Kepler Science conference, Mountain View, calif., DeceMber 5–9
312
candidates
1,235
candidates
Possible planets
found by Kepler
as of June 2010
Possible planets
found by Kepler as
of February 2011
2,326
candidates
Possible planets
found by Kepler as
of December 2011
extrasolar orb
occupies just
right spot for life
Planet hunters add more
than 1,000 new candidates
Kepler-22b, illustrated here, is the
smallest exoplanet
confirmed to be
well within a sunlike star’s habitable
zone, where liquid
water can exist.
by nadia Drake
One of the many worlds circling faraway
suns is tracing a most Earthlike path:
The planet Kepler-22b has a 290-day
orbit that parks it firmly within its star’s
life-friendly zone, scientists announced
December 5.
With a radius 2. 4 times Earth’s, Kepler-
22b is the smallest planet confirmed to
sit comfortably in a sunlike star’s “
habitable zone,” where temperatures allow
liquid water to exist. Kepler-22b’s home
star — shining 600 light-years away near
the constellation Cygnus — is very similar
to the sun, though a bit cooler.
“Today’s discovery is a tantalizing
indication that, with time, Kepler might
find true Earth analogs, if they exist,”
said Pete Worden, the director of NASA’s
Ames Research Center in Mountain
View. The Kepler team’s goal is to detect
Earth-sized exoplanets within the habitable zones of their parent stars, a task
that requires years of observing time.
The space telescope stares at a patch
of sky near the constellations Cygnus
and Lyra that sparkles with more than
150,000 stars. A planet passing in front
of one of these stars produces a telltale
dimming in the star’s light, and scientists
use these flickers to calculate a planet’s
radius and orbital period. The team waits
for three of these dimming events before
considering whether a planet might be
the culprit, which is why detecting longer, Earthlike orbits takes years.
The telescope spotted Kepler-22b’s
first transit just after observations began
in 2009, and the third transit occurred
just before Christmas 2010. “It’s a great
gift,” said Kepler principal investigator
Bill Borucki of Ames. “We consider this
sort of our Christmas planet.”
But Kepler-22b isn’t everything planet
hunters could wish for. Astronomer Sara
Seager of MI T said the planet’s size sug-
gests the presence of an atmosphere
with a massive greenhouse effect. “It’s
going to be hot,” she says. “Too hot at the
surface for life to survive.”
Planetary scientist Abel Mendez of
the University of Puerto Rico in Arecibo
has used Kepler-22b’s radius to calculate
possible masses and densities. Mendez
says the most optimistic habitable sce-
nario would be if Kepler-22b were a sort
of water planet, with a global ocean and
some clouds. But even that is only mar-
ginally life-friendly, he noted. “I’m not
optimistic,” Mendez said. “But I would
love to be wrong.”
Scientists initially speculated that tem-
peratures on Kepler-22b would hover
around 72 degrees Fahrenheit. But that
calculation was based on an atmosphere
with Earthlike properties and warm-
ing effects. Scientists can’t determine
whether the planet is truly Earthlike in
composition without knowing its mass.
If they did know how heavy Kepler-22b
is, they could determine the planet’s
composition — rocky, watery or gassy.
Seager’s scenario assumes that
Kepler-22b is a gassy planet, a mini-Neptune, the likes of which are absent
from Earth’s own solar system. But it’s
possible that Kepler-22b is just a dense,
rocky planet. If it is really dense, with a
thin atmosphere, then there’s a chance it
could be habitable, Seager said.
So far, the telescope’s 1,000-day
stare has allowed it to glimpse thousands of potential planets, including
1,094 new candidates just announced.
These newbies bring the total number
of exotreasures in the Kepler clutch to
2,326, including 207 Earth-sized — and
smaller — planets. “Not only do we have
Earth-size planets, we have planets that
are significantly smaller than Earth,”
said Kepler scientist Natalie Batalha of
San Jose State University in California.
Providing tantalizing targets for those
in search of intelligent life are another
10 potential Earth-sized candidates in
their stars’ habitable zones. Five are
“very, very viable candidates,” Batalha
said, noting that about 20 percent of
the candidate planets are organized
in planetary systems, making it easier
to confirm the presence of the littlest,
most Earthlike orbs.
Astronomer Jill Tarter of the SETI
Institute in Mountain View said she and
her team will be using the Allen Telescope Array in California to eavesdrop
on the entire Kepler field and listen for
radio signals of intelligent origin. “We’re
once again looking at all the Kepler exoplanets,” she said. But she added, “We
will give a higher priority to those worlds
that our colleagues tell us are not too
warm and not too cold, but just right.” s
