But It's Noplace Like Home
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.