three years ago got an inkling that there might be a smaller, third
planet orbiting the star. In fact, if they hadn't taken account of the
resonant interaction between the two known planets, they never would
have seen the third planet.

"We had a model for the two planets interacting with one another, but
when we looked at the difference between the two-planet model and the
actual data, we found a signature that could be interpreted as a third
planet," Lissauer said.

A three-planet model consistently gave a better fit to the data, added
Rivera. "But because the signal from this third planet was not very
strong, we were very cautious about announcing a new planet until we
had
more data," he said.

Recent improvements to the Keck Telescope's high-resolution
spectrometer
(HIRES) provided crucial new data. Vogt, who designed and built HIRES,
worked with the technical staff in the UC Observatories/Lick
Observatory
Laboratories at the University of California, Santa Cruz, to upgrade
the
spectrometer's CCD (charge-coupled device) detectors last August.

"It is the higher precision data from the upgraded HIRES that gives us
confidence in this result," Butler said.

The team now has convincing data for the planet orbiting very close to
the star, at a distance of about 10 stellar radii. That's less than
one-tenth the size of Mercury's orbit in our solar system.

"In a two-day orbit , it's about 200 degrees Celsius too hot for liquid
water," Butler said. "That tends to lead us to the conclusion that the
most probable composition of this thing is like the inner planets of
this solar system--a nickel-iron rock, a rocky planet, a terrestrial
planet."

"A planet seven and a half times the mass of the Earth could easily
hold
onto an atmosphere," noted Laughlin, an assistant professor of
astronomy
at UC Santa Cruz. "It would still be considered a rocky planet,
probably
with an iron core and a silicate mantle. It could even have a dense
steamy water layer. I think what we are seeing here is something that's
intermediate between a true terrestrial planet like the Earth and a hot
version of the ice giants Uranus and Neptune."

Combined with improved computer software, the new CCD detectors
designed
by this team for Keck's HIRES spectrometer can now measure the Doppler
velocity of a star to within one meter per second--human walking
speed--instead of the previous precision of three meters per second.
This improved sensitivity will allow the planet-hunting team to detect
the gravitational effect of an Earth-like planet within the habitable
zone of M dwarf stars like Gliese 876.

"We are pushing a whole new regime at Keck to achieve one meter per
second precision, triple our old precision, that should also allow us
to
see Earth-mass planets around sun-like stars within the next few
years,"
Butler said.

"Our UC Santa Cruz and Lick Observatory team has done an enormous
amount
of optical and technical and detector work to make the Keck telescope a
rocky planet hunter, the best one in the world," Marcy added.

Lissauer also is excited by another feat reported in the paper
submitted
to the journal. For the first time, he, Rivera and Laughlin have
determined the line-of-sight inclination of the orbit of the stellar
system solely from the observed Doppler wobble of the star. Using
dynamical models of how the two Jupiter-size planets interact, they
were
able to calculate the masses of the two giant planets from the observed
shapes and precession rates of their oval orbits. Precession is the
slow
turning of the long axis of a planet's elliptical orbit.

They showed that the orbital plane is tilted 40 degrees to our line of
sight. This allowed the team to estimate the most likely mass of the
third planet as seven and a half Earth masses.

"There's more dynamical modeling involved in this study than any
previous study, much more," Lissauer said.

The team plans to continue to observe the star Gliese 876, but is eager
to find other terrestrial planets among the 150 or more M dwarfs they
observe regularly with Keck.

"So far we find almost no Jupiter-mass planets among the M dwarf stars
we've been observing, which suggests that, instead, there is going to
be
a large population of smaller mass planets," Butler noted.

-NSF-

Media Contacts
M. Mitchell Waldrop, NSF (703) 292-7752
Tim Stephens, University of California, Santa Cruz (831) 459-4352

Susanne Garvey, Carnegie Institution of Washington (202) 939-1128

Robert Sanders, University of California, Berkeley (510) 643-6998


Program Contacts
Michael Briley, NSF (703) 292-4901
Michael S. Turner, NSF (703) 292-8800

Images/B-Roll
Dena Headlee, NSF (703) 292-7739

Co-Investigators
Paul Butler, Carnegie Institution of Washington (202) 478-8866

Steven Vogt, University of California, Santa Cruz (831) 459-2151

Jack Lissauer, NASA Ames Research Center (650) 604-2293

Geoffrey Marcy, University of California, Berkeley (510) 642-8400

Eugenio Rivera, NASA Ames Research Center and Lick Observatory (831)
459-2277
Gregory Laughlin, Lick Observatory and University of California, Santa
Cruz: (831) 459-3208

Related Websites
NASA's PlanetQuest site: http://planetquest.jpl.nasa.gov/
The California & Carnegie Planet Search site: http://exoplanets.org/
Geoffery Marcy's web page on the new planet:
http://www.exoplanets.org/index_gl.html

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the discovery of the smallest extrasolar planet yet detected. About
seven-and-a-half times as massive as Earth, with about twice the
radius,
it may be the first rocky planet ever found orbiting a normal star not
much different from our Sun.


If the equation I found for surface gravity is any good

g=GMe/Re^2

(looks like G=1 if Me and Re are normalized to Earth's,
plugging in values for the moon 1/81 Me and 1/4 Re
gets me a g of 1/5, which isn't that far off. Also
for the Sun 365000Me and 100Re yields gravity of 34,
which ain't that far off the number of 27g I remember
reading somewheres.)

Then the surface gravity of that planet would be
about 1.8 times Earth's. Someone in good health could
deal with that (assuming normal Earthlike conditions
otherwise).