The lightest exoplanet yet discovered — only about twice the mass of Earth — has been detected, astronomers announced Tuesday.
"With only 1.9 Earth-masses, it is the least massive exoplanet ever detected and is, very likely, a rocky planet," Xavier Bonfils of Grenoble Observatory in France, a member of the team that made the discovery, said in a statement from the European Southern Observatory.
The find was announced at the European Week of Astronomy and Space Science at the University of Hertfordshire in Britain.
The planet was found in the well-known planetary system Gliese 581 and has been dubbed "Gliese 581 e." It was detected using the HARPS spectrograph, an instrument attached to the 11.8-foot (3.6-meter) ESO telescope at La Silla, Chile.
Measurements with the telescope also helped to refine the orbit of the new planet's solar system sibling, a previously detected planet called Gliese 581 d. The new measurements place that planet well within the habitable zone where liquid water oceans could exist.
"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, who led the European team that made the finding.
Planet Gliese 581 e orbits its host star — located only 20.5 light-years away in the constellation Libra — in just 3.15 days. Being so close to its host star, the planet is not in the habitable zone.
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). Gliese 581 d, the farthest-out known planet, makes a complete orbit in 66.8 days.
"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 of Geneva University in Switzerland. "'D' could even be covered by a large and deep ocean — it is the first serious 'water world' candidate."
Low-mass red dwarf stars such as Gliese 581 are potentially fruitful hunting grounds for low-mass exoplanets in the habitable zone. The gravitational pull of orbiting exoplanets introduces a slight wobble to the star's motion. Because the habitable zone of cool stars like Gliese 581 is so close to the star, the planets within this zone exert a stronger pull, and so the wobble of the star is more pronounced, though detecting the signal is still a challenge.
Over the last two decades, scientists have spotted more than 300 extrasolar planets circling other stars in our Milky Way galaxy. Most of these planets have been about the size of Jupiter or larger.
"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," said Mayor, who helped find that planet. "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 team plans to continue looking for Earthlike, rocky planets around other stars.
"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."
And HARPS isn't the only instrument looking for low-mass, Earthlike planets. NASA's new Kepler space telescope will also be peering through the galaxy in search of smaller alien worlds. It was launched on March 6 and sent back its first images last Thursday.