The
discovery of the lightest exoplanet ever found, less than twice the mass of the
Earth, has electrified a week-long meeting on astronomy and space science in
Europe.
The
stunning finding was made by a team headed by Michel Mayor of the Geneva
Observatory. The icing on the cake is a related discovery that a previously
discovered "super-Earth" orbiting the same star appears to reside in the
habitable zone.
The finding
portends the discovery of a true Earth-mass planet, which could come in about
two years, Mayor said.
Mayor made
the very first discovery of an exoplanet, a Jupiter-sized world that orbits the
star 51 Pegasi, in 1994. Among his many planet discoveries since then at
ESO's La Silla Observatory in Chile, Mayor has made a specialty of observing
the star Gliese 581. Located 20.5 light-years away in the
constellation Libra ("the Scales"), Gliese 581 is a red dwarf star with only
one-third of the mass of our sun.
Two years
ago, Mayor discovered a planet the size of Neptune and two super-Earths
orbiting this star. The newly discovered planet, named Gliese 581 e, is
now the fourth known planet in this solar system and the lightest, weighing in
at only 1.94 Earth masses. It flies
round the star at dizzying speed, taking just 3.15 days to complete an
orbit. "The surprise for me was to discover a planet with by far the lowest
mass seen to date," says Mayor.
This new
planet orbits so close to the star that its water would have boiled away long
ago. It is therefore not in
the habitable zone – the region of a solar system where water can stay
liquid on the surface of a rocky planet, and, consequently, where scientists
expect life can occur. In our solar system, the habitable zone is roughly
between the orbits of Venus and Mars (with Earth sitting not quite in the
middle).
In finding the
new planet, Mayor has been able to more accurately determine the orbit for the
outermost planet, Gliese 581 d. One of the super-Earths in the solar
system, this planet is closer to the host star than was thought when it was
discovered in 2007. And that provided the second great surprise. "It is
the only (Earth-like) exoplanet found inside the habitable water zone of the
parent star," says Mayor.
Gliese 581
d is 7 Earth-masses, and team member Stephane Udry says the planet is probably too
massive to be made only of rocky material. "We can speculate that it is
an icy planet that has migrated closer to the star," he says. At the European
meeting, Mayor added the latest news indicated, "No icebergs, but there may be
an ocean at the surface, meaning this is a new class of ocean planet."
To detect
exoplanets, Mayor's team studies a star's radial velocity, in which the tiny
tugs exerted by orbiting exoplanets produce a complex wobble in the star. This
wobble can be analyzed to learn about properties of the planets in the solar
system. The velocity of a star with multiple planets has to be followed
for several years to discover the different properties of its orbiting planets,
and this requires instrumentation that is extremely stable from year to year --
one of the big challenges in detecting exoplanets through the radial velocity
technique.
The team's
observing program began back in 2004 with a sample of 400 sun-like stars.
Mayor is
now scooping up small exoplanets that have been missed by a rival search
technique (called transit photometry) which involves measuring the tiny fall in
a star's magnitude when an exoplanet passes between the star and the Earth.
Both techniques, transit photometry and radial velocity, are strongly biased to
catch giant planets with the mass of Jupiter or more, as well as smaller
planets that orbit very close to their star. But to find small planets orbiting
within a star's habitable zone, Mayor's approach now seems to have the edge.
The team
has found that one-third (30%) of exoplanet systems discovered to date include
small bodies. "We have discovered a new category of small exoplanets," says
Mayor. "Within a couple of years we will drive down our lower limit of
detection to the mass of the Earth. The next challenge after that is to detect a twin of the
Earth in the habitable zone of a solar-type star."
The next
stage for Mayor's team is to migrate the detection technology from the current
3.6-meter telescope to ESO's 8-meter Very Large Telescope in order to improve
the precision of observations. After that, Mayor looks forward to using the
European Extremely Large Telescope (E-ELT), a 42-meter eye-on-the-sky that is
planned to be operating by 2018.
Currently
in the later stages of design, this facility will be capable of directly
imaging larger exoplanets, and possibly will be able to search their
atmospheres for biosignatures. E-ELT will answer fundamental questions on the
formation and evolution of exoplanets, bringing us one step closer to answering
the question: are we alone?
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