Astronomers
announced today the discovery of what is possibly the smallest planet known
outside our solar system orbiting a normal star.
Its orbit is
farther from its host star than Earth is from the Sun. Most known extrasolar
planets reside inside the equivalent of Mercury's orbit.
The planet
is estimated to be about 5.5 times as massive as Earth and thought to be rocky.
It orbits a red dwarf star about 28,000 light-years away. Red dwarfs are about
one-fifth as massive as the Sun and up to 50 times fainter. But they are among
the most
common stars in the universe.
So the
finding suggests rocky worlds may be common.
"The
team has discovered the most Earth-like planet yet," said Michael Turner, assistant
director for the mathematical and physical sciences directorate at the National
Science Foundation, which supported the work.
The
discovery is detailed in the Jan. 26 issue of the journal Nature.
More to
come
Prior
to this discovery, the smallest extrasolar planet found around a normal star
was about 7.5 Earth
masses. Earth-sized planets have been detected, but only around dying
neutron stars.
The
newfound planet, named OGLE-2005-BLG-390Lb, is probably too cold to support
life as we know it, astronomers said. With a surface temperature of -364
degrees Fahrenheit (-220 degrees Celsius), it is nearly as frigid
as Pluto.
It
was discovered using a technique called "gravitational
microlensing," whereby light from a distant star is bent and magnified by
the gravitational field of a foreground star. The presence of a planet around
the foreground star causes light from the distant star to become momentarily
brighter.
Astronomers
hailed the discovery as the first of a new class of small, rocky worlds located
at far-out distances from their stars.
The planet
and star are separated by about 2.5 astronomical units (AU). One AU is equal to
the distance between the Earth and the Sun. Until now, no small planet had been
found farther than 0.15 au from its parent star.
The
finding means planet hunters are one step closer to detecting their holy
grail: a habitable Earth-like planet that can sustain liquid water and
support life.
"We
may predict with reasonable probability that microlensing will discover planets
with masses like that of Earth at a similar distance from their stars and with
comparable surface temperature," said study co-author Bohdan Paczynski from Princeton University.
New
outlook
Of the more
than 150 planets have been discovered so far, most were found using the Doppler
technique, in which astronomers look for wobbles in a star caused by the
gravitational pull of a planet. This method has uncovered dozens of huge worlds
but can't spot small planets that are far from their stars.
Microlensing
can detect small planets, but it is 50 times more likely to find a gas-giant
planet like Jupiter.
"Microlensing
should have discovered dozens of Jupiters by now if they were as common as
these five-Earth-mass planets," said study co-author David Bennett.
That
suggests most of our galaxy's planets are small and rocky.
This
prediction agrees with the standard model for solar system formation, known as
the "core
accretion" model. It goes like this: Dust around newborn stars forms clumps
that stick together and eventually become asteroids, comets and planet
precursors. In this scheme, relatively few planets successfully become gas
giants, and they are outnumbered by small, rocky worlds.
"It's
incredible to think that we went from 10 years ago having no planet to now
having over 100 gas giants and even starting to find the first terrestrial
planets," said Alan Boss, a theorist at the Carnegie Institution of Washington
who did not participate in the discovery. "That's just an amazing leap."
Shortcoming
Since star
alignments are unique events, a microlensing experiment can never be repeated.
Todd Henry, an astronomer at Georgia State University who was not involved in
the study, said the discovery was an "intriguing result from this particular
technique, but unfortunately you can't follow it up."
Many
astronomers view the lack of repeatability as an acceptable trade-off, however,
because thousands of star systems can be screened in a relatively short period
of time compared to other techniques.
"You can't
learn a whole lot about the details of individual systems ... but it's a
wonderful alternative for learning about what the mass distribution of
extrasolar planets might be and the frequency at which they occur," said David
Latham, an astronomer at the Harvard-Smithsonian Center for Astrophysics who
was not part of the study.
In a
telephone interview, Jean-Phillipe Beaulieu, a co-author in the study, said
that while the observations can never be repeated, the discovery was
simultaneously verified by different telescopes around the world.
The
microlensing event was detected July 11 by telescopes in the OGLE
(Optical Gravitational Lensing Experiment) project. Planet-hunters are very
protective of their data and cooperation between different teams is rare. But
astronomers around the globe were alerted so the event could be detected by
multiple telescopes.
"The only
way to realize the full scientific benefit of our observations is to share the
data with our competition," said Paczynski, an OGLE co-founder.
Overall,
the study involved 73 researchers from 32 institutions worldwide.
"The fact
that they've got a whole bunch of folks using multiple telescopes all observing
the same event and calibrating themselves self-consistently makes the data look
very sound," Boss said. "I think it's a pretty solid detection."
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