Steven Vogt of UC Santa Cruz and UC Observatories and Paul Butler of the Carnegie Institution of Washington join NSF’s Lisa-Joy Zgorski to announce the discovery of the first exoplanet that has the potential to support life. Video courtesy of: National Science Foundation

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This artist's conception shows the inner four planets of the Gliese 581 system and their host star. Image Credit: Lynette Cook.

A team of planet-hunting astronomers, utilizing the HIRES spectrometer on the W.M. Keck Observatory’s Keck I Telescope, has announced the discovery of an Earth-sized planet orbiting a nearby star. The new planet, known as Gliese 581g, is 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. If confirmed, this would be the most Earth-like exoplanet and the first bona fide potentially habitable one yet discovered.

The discovery by the team, led by astronomers at the University of California, Santa Cruz, and the Carnegie Institution of Washington D.C., is based on 11 years of observations made at the Keck Observatory atop Mauna Kea mountain on the Big Island of Hawaii. “Our 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.”

“Advanced techniques combined with old-fashioned ground-based telescopes continue to lead the exoplanet revolution,” added Paul Butler of the Carnegie Institution. “Our ability to find potentially habitable worlds is now limited only by our telescope time.”

Vogt and Butler lead the Lick-Carnegie Exoplanet Survey. The team’s new findings are reported in a paper to be published in the Astrophysical Journal and posted online at arXiv.org. Coauthors include UCSC associate research scientist Eugenio Rivera; associate astronomer Nader Haghighipour of the University of Hawaii-Manoa; and research scientists Gregory Henry and Michael Williamson of Tennessee State University.

The planetary orbits of the Gliese 581 system compared to those of our own solar system. Image Credit: National Science Foundation.

The paper reports the discovery of not one but two new planets around the nearby red dwarf star known as Gliese 581. This brings to six the number of known planets around this star, 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. Gliese 581g has a mass 3 to 4 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.

Keck’s HIRES 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.

“It’s 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.

The 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 ten 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.”

Gliese 581, located 20 light years away from Earth in the constellation Libra, has a somewhat checkered history of habitable-planet claims. Two 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.

“It’s the Goldilocks planet,” Vogt said. “That’s a well-worn analogy, but in this case it fits. 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.”

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.

The W. M. Keck Observatory operates two 10-meter optical/infrared telescopes on the summit of Mauna Kea. The twin telescopes feature a suite of advanced instrumentation including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectroscopy and a world-leading laser guide star adaptive optics system. The Observatory is a private 501(c) 3 organization and a scientific partnership of the California Institute of Technology, the University of California and NASA.