Chart of Kepler planet candidates as of January
2014.
Image Credit: NASA Ames
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More than three-quarters of the planet candidates discovered by NASA's Kepler
spacecraft have sizes ranging from that of Earth to that of Neptune, which is
nearly four times as big as Earth. Such planets dominate the galactic census but
are not represented in our own solar system. Astronomers don’t know how they
form or if they are made of rock, water or gas.
The Kepler team today reports on four years of ground-based follow-up
observations targeting Kepler's exoplanet systems at the American Astronomical
Society meeting in Washington. These observations confirm the numerous Kepler
discoveries are indeed planets and yield mass measurements of these enigmatic
worlds that vary between Earth and Neptune in size.
Included in the findings are five new rocky planets ranging in size from ten
to eighty percent larger than Earth. Two of the new rocky worlds, dubbed
Kepler-99b and Kepler-406b, are both forty percent larger in size than Earth and
have a density similar to lead. The planets orbit their host stars in less than
five and three days respectively, making these worlds too hot for life as we
know it.
A major component of these follow-up observations were Doppler measurements
of the planets' host stars. The team measured the reflex wobble of the host
star, caused by the gravitational tug on the star exerted by the orbiting
planet. That measured wobble reveals the mass of the planet: the higher the mass
of the planet, the greater the gravitational tug on the star and hence the
greater the wobble.
"This marvelous avalanche of information about the mini-Neptune planets is
telling us about their core-envelope structure, not unlike a peach with its pit
and fruit," said Geoff Marcy, professor of astronomy at University of
California, Berkeley who led the summary analysis of the high-precision Doppler
study. "We now face daunting questions about how these enigmas formed and why
our solar system is devoid of the most populous residents in the galaxy."
Using one of the world's largest ground-based telescopes at the W. M. Keck
Observatory in Hawaii, scientists confirmed 41 of the exoplanets discovered by
Kepler and determined the masses of 16. With the mass and diameter in-hand,
scientists could immediately determine the density of the planets,
characterizing them as rocky or gaseous, or mixtures of the two.
These density measurements dictate the possible chemical composition of these
strange, but ubiquitous planets. The density measurements suggest that the
planets smaller than Neptune – or mini-Neptunes – have a rocky core but the
proportions of hydrogen, helium and hydrogen-rich molecules in the envelope
surrounding that core vary dramatically, with some having no envelope at
all.
The ground-based observation research validates 38 new planets, six of which
are non-transiting planets only seen in the Doppler data. The paper detailing
the research is published in the Astrophysical Journal today.
A complementary technique used to determine mass, and in turn density of a
planet, is by measuring the transit timing variations (TTV). Much like the
gravitational force of a planet on its star, neighboring planets can tug on one
another causing one planet to accelerate and another planet to decelerate along
its orbit.
Ji-Wei Xie of the University of Toronto, used TTV to validate 15 pairs of
Kepler planets ranging from Earth-sized to a little larger than Neptune. Xie
measured masses of the 30 planets thereby adding to the compendium of planetary
characteristics for this new class of planets. The result also was published in
the Astrophysical Journal in Dec. 2013.
"Kepler's primary objective is to determine the prevalence of planets of
varying sizes and orbits. Of particular interest to the search for life is the
prevalence of Earth-sized planets in the habitable zone," said Natalie Batalha,
Kepler mission scientist at NASA’s Ames Research Center in Moffett Field, Calif.
"But the question in the back of our minds is: are all planets the size of Earth
rocky? Might some be scaled-down versions of icy Neptunes or steamy water
worlds? What fraction are recognizable as kin of our rocky, terrestrial
globe?"
The dynamical mass measurements produced by Doppler and TTV analyses will
help to answer these questions. The results hint that a large fraction of
planets smaller than 1.5 times the radius of Earth may be comprised of the
silicates, iron, nickel and magnesium that are found in the terrestrial planets
here in the solar system.
Armed with this type of information, scientists will be able to turn the
fraction of stars harboring Earth-sizes planets into the fraction of stars
harboring bona-fide rocky planets. And that's a step closer to finding a
habitable environment beyond the solar system.
Ames is responsible for the Kepler mission concept, ground system
development, mission operations, and science data analysis. NASA's Jet
Propulsion Laboratory in Pasadena, Calif., managed Kepler mission development.
Ball Aerospace & Technologies Corp. in Boulder, Colo., developed the Kepler
flight system and supports mission operations with the Laboratory for
Atmospheric and Space Physics at the University of Colorado in Boulder. The
Space Telescope Science Institute in Baltimore archives, hosts and distributes
Kepler science data. Kepler is NASA's 10th Discovery Mission and was funded by
the agency's Science Mission Directorate.
For more information about the Kepler space telescope, visit:
Guillermo Gonzalo Sánchez Achutegui
ayabaca@gmail.com
ayabaca@hotmail.com
ayabaca@yahoo.com
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