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First Earth-sized
planets netted
Kepler-20e
Venus
Earth
Kepler-20f
By Nadia Drake
The newest exo-apples of the planet-hunting Kepler telescope’s unblinking
eye are two rocky, Earth-sized planets
hovering around Kepler- 20, a sunlike
star 950 light-years away.
Though snuggled too close to their star
to be habitable, these first Earth-sized
worlds confirmed by the Kepler team are
another big step forward for the planet
hunters, who recently found a planet
somewhat larger than Earth orbiting
a sunlike star at a distance hospitable
to life (SN: 12/31/11, p. 11). Finding
habitable distant worlds — Earth-sized
planets at the right distance from their
stars to allow the presence of liquid
water — is the team’s ultimate goal.
“The hunt is on to find a planet that
combines the best of both of these
worlds — a true Earth twin,” says David
Charbonneau, an astronomer at the
Harvard-Smithsonian Center for
The first two Earth-sized exoplanets found, Kepler-20e and Kepler-20f, are slightly
smaller than Venus (20e) and slightly larger than Earth (20f).
Astrophysics in Cambridge, Mass., and
a coauthor of the study describing the
small planets, which appears online
December 20 in Nature.
One of the planets, Kepler-20e, is a bit
smaller than Venus — 0.87 times as wide
as Earth — and completes a trip around
the star every 6. 1 days. The other, Kepler-
20f, is 1.03 times as wide as Earth, and a
year on that planet would last just 19. 6
days. Because the planets are so small,
they’re probably made of ingredients
similar to Earth’s.
Depending on where and how it
formed, Kepler-20f could even have
developed a water vapor atmosphere,
says planetary scientist Jonathan
Fortney of the University of California,
Santa Cruz. “If it started out with the
amount of water we had on Earth and
Venus, it’s probably long gone — just like
it is on Venus,” he says. “But if that planet
had a tremendous amount more water,
then it might have some left over.”
The Kepler- 20 system is a quintet
comprising three large planets (Kepler-
20b, c and d) and the two Earth-sized
ones, all tucked in nearer to their star
than Mercury is to the sun. Moving
out from Kepler- 20, the five spheres
alternate in size, with the runts of the
planetary litter bracketed on either side
by their bigger siblings.
Behemoth black holes
Two galaxies are racing to bust the record for harboring the
biggest, baddest central black hole. The galaxies NGC 3842
and NGC 4889 surround black holes nearly 10 billion times
as massive as the sun, a team of U.S. astronomers reports
in the Dec. 8 Nature. One of the bruisers, churning away in
NGC 3842, has a radius seven times bigger than that of Pluto’s orbit. The previous record-holding black hole, Messier 87,
comes in at a comparably paltry 6. 3 billion solar masses.
Scientists had predicted the existence of such enormous
mass gobblers (SN: 10/25/08, p. 18) but hadn’t found any until
now. Such large black holes might have formed after several
smaller galaxies merged. The behemoths (an artist’s conception of the region surrounding one is at left) are believed to be
the relics of quasar-powering monsters—the enormous black
holes that fueled extremely bright active galactic nuclei in the
early universe. — Nadia Drake
FROM TOP: TIM P YLE; LYNET TE COOK, GEMINI OBSERVATORY/AURA
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