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Scientists discover lightest exoplanet yet

The planet orbiting Gliese 581 has a mass twice that of Earth.
Provided by ESO, Garching, Germany
Gliese 581 planetary system
After more than 4 years of observations using the most successful low-mass-exoplanet hunter in the world, the HARPS spectrograph attached to the 3.6-meter ESO telescope at La Silla, Chile, astronomers have discovered in this system the lightest exoplanet found so far: Gliese 581 e (foreground) is only about twice the mass of our Earth. The Gliese 581 planetary system now has four known planets, with masses of about 1.9 (planet "e," left in the foreground), 16 (planet "b," nearest to the star), 5 (planet "c," center), and 7 Earth masses (planet "d," with the bluish colour). The planet furthest out, Gliese 581 d, orbits its host star in 66.8 days while Gliese 581 e completes its orbit in 3.15 days.
ESO/L. Calçada
April 21, 2009
Well-known exoplanet researcher Michel Mayor today announced the discovery of the lightest exoplanet found so far. The planet "e" in the famous system Gliese 581 is only about twice the mass of Earth. The team also refined the orbit of the planet Gliese 581 d, first discovered in 2007, placing it well within the habitable zone, where liquid water oceans could exist. These amazing discoveries are the outcome of more than 4 years of observations using the most successful low-mass-exoplanet hunter in the world, the High Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph attached to the 3.6-meter European Southern Observatory (ESO) telescope at La Silla, Chile.

"The holy grail of current exoplanet research is the detection of a rocky, earthlike planet in the 'habitable zone' — a region around the host star with the right conditions for water to be liquid on a planet's surface," said Michel Mayor from the Geneva Observatory in Switzerland, who led the European team to this stunning breakthrough.

Planet Gliese 581 e orbits its host star — located only 20.5 light-years away in the constellation Libra — in just 3.15 days. "With only 1.9 Earth masses, it is the least massive exoplanet ever detected and is, very likely, a rocky planet," said co-author Xavier Bonfils from Grenoble Observatory in France.

Being so close to its host star, the planet is not in the habitable zone. But another planet in this system appears to be. From previous observations — also obtained with the HARPS spectrograph at ESO's La Silla Observatory and announced 2 years ago — this star was known to harbor a system with a Neptune-sized planet and two super-Earths. With the discovery of Gliese 581 e, the planetary system now has four known planets, with masses of about 1.9 Earth masses (planet "e"), 16 Earth masses (planet "b"), 5 Earth masses (planet "c"), and 7 Earth masses (planet "d"). The planet farthest out, Gliese 581 d, orbits its host star in 66.8 days.
Habitable zone Gliese 581
By refining the orbit of the planet Gliese 581 d, first discovered in 2007, a team of astronomers has shown that it lies well within the habitable zone, where liquid water oceans could exist. This diagram shows the distances of the planets in our solar system (upper row) and in the Gliese 581 system (lower row), from their respective stars (left). The habitable zone is indicated as the blue area, showing that Gliese 581 d is located inside the habitable zone around its low-mass red star. Based on a diagram by Franck Selsis, University of Bordeaux.
ESO
"Gliese 581 d is probably too massive to be made only of rocky material, but we can speculate that it is an icy planet that has migrated closer to the star," said team member Stephane Udry. The new observations have revealed that this planet is in the habitable zone, where liquid water could exist. "[Planet] 'd' could even be covered by a large and deep ocean — it is the first serious 'water world' candidate," Udry said.

The gentle pull of an exoplanet as it orbits the host star introduces a tiny wobble in the star's motion — only about 4 mph (7 km/h), corresponding to brisk walking speed — that can just be detected on Earth with today's most sophisticated technology. Low-mass red dwarf stars such as Gliese 581 are potentially fruitful hunting grounds for low-mass exoplanets in the habitable zone. Such cool stars are relatively faint. Their habitable zones lie close in, where the gravitational tug of any orbiting planet found there would be stronger, making the telltale wobble more pronounced. Even so, detecting these tiny signals is still a challenge, and the discovery of Gliese 581 e and the refinement of Gliese 581 d's orbit were only possible due to HARPS's unique precision and stability.

"It is amazing to see how far we have come since we discovered the first exoplanet around a normal star in 1995 — the one around 51 Pegasi," Mayor said. "The mass of Gliese 581 e is 80 times less than that of 51 Pegasi b. This is tremendous progress in just 14 years."

The astronomers are confident that they can still do better. "With similar observing conditions, an earthlike planet located in the middle of the habitable zone of a red dwarf star could be detectable," Bonfils said. "The hunt continues."
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