Kepler's Insight to Physical Nature of Planetary Systems

It’s been 808 days since the launch of one of NASA’s most prolific space observatories: the Kepler exoplanet-hunting space telescope. Kepler team members met in Boston this week at the meeting of the American Astronomical Society (AAS) to give a status report on their progress toward answering one of the most timeless questions in astronomy: how abundant are Earth-sized planets in the Galaxy?

They are methodically closing in on the answer, but certainly not there yet.

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For the past two years, the spacecraft has kept a steady gaze on 165,000 stars in the summer constellation Cygnus. To date, Kepler has tallied 1235 candidate exoplanets with orbits inclined such that they can be seen passing in front of their stars. This trawl has required a staggering 5.5 billion separate brightness measurements.

“Hundreds more planets are expected to be found,” said Kepler principal investigator Bill Borucki of NASA’s Ames Research Center.

Geoff Marcy of the University of California at Berkeley said that Kepler is striving to answer two key questions: what fraction of stars harbor planets? How many Jupiter–sized, super-Earths, and Earth-sized planets are there?

There has been building anticipation for Kepler finding a true “Earth-clone” planet, but Borucki reiterated: "Kepler is a statistical mission to determine frequency of Earth sized planets in habitable zones, understand sizes of potentially habitable planets, and get a feel for their temperatures."

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Flat Land

Kepler has recorded transits by several planets around a single star. To date 170 multiple systems of two or more plants have been found. The poster child for these is the "six-pack" planet system Kepler 11, which I reported on back in February. What this means is that the systems are remarkably "coplanar." The planets are laid out like peas on a flat dinner plate.

By contrast, the orbits of the major planets in our solar system are tilted by as much as seven degrees from the plane of Earth’s orbit. But some Kepler systems vary by only one degree offset.

However, one caution with this interpretation is that the there still could be undiscovered planets much farther out that are tilted relative to the flat inner planet population.

It’s been long held that planets agglomerate inside a very flat disk of material encircling a newborn star like the rings of Saturn. So their orbits should be coplanar. But later perturbations by the planets’ gravitational tug may warp orbits. The star Beta Pictoris is a classic example with a planet that is orbiting several degrees off from the bright edge-on disk plane.

Warped disks could offer forensic evidence of disrupted planetary systems. Kepler found that the least coplanar systems are those with a hot Jupiter –- a giant planet that has migrated precariously close to its star. The spiraling-in toward the star left a rough road behind for other planetary orbits.

Even more intriguing is that multiple planet systems push and pull on each other. This slightly changes the timetable as Kepler follows recurring transits. These data can be used to roughly measure the mass of a planet. The diameter of the planet is loosely estimated from how much starlight the planet blocks out during transit and the density of a planet can be calculated from these observations.

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Astronomers may be able to distinguish between sold rocky “bowling ball planets” and mushy lower density planets. However these data alone could not definitively say if a planet has oceans.

Finally, as reported for Discovery News by Irene Klotz on Monday, Kepler is even being used to estimate the ages of the host stars by measuring their spin rates. Like an unwinding spring stars will spin-down due to drag induced by bleeding matter into space via a stellar wind.

But in a presentation at the AAS meeting by Jeremy Drake of the Harvard-Smithsonian Center for Astrophysics, he said that planets actually inhibit a parent star’s spin-down. In particular, close-up planet keeps stars rotating faster for longer.

What’s Next?

The next step in Kepler’s survey is to confirm the planet candidates with ground-based telescopes that will look for the telltale wobble of a star due to a planet’s gravitational pull. Planet-hunting season is opening as the summer constellation Cygnus the Swan returns to optimum visibility in the northern sky.

Marcy did a cursory estimate, based on preliminary Kepler data to date, that there could be 100 Earth-sized planets per 1000 stars. “If confirmed this would be very profound discovery,” he said. But he cautioned that more observations are needed to reach a statistically significant sample of Earth-diameter planets.

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