Surprise find in Kepler planet hunt: lots of multi-planet systems

So many planets, so little time.

Two years into a 3-1/2-year mission, NASA’s Kepler spacecraft, hunting for planets orbiting some 165,000 stars in the constellation Cygnus, is uncovering planet candidates by the hundreds.

Many of these inhabit multi-planet systems that are unexpectedly flat – the inclination of the planets’ orbits within each of these systems are essentially the same, a feature that may hold clues about how these systems formed and evolved.

Not only is the team uncovering many more multi-planet systems than it had anticipated. But the systems hold the promise of allowing researchers to gain valuable information that can lead to an initial estimation of each planet's density and hence it's bulk composition – is it rocky, a water world, or something else? And they potentially can estimate these traits more quickly than previously thought.

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These multi-planet-system discoveries “are very important to the success of the mission,” said David Latham, a researcher at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and a member of the Kepler team. He spoke at a briefing May 23 at the American Astronomical Society's spring meeting underway in Boston.

Kepler’s ultimate goal is to gather a census of Earth-mass planets orbiting in the habitable zones of sun-like stars – orbits at distances where the star warms a planet just enough to allow liquid water to remain stable on the surface.

So far, the planets Kepler is finding are well inside that zone, where hot is the order of the day.

Spitzer telescope helps confirm find

One of these is Kepler 10c, a new planet orbiting a 10-billion-year-old sun-like star some 564 light years from Earth. The team announced the discovery of its sibling, Kepler 10b, in January. This earlier discovery was striking because at 1.4 times Earth's mass, 10b was the smallest rocky planet found to date.

Evidence for 10c turned out to be buried in 10b's discovery data – the fluctuations in the host star's light as the planet briefly eclipsed, or transited, the star with each orbit.

But 10c didn't emerge as a strong planet candidate until the team used a second space telescope – NASA's Spitzer Space Telescope, an infrared observatory that is orbiting Earth – to track the dip the planet imposed on the star's light.

The case for 10c's candidacy resulted from this dual observation, in which the dip in light the two spacecraft recorded was able to rule out the possibility that 10c had been another, perhaps dimmer, star.

Comparing the data with what one might expect to see with a binary-star system rather then a planetary system, the team is 99.9998 percent certain they have a planet, according to Francois Fressin, another researcher at the Center for Astrophysics.

Many multi-planet systems found

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