29, 2010-- Discovery suggests our galaxy may be teeming with potentially
of planet hunters led by astronomers at the University of
California, Santa Cruz (UC Santa Cruz), and the Carnegie
Institution of Washington, and supported by the National
Science Foundation (NSF) and NASA, has announced the discovery
of an Earth-sized planet (three times the mass of Earth)
orbiting a nearby star at a distance that places it squarely
in the middle of the star's "habitable zone," where liquid
water could exist on the planet's surface.
star Gliese 581 hosts an Earth-sized planet that orbits
in the star's habitable zone.
confirmed, this would be the most Earth-like exoplanet yet discovered
and the first strong case for a potentially habitable one. "This
is clearly one of the most exciting areas of science these days"
said Ed Seidel, assistant director for NSF's Mathematical and
Physical Sciences directorate. "If we do discover life outside
our planet, it would perhaps be the most significant discovery
of all time."
astronomers, a "potentially habitable" planet is one that could
sustain life, not necessarily one that humans would consider a
nice place to live. Habitability depends on many factors, but
liquid water and an atmosphere are among the most important.
findings offer a very compelling case for a potentially habitable
planet," said Steven Vogt, professor of astronomy and astrophysics
at UC Santa Cruz. "The fact that we were able to detect this planet
so quickly and so nearby tells us that planets like this must
be really common."
modern techniques, it is now possible to actually search for worlds
that might be able to support life as we understand it," added
Seidel. "Just a few years back I wouldn't have thought this could
have advanced so fast." This discovery was the result of over
a decade of observations on the W. M. Keck Observatory in Hawaii.
techniques combined with old-fashioned ground-based telescopes
continue to lead the exoplanet revolution," said Paul Butler of
the Carnegie Institution. "Our ability to find potentially habitable
worlds is now limited only by our telescope time."
of the three main science objectives of the Astronomy and Astrophysics
Decadal Survey released last month is labeled 'New Worlds: Seeking
nearby habitable planets,'" added NSF Astronomy Division Director
The team's new findings are reported in a paper to be published
in the Astrophysical Journal and posted online today at arXiv.org.
Coauthors include associate research scientist Eugenio Rivera
of UC Santa Cruz; associate astronomer Nader Haghighipour of the
University of Hawaii-Manoa; and research scientists Gregory Henry
and Michael Williamson of Tennessee State University.
paper reports the discovery of two new planets around the nearby
red dwarf star Gliese 581. This brings the total number of known
planets around this star to six, the most yet discovered in a
planetary system other than our own solar system. Like our solar
system, the planets around Gliese 581 have nearly circular orbits.
The most interesting of the two new planets is Gliese 581g, with
a mass three to four times that of the Earth and an orbital period
of just under 37 days. Its mass indicates that it is probably
a rocky planet with a definite surface, and that it has enough
gravity to hold on to an atmosphere, according to Vogt. Gliese
581, located 20 light years away from Earth in the constellation
Libra, has a somewhat checkered history of habitable-planet claims.
previously detected planets in the system lie at the edges of
the habitable zone, one on the hot side (planet c) and one on
the cold side (planet d). While some astronomers still think planet
d may be habitable if it has a thick atmosphere with a strong
greenhouse effect to warm it up, others are skeptical. The newly
discovered planet g, however, lies right in the middle of the
habitable zone. "We had planets on both sides of the habitable
zone--one too hot and one too cold--and now we have one in the
middle that's just right," Vogt said. The planet is tidally locked
to the star, meaning that one side is always facing the star and
basking in perpetual daylight, while the side facing away from
the star is in perpetual darkness. One effect of this is to stabilize
the planet's surface climates, according to Vogt. The most habitable
zone on the planet's surface would be the line between shadow
and light (known as the "terminator"), with surface temperatures
decreasing toward the dark side and increasing toward the light
side. "Any emerging life forms would have a wide range of stable
climates to choose from and to evolve around, depending on their
longitude," Vogt said.
The researchers estimate that the average surface temperature
of the planet is between -24 and 10 degrees Fahrenheit (-31 to
-12 degrees Celsius). Actual temperatures would range from blazing
hot on the side facing the star to freezing cold on the dark side.
If Gliese 581g has a rocky composition similar to the Earth's,
its diameter would be about 1.2 to 1.4 times that of the Earth.
The surface gravity would be about the same or slightly higher
than Earth's, so that a person could easily walk upright on the
planet, Vogt said.
new findings are based on 11 years of observations of Gliese 581
using the HIRES spectrometer (designed by Vogt) on the Keck I
Telescope at the W. M. Keck Observatory in Hawaii. The spectrometer
allows precise measurements of a star's radial velocity (its motion
along the line of sight from Earth), which can reveal the presence
of planets. The gravitational tug of an orbiting planet causes
periodic changes in the radial velocity of the host star. Multiple
planets induce complex wobbles in the star's motion, and astronomers
use sophisticated analyses to detect planets and determine their
orbits and masses.
really hard to detect a planet like this," Vogt said. "Every time
we measure the radial velocity, that's an evening on the telescope,
and it took more than 200 observations with a precision of about
1.6 meters per second to detect this planet." To get that many
radial velocity measurements (238 in total), Vogt's team combined
their HIRES observations with published data from another group
led by the Geneva Observatory (HARPS, the High Accuracy Radial
velocity Planetary Search project). In addition to the radial
velocity observations, coauthors Henry and Williamson made precise
night-to-night brightness measurements of the star with one of
Tennessee State University's robotic telescopes. "Our brightness
measurements verify that the radial velocity variations are caused
by the new orbiting planet and not by any process within the star
itself," Henry said.
researchers also explored the implications of this discovery with
respect to the number of stars that are likely to have at least
one potentially habitable planet. Given the relatively small number
of stars that have been carefully monitored by planet hunters,
this discovery has come surprisingly soon. "If these are rare,
we shouldn't have found one so quickly and so nearby," Vogt said.
"The number of systems with potentially habitable planets is probably
on the order of 10 or 20 percent, and when you multiply that by
the hundreds of billions of stars in the Milky Way, that's a large
number. There could be tens of billions of these systems in our
National Science Foundation
Contacts Lisa-Joy Zgorski, NSF (703) 292-8311 email@example.com
Tim Stephens, UC, Santa Cruz (831) 459-2495 firstname.lastname@example.org
Investigators Steve Vogt, UC, Santa Cruz email@example.com
Websites UC, Santa Cruz News: www.ucsc.edu/news_events
Carnegie Institution of Science: http://carnegiescience.edu/
Ground-based Astronomy at NSF: http://www.nsf.gov/eyesonthesky