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Two Planets Identified as Most Similar to Earth
By Thomas H. Maugh II, Los Angeles Times
April 22, 2009
Both orbit the same star, 20 light-years from Earth, called Gliese 581. The larger of the
two orbits in the 'habitable zone,' and could have oceans.
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An illustration shows two exoplanets, Gliese 581ƒ|e in the left foreground and Gliese
581ƒ|d just to its right, orbiting a star that is 20 light-years from Earth.
After locating more than 340 planets orbiting other stars, astronomers have identified
two that are the most similar to Earth so far.
The most recently discovered one is almost twice as large as Earth, making it the
smallest exoplanet -- for extra-solar planet -- found to date. The second one was found
in 2007, but new observations have shown that it is the only exoplanet to date that orbits
its star in the so-called habitable zone, where water remains a liquid. Thus, it is the only
exoplanet discovered that is likely to have oceans.
Intriguingly, both orbit the same star, a dwarf 20 light-years from Earth called Gliese 581,
European researchers said Tuesday.
The identification of the small planet "is a remarkable discovery and bodes well for our
eventual discovery of a true Earth-like, habitable planet," astronomer Alan Boss of the
Carnegie Institution of Washington wrote in an e-mail.
It "is the most exciting discovery in exoplanets so far," added astronomer Geoffrey W.
Marcy of UC Berkeley via e-mail. "It shows that nature makes such small planets,
probably in large numbers."
The small planet is the fourth discovered circling Gliese 581 by a team of astronomers
working with the European Southern Observatory's 3.6-meter telescope at La Silla,
Chile. They identified the planets by detecting and analyzing slight wobbles in the star's
path as the planets orbit it.
The small planet, called Gliese 581 e, has an estimated mass equal to 1.9 Earths and
orbits its sun every 3.15 days, the team reported at an astronomical meeting at the
University of Hertfordshire in Britain. Because it is so close to Gliese 581, it is blisteringly
hot, and any gases or liquids that it might have carried have long since dissipated,
leaving only uninhabitable rock.
In February, French astronomers said they had discovered an even smaller planet,
called CoRoT-Exo-7b, that has an estimated mass equal to 1.7 Earths, circling a
different star. But experts said the data for Gliese 581 e is more convincing.
The other three planets in the Gliese system have masses of 16, five and seven Earths.
The one with a mass of seven Earths, called Gliese 581 d, was initially thought to have
an orbital period of 80 days, which would put it just on the outer edge of the habitable
Recent refinements of the data, however, show that it has an orbit of only 66.8 days,
which places it well within the habitable zone, astronomer Stephane Udry of Geneva
University told the meeting. Because of its distance from Gliese 581, moreover, it must
have a significant amount of water and other gases, he added. It could have oceans
thousands of meters deep, he said.
The team is continuing to monitor Gliese 581 in hopes that the orbital planes of the
planets will bring them between the star and Earth, which will allow astronomers to learn
more about their composition.
Astronomers Find Planet Closer to Size of Earth
By Dennis Overbye, The New York Times
April 21, 2009
European astronomers said Tuesday that they had discovered the smallest planet yet
found orbiting another star. The planet could be as little as only 1.9 times as massive as
the Earth and belongs to a dim red star known as Gliese 581, which lies about 20 lightyears from Earth in the constellation Libra.
The star was already know to harbor at least three more massive planets. The new
planet, known as Gliese 581e, is probably rocky like the Earth, but it lies in such a close
orbit — only three million miles from its star — that it is surely blasted with too much
radiation and heat to be livable.
Michel Mayor, of Geneva Observatory, and his colleagues announced their results at a
conference at the University of Hertfordshire in Britain and in a paper submitted to the
journal Astronomy and Astrophysics.
Astronomers said the discovery was more encouragement that the galaxy was full of
small-mass planets and that with more time and improved instruments like the Kepler
satellite, recently launched by NASA, they would eventually find Earth-like planets in
orbits suitable for life around other stars.
“Finding Earth-like planets with lukewarm temperatures is the next great goal,” Geoff
Marcy, of the University of California, Berkeley, a planet-hunting rival of Dr. Mayor’s, said
in an e-mail message.
“This is the most exciting discovery in exoplanets so far,” Dr. Marcy said.
Dr. Mayor’s group also discovered the first exoplanet, a gas giant 160 times the mass of
the Earth, in 1995, using a technique known popularly as the “wiggle” method that
detects planets by a slight gravitational tug they give their stars. The method is most
sensitive to massive planets in close orbits. In a statement, Dr. Mayor noted that the new
planet is only one-eightieth of the mass of the first one, saying, “This is tremendous
progress in 14 years.”
The discovery also cements the Gliese system as one of the most promising exoplanet
systems. Two years ago, the third planet from that star was hailed as a ”Goldilocks”
planet, where liquid water and thus life might be possible, until calculations showed that
the greenhouse effect would broil it.
But the new data also shifted the orbit of the star’s outermost planet inward so that it
now appears to revolve in the so-called habitable zone where liquid water is possible,
according to Stéphane Udry of Geneva University, one of the team members.
That planet, 581d, is about seven times as massive as the Earth, Dr. Udry explained,
which is too big to be just rock. It probably formed as a combination of ice and rock
farther out in the Gliese system and then migrated inward, according to various
planetary formation models, and melted. He called it the first serious “water world
Sara Seager, a planet theorist at the Massachusetts Institute of Technology, said in an
e-mail message that the Gliese planetary system “is like the gift that keeps on giving.”