With Two New Saturns, Kepler Starts Its 'Top 400' Countdown

NASA came under criticism in June when it announced that its space-based telescope Kepler had detected 706 potential new exoplanets, but only released data for 306 of the candidate planets. To rule out “false alarms” from bona fide exoplanet discoveries, Kepler's scientific team wanted to give themselves time to do follow-up observations on the most promising targets. But it turns out that we didn’t have to wait too long. Today, researchers have announced two confirmed Saturn-sized planets from Kepler’s “top 400.”

The new telescope has also made the first concrete observation of a gravitational tug-of-war between neighboring exoplanets, which will allow astronomers to pinpoint the bodies’ masses in unprecedented detail, and probe how these alien solar systems formed and evolved.

The Kepler telescope, launched in March 2009, detects exoplanets by looking for a periodic dimming in stellar brightness, caused by a planet transiting (passing in front of) a star. Previously, astronomers relied on follow-up radial velocity observations, which look for the degree to which the gravitational pull of the planet perturbs the star that it orbits, to determine the masses of such planets. Such measurements have a lot of uncertainty, however, as they depend on the inclination of the planet’s orbit, which isn’t well known.

But if two exoplanets orbiting the same star have a strong gravitational influence on each other, the time of transit is not periodic, and astronomers can use this information to more accurately calculate their masses.

In a paper published online today in Science Express, Kepler researchers announce the first definitive observation of such transit time variations in planets called Kepler-9b and 9c. Combining Kepler’s initial transit observations with traditional radial velocity measurements, the scientists have been able to calculate that the two planets are just slightly less massive than Saturn. Additional observations in the years ahead should ultimately give much more accurate masses. “We will know the masses [of these planets] better than any planet outside of our Solar System,” says lead author Matthew Holman from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. Holman says the changes in the transit times of these planets were enhanced by the fact that one of the planets orbits the star in almost exactly half of the time that it takes the other, as such ‘orbital resonances’ increase their gravitational interaction.

Astronomer David Nesvorny from the Southwest Research Institute in Boulder, Colorado, who was not involved in the study, calls the detection of transit time variations “solid” and believes it’s the beginning of many important discoveries to come from the Kepler mission. “Gravitational interaction of planets in our solar system is central to our understanding of its formation and evolution, yet this interaction had been elusive in exoplanet systems,” Nesvorny says. Kepler will probably obtain similar data on hundreds of other solar systems, he says, which could then be characterized to an unprecedented detail. "This could be a true revolution.”