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

The most striking multi-planet system so far is a system dubbed Kepler 11, whose discovery the team announced in February. Its six planets are orbiting a yellow dwarf star some 2,000 light years from Earth. These planets have been confirmed as such, and they seem to be made mostly of rock by mass, although gas appears to take up most of their volume.

But Kepler's cosmic census-takers have found far more of these than anticipated. Of 1,235 planet candidates reported so far, 408 are divvied up among 107 multi-planet systems.

“We didn’t anticipate that we would find so many…. We thought we might see two or three,” Dr. Latham says.

Given the candidates found so far, it looks as though the number of planets of Neptune's mass or less are the most common, while planets with Jupiter's heft or greater are less common, although they are easier to find, Latham says.

As the team gathers more data, it’s becoming clear that it is discovering new tools for turning observations into results.

Up to now, for instance, the team has relied on ground-based telescopes to use a second planet-detection method to confirm that a transit that Kepler spots is indeed the result of a planet eclipsing its host star.

With ground-based telescopes, it takes time to carry out the observations needed for those confirmations. Kepler is observing its patch of the sky in Cygnus 24/7/365. But the big telescopes needed to confirm Kepler's observations tend to be in the Northern Hemisphere, whose night sky Cygnus vacates for part of the year. Telescopes such as Spitzer can conduct follow-up transit observations more quickly.

In other cases, Kepler data themselves are opening the possibility for new tools – “Kepler helping itself,” as astrophysicist Soren Meibom describes it.

Dr. Meibom, another researcher at the Center for Astrophysics and a Kepler team member, is working with Kepler data to develop a more-precise way of telling how old stars are in Kepler's field of view. Essentially, his team is using a star’s rotation rate as a clock.

Star's age impacts chance for life

The issue bears directly on the chance that an Earth-like planet in the habitable zone of another star could harbor life. If a star is a billion years old, don't bet on much more than microbes. At 3 billion years, far more complex life could be present, but not so developed that it could phone home. At 4.6 billion years, however, life could evolve into technologically advanced civilizations, if Earth is any indication.

But “determining the age of a star is one of the most difficult tasks in astronomy; stars don't have birth certificates,” Meibom says.

The problem: After a sun-like star's first 500 million years or so, its mass, brightness, and temperature don't change much until billions of years later, when it teeters on the brink of its self-destruction, explains Dr. Meibom,

But a star's rotation rate does slow with time as its stellar wind carries off material, in effect acting like the spinning ice skater who extends his or her arms to slow the spin.

Astronomers can track a star's rotation by patterns in the periodic dimming that occurs with the coming and going of dark patches on stars – essentially their equivalents of sunspots. Younger stars tend to have larger spots and rotate faster than older stars, so their dimming is deeper and more frequent than that in older stars.

Kepler scientists are using stars in star clusters of known ages to build a temporal yardstick for determining the ages of the stars that host planets.

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