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Planetary News: Kepler (2010)

Kepler Discovers Hundreds of New Planets

Click to enlarge > Kepler Credit: NASA / ARC

Kepler, the NASA spacecraft charged with searching for distant worlds like ours, follows in the Earth’s wake as it orbits the Sun, its sights focused permanently on a single patch of sky. With its unblinking gaze, the spacecraft searches among 156,000 stars for slight but regular dips in a star’s brightness.  Such dimmings could indicate a transit – a planet moving across the face of its home star, announcing its presence by blocking out a small portion of starlight. If scientists’ expectations prove true, over its three and a half year mission Kepler could discover hundreds or perhaps thousands of transiting planets.

It’s been a year now since Kepler completed its first round of observations, and the early results are now in. Over its first month and a half of operation, between May 2 and June 15, 2009, Kepler detected over 850 stars that showed the signature dips in luminosity that characterize a transiting planet. Of these about 150 were shown to be “false positives,” that is stars that display the effect for other reasons, and almost certainly do not have a transiting planet. This leaves 706 stars that at this point appear to have transiting planets, although it is almost certain that further studies will show that a good portion of them will ultimately turn out to be false positives as well. But even in the extreme case that a full half of these “candidate planets” will prove to be no planets at all, that still leaves Kepler with a haul of over 350 new planets.

To appreciate this number, consider that since the discovery of the first exoplanet in 1995, astronomers have detected roughly 400 exoplanets using a slew of different methods. This means that in its first month and a half of operation Kepler has found almost as many planets as all other methods combined in the preceding 15 years!

In order to separate true transiting planets from other phenomena that mimic the transit effect, scientists try to follow up on each of Kepler’s discoveries with radial velocity observations of each star. This method measures the slight shifts in a star’s spectrum as it rocks back and forth to the tug of an orbiting planet, and has been responsible for the majority of exoplanets discoveries to date. Significantly, whereas transit detections like Kepler’s provide a good estimate of a planet’s diameter, radial velocity measurements provide an accurate estimate of a planet’s mass. If the RV study shows that a planetary mass object is orbiting a star where Kepler detected a candidate planet, then the candidate is almost certainly a true exoplanet. If, however, the orbiting object turns out to have the mass of a star, then Kepler’s “transiting exoplanet” is, in fact, no planet at all but a binary star whose two components periodically eclipse each other.

From its station in space Kepler can observe its star field continuously, but that is not the case for planet hunters working to confirm its discoveries from Earth. RV measurements require large telescopes that are available only at the world’s top observatories, such as the Keck Observatory in Hawaii and the European Southern Observatory in Chile. From these locations, as it happens, Kepler’s star field is visible for only half the year, during the summer months. In the summer of 2009, with unanalyzed data streaming in from a new and untested spacecraft, astronomers only had time to confirm 5 of Kepler’s planetary candidates. This summer they are working to obtain RV data on hundreds more before the stars sink below the horizon once again in September.

Among Kepler’s candidates are what appear to be several multi-planet systems which, if confirmed, would be the first systems with more than one transiting planet ever detected. A report by Jason Steffen and Kepler Team collaborators analyzes five of these systems, one of them composed of at least three orbiting objects, the other four seemingly with two planets each. According to Darin Ragozzine of the Harvard Smithsonian Center for Astrophysics, who has been studying the Kepler data, such planetary systems are especially valuable to scientists.

Click to enlarge > A Planetary Transit An artist's impression of a Jupiter size extrasolar planet passing in front of its parent star. Credit: NASA, ESA, G. Bacon (STSci)

In systems with more than one transiting planet, Ragozzine explained in a recent article submission with Matthew J. Holman, it is sometimes possible not only to measure each planet’s orbit and size, but also to detect the interaction between the different planets. As the data from Kepler accumulates in the coming months and years, astronomers expect to find slight but measurable variations in the exact timing and duration of the transits. These anomalies are caused by the planets’ gravitational pull on each other, and they will enable scientists to calculate the planets’ mass, the inclination of their orbits, and sometimes provide an independent measure of the mass of the home star. Ragozzine goes on to point out that when two planets pass in front of their star at the same time, they might cross over one another as well. This is not just a cool exoplanetary alignment, but can also provide valuable information about the orientation of the planetary orbits. Overall, Ragozzine concludes, multi-transiting system will be the most information-rich planetary systems besides our own solar system.

Kepler has been in orbit for just over a year, and it has already proven itself as by far the most effective and efficient planet-hunting instrument ever built. But Kepler’s work has barely begun: the spacecraft will continue its sleepless watch over the same patch of sky for at least another two and a half years, supplying scientists with enough data to study and analyze for many years to come. If predictions hold true, somewhere within that mountain of data is hidden the holy grail of planet hunting: A small rocky planet orbiting within the habitable zone of its star, where water can exist in liquid state – another Earth in the depths of space.