The
lightest exoplanet yet discovered — only about twice the mass of Earth — has
been detected, astronomers announced today.
"With
only 1.9 Earth-masses, it is the least massive exoplanet ever detected and is,
very likely, a rocky planet,"said Xavier Bonfils of Grenoble Observatory in
France, a member of the team that made the discovery, which was announced at
the European Week of Astronomy and Space Science at the University of
Hertfordshire in the United Kingdom.
The planet
was found in the famous system Gliese 581 and has been dubbed "Gliese 581
e." It was detected using the low-mass-exoplanet hunter HARPS spectrograph
attached to the 3.6-metre 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 planet called Gliese
581 d, placing it well within the habitable
zone, where liquid water oceans could exist.
"The
holy grail of current exoplanet research is the detection of a rocky,
Earth-like 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, which orbits the host star in 66.8 days, 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 Earth-like, rocky planets around other stars.
"With
similar observing conditions an Earth-like 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, Earth-like 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.
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