Download Kepler Team Finds System with Two Potentially Habitable Planets

Kepler Team Finds System with Two
Potentially Habitable Planets
by Nancy Atkinson on April 18, 2013
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The newly discovered planets named Kepler-62e and -f are super-Earths in
the habitable zone of a distant sun-like star. The largest planet in the image,
Kepler-62f, is farthest from its star and covered by ice. Kepler-62e, in the
foreground, is nearer to its star and covered by dense clouds. Closer in orbits
a Neptune-size ice giant with another small planet transiting its star. Both
habitable-zone planets may be capable of supporting life.
Credit: David A. Aguilar (CfA)
This might be the most exciting exoplanet news yet. An international team of
scientists analyzing data from NASA’s Kepler mission has found a planetary
system with two small, potentially rocky planets that lie within the habitable
zone of their star. The star, Kepler-62, is a bit smaller and cooler than our
Sun, and is home to a five-planet system. Two of the worlds, Kepler-62e and
Kepler-62f are the smallest exoplanets yet found in a habitable zone, and they
might both be covered in water or ice, depending on what kind of atmosphere
they might have.
“Imagine looking through a telescope to see another world with life just a few
million miles from your own. Or, having the capability to travel between them
on a regular basis. I can’t think of a more powerful motivation to become a
space-faring society,” said Harvard astronomer Dimitar Sasselov, who is coauthor of a new paper describing the discovery.
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Masses and sizes for selected planets. The curves show the mass-radiusrelation (average density) for different types of planets: The blue line indicates
the loci of planets made mostly (75%) of water, the black line that of planets
like our Earth that consist almost exclusively of rock (represented here by the
mineral Enstatite, MgSiO3, a member of the pyroxite silicate mineral series
that makes up most of the Earth’s mantle), and so on. The measured radii of
Kepler-62e and Kepler-62f plus an estimate of their mass places them in a
region (blue areas) where it is highly probable for them to be earth-like
planets, that is: planets with a solid (if possibly covered in water) surface.
Kepler-11f, on the other hand, is a Mini-Neptune, showing clearly that a
comparatively low mass does not necessarily make for a solid planet. Image:
L. Kaltenegger (MPIA)
Kepler-62 in the constellation Lyra, and is about 1,200 light-years from Earth.
62e is 1.61 times Earth’s size, circles the star in 122.4 (Earth) days. 62f is 1.4
times the size of Earth, and orbits its star in 267.3 days. Previously, the
smallest planet with known radius inside a habitable zone was Kepler-22b,
with a radius of 2.4 times that of the Earth.
A third planet in another star system was also announced at a press briefing
today. Kepler-69c is 70 percent larger than the size of Earth, and orbits in the
habitable zone of a star similar to our Sun. Researchers are uncertain about
the composition of Kepler-69c, but astronomer Thomas Barclay from the
BAER Institute said its closer orbit of 242 days around a Sun-like star means it
is likely more like a super-Venus rather than a super-Earth.
The habitable zone (in which liquid water on a planet’s surface can exist) for
different types of stars. The inner planets of our Solar System are shown on
top, with Earth and Mars in the habitable zone. Kepler-62 is a notably cooler
star, and Kepler-62e and -62f are in its habitable zone. For Kepler-69c,
another planet announced today by NASA, the error bars for the star’s
radiation are such that it could possibly in the habitable zone as well. Kepler22b, the smallest planet found in a habitable zone before the recent
discoveries, is very likely a Mini-Neptune, and not a solid planet. In what is
denoted the empirical habitable zone, liquid water can exist on the surface of
a planet if that planet has sufficient cloud cover. In the narrow habitable zone,
liquid water can exist on the surface even without the presence of a cloud
cover. Image: L. Kaltenegger (MPIA)
The team says that while the sizes of Kepler 62e and 62f are known, their
mass and densities are not. However, every planet found in their size range
so far has been rocky, like Earth.
Kepler-62 system. Five planets, two of which are in the Habitable Zone.
Credit: NASA
“These planets are unlike anything in our solar system. They have endless
oceans,” said lead author Lisa Kaltenegger of the Max Planck Institute for
Astronomy and the Harvard Smithsonian Center for Astrophysics. “There may
be life there, but could it be technology-based like ours? Life on these worlds
would be under water with no easy access to metals, to electricity, or fire for
metallurgy. Nonetheless, these worlds will still be beautiful blue planets
circling an orange star — and maybe life’s inventiveness to get to a
technology stage will surprise us.”
As the warmer of the two worlds, Kepler-62e would have a bit more clouds
than Earth according to computer models. More distant Kepler-62f would need
the greenhouse effect from plenty of carbon dioxide to warm it enough to host
an ocean. Otherwise, it might become an ice-covered snowball.
“Kepler-62e probably has a very cloudy sky and is warm and humid all the
way to the polar regions. Kepler-62f would be cooler, but still potentially lifefriendly,” said Harvard astronomer and co-author Dimitar Sasselov. “The good
news is — the two would exhibit distinctly different colors and make our
search for signatures of life easier on such planets in the near future. “
The Kepler spacecraft is able to detect planets that transit or cross the face of
their host star. Measuring a transit tells astronomers the size of the planet
relative to its star.
“All of the other interesting planets in the habitable zone were until now
discovered by what is known as the radial velocity method,” said Kaltenegger.
“This method gives you a lower limit for the planet’s mass, but no information
about its radius. This makes it difficult to assess whether or not a planet is
rocky, like the Earth. A small radius (less than 2 Earth radii), on the other
hand, is a strong indicator that a planet around is indeed rocky – unless we
are talking about a planet around a very young star.”
“What makes Kepler-62e and Kepler-62f so exciting is a combination of two
factors,” Kaltenegger added. “We know their radius, which indicates that these
are indeed rocky planets, and they orbit their star in the habitable zone. That
makes them our best candidates for habitable planets out there yet.
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