Editor's Note: This article was updated at 2:55 p.m. ET to reflect additional perspective from another astronomer, questioning the claim that this planet is the smallest exoplanet known.
What may be
the smallest extrasolar planet, measuring less than twice the size of Earth,
has been discovered orbiting a sun-like star.
The world
is far hotter than ours, however. And controversy over the size claim has heated up, too.
Astronomers
used the COROT space telescope (a mission led by the French Space Agency, and
also involving the European Space Agency and others) to detect the new
planet as it transited its parent star, dimming the light from the star as
it passed in front of it. The host star is located 457 light-years from Earth,
where one
light-year is the distance light will travel in a year, or about 6 trillion
miles (10 trillion kilometers).
"For
the first time, we have unambiguously detected a planet that is 'rocky' in the
same sense as our own Earth," said Malcolm Fridlund, ESA's COROT Project
Scientist. "We now have to understand this object further to put it into
context, and continue our search for smaller, more Earth-like
objects with COROT."
He added,
"This discovery is a very important step on the road to understanding the
formation and evolution of our planet."
Sara
Seager, an astrophysicist at MIT who was not involved in the discovery said,
"My first thought is that it's extremely exciting because we've been
waiting to find a planet that we can really call rocky. I would just caution
that more information, more data, is needed."
For
instance, the discovery has yet to be published in a peer-reviewed journal, and
not much information about the planet has been released by COROT scientists.
Seager says in order to confirm an exoplanet is rocky, scientists need to nail
down its mass and radius (or the combination of size and density, or mass and density).
"It looks
like the mass is not well-determined and so that's why they're saying they're
not sure what the density is," Seager told SPACE.com. "They
think it is terrestrial-like. It might have water ice, or it might have rocks,
but it's certainly not a gas giant."
COROT
scientists estimate the planet ranges from 5.7 to 11 Earth masses.
Hot
discovery
One big
difference in the newfound planet compared to Earth: COROT-Exo-7b is located
very close to its star, orbiting once every 20 hours. Its temperature is so
high, ranging from 1,832 to 2,732 degrees Fahrenheit (1,000 to 1,500 degrees
Celsius) that the researchers say the exoplanet could be covered in lava or
water vapor.
The density
of the planet is still under investigation, though scientists say it may be rocky
like Earth and covered in liquid lava. COROT-Exo-7b may also belong to a
class of planets that are thought to be made up of water and rock in almost
equal amounts. Given the high temperatures measured, the planet would likely be
a very hot and humid place.
"Finding
such a small planet was not a complete surprise," said Daniel Rouan,
researcher at the Observatoire de Paris Lesia, who coordinates the project with
Alain Leger, from Institut d'Astrophysique Spatiale. "COROT-Exo-7b belongs
to a class of objects whose existence had been predicted for some time."
Small
and odd
Very few of
the more
than 300 exoplanets found so far have a mass comparable to that of Earth
and the other terrestrial planets — Venus, Mars and Mercury. That's because
terrestrial planets are extremely difficult to detect.
Of the
Earth-like planets detected, this is the first one spotted using the so-called
transit method, which can yield both the planet's mass and radius. Other
methods just reveal the planet's mass, Seager said.
The newfound planet's size status is also questioned. When astronomers study planets, they're interested in both mass and diameter.
"The
claim that it is the 'smallest exoplanet' found to date is not correct," said
planet-formation theorist Alan Boss of the Carnegie Institution of Washington.
"It is the smallest mass exoplanet found to date that transits, but other
hot super-Earths have been found that do not transit but have lower masses."
Boss was not involved in the current discovery.
For
instance, he adds Gliese 876 d has "a minimum mass of 5.9 Earth masses and
a best estimate for the true mass of 7.5 Earth masses."
Most of the
methods used so far are indirect and sensitive to the mass of the planet, which
is why bigger worlds are easier to detect. COROT can directly measure the size
of a planet's surface, which is an advantage to astronomers. In addition,
because the probe is in space, it has longer periods of uninterrupted
observation than from the ground.
The
internal structure of COROT-Exo-7b particularly puzzles scientists, as they are
unsure whether it is an "ocean planet," a kind of planet whose
existence has never been proved so far. In theory, such planets would initially
be covered partially in ice, and they would later drift toward their star, with
the ice melting to cover it in liquid.
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