First Potentially Habitable Earth-like Planet Found
WASHINGTON, DC, October 4, 2010 (ENS) – A team of U.S. planet hunters has discovered an Earth-sized body orbiting a nearby star in the middle of the star’s habitable zone, where liquid water could exist on the planet’s surface and the planet could sustain life as we know it.
The planet, named Gliese 581g, is about three to four times the mass of Earth. The scientists say this mass indicates a rocky planet with a definite surface and enough gravity to hold onto an atmosphere.
More than 500 planets have been found outside our solar system, but discoverer Steven Vogt, professor of astronomy and astrophysics at UC Santa Cruz says, “This is the first exoplanet that really has the right condititions to allow liquid water to exist on its surface.”
Artist’s conception shows the inner four planets of the Gliese 581 system and their host star. (Artwork by Lynette Cook courtesy NSF)
Every 37 days, it orbits the red dwarf star Gliese 581, located 20 light years away from Earth in the constellation Libra.
“If we do discover life outside our planet, it would perhaps be the most significant discovery of all time,” said Ed Seidel, assistant director for National Science Foundation’s Mathematical and Physical Sciences directorate.
“With modern techniques, it is now possible to actually search for worlds that might be able to support life as we understand it,” said Seidel. “Just a few years back I wouldn’t have thought this could have advanced so fast.”
The discovery is the result of over a decade of observations at the W.M. Keck Observatory on the island of Hawaii. It was found by astronomers from the University of California, Santa Cruz and the Carnegie Institution of Washington, financially supported by the National Science Foundation and NASA.
“Advanced 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.”
If the team’s discovery is confirmed, this would be the most Earth-like exoplanet yet found and the first strong case for a potentially habitable planet.
“Our findings offer a very compelling case for a potentially habitable planet,” said Vogt. “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.”
“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 galaxy. This is a new age of discovery.”
To 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.
“Any discussion of life at this point is speculative,” said Butler. “That being said on the Earth anywhere you find liquid water, you find life in overwhelming abundance. The question should be how can you rule out that life doesn’t exist.
“At this point we can’t say anything about the physical conditions on the planet,” said Butler. “What we observe is the gravitational tugging of the planet on its star. We know it exists we know its orbital distance and we know its mass. We can say the mass of the planet would be sufficient to hold a nice strong atmosphere like that on Earth an maybe even a deeper atmosphere and there are places on the surface of the planet that would be proper for liquid water.”
“It’s pretty hard to image that water wouldn’t be there,” said Vogt. “Look around on our own solar system. We know there’s water on the moon and at the poles of the moon, a dry arid body. We know there’s plenty of water on Earth; we know there was water on mars, still is, we can see it on an amateur telescope.”
Water has been discovered on moons of Jupiter and Saturn and even much farther away in the Orion Nebula where stars and planets being born, said Vogt. “Water is a very common consitituent. it’s everywhere in the universe.”
The scientists report the discovery of two new planets around 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 planet Gliese 581g is tidally locked to the star, meaning that one side is always facing the star in perpetual daylight, while the side facing away from the star is in perpetual darkness.
Steven Vogt, professor of astronomy and astrophysics at UC Santa Cruz (Photo courtesy NSF)
One effect of this is to stabilize the planet’s surface climates, said Vogt. The most habitable zone on the planet’s surface would be the line between shadow and light, 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 Gliese 581g 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.
Two previously detected planets in the Gliese 581 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. But the newly discovered planet g 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.
Paul Butler of the Carnegie Institution (Photo courtesy NSF)
The new findings are based on 11 years of observations of Gliese 581 using the HIRES spectrometer, designed by Vogt, on the Keck I Telescope in Hawaii.
The spectrometer can reveal the presence of planets by allowing precise measurements of a star’s motion along the line of sight from Earth, called its radial velocity.
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.
“It’s really hard to detect a planet like this,” Vogt explained. “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, Vogt’s team combined their HIRES observations with published data from another group led by the Geneva Observatory’s High Accuracy Radial velocity Planetary Search project.
In addition to the radial velocity observations, coauthors Gregory Henry and Michael Williamson of Tennessee State University 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.
Jim Ulvestad, Astronomy Division director with the National Science Foundation, says that one of the three main science objectives of the Astronomy and Astrophysics Decadal Survey released last month is “seeking nearby habitable planets.”
The several million dollars of funding that has gone into this discovery over the past 25 years have come from the NSF and NASA and from private donations, the scientists say.
“It is very gratifying to see that long-term scientific investments by NSF and NASA toward meeting this objective are paying off,” said Ulvestad, “and we expect continued discoveries in this area as nearby stars are monitored for longer periods.”
Butler said, “This star is one of the very nearest stars in the sky; it’s only about 20 light years away. Only roughly 100 stars that are this close, so this star presents the possibility over the next decade or two for both astrometry and direct imagery work on this system to get the whole three dimensional orbit. We are hopeful these technologies will lead to confirmation of this discovery.”
Further confirmation could also come from Terrestrial Planet Finder, a future mission NASA is considering that would study all aspects of exoplanets from their formation and development in disks of dust and gas around newly forming stars to their suitability as abodes for life.