The Kepler spacecraft's mission is a straightforward one: keep a vigilant watch on a large patch of stars to see if they dim, even just slightly, on a regular basis. The idea is that a planet passing in front of its host star will reveal itself to Kepler by blotting out a fraction of the star's light. This transit method has already borne fruit: NASA's Kepler spotted five planets in the first few weeks after its 2009 launch, and dozens more have been detected over the past decade from the ground and from other spacecraft. But Kepler's strength lies in its unique sensitivity to Earth-like planets—small, terrestrial worlds in temperate orbits that allow liquid water to persist.



That same sensitivity, according to a new study, might allow Kepler to identify objects in the Oort cloud—the massive sea of comets widely believed to fill the outer reaches of the solar system but never directly observed. Dutch astronomer Jan Oort predicted its existence in 1950 as a way of explaining the origin of comets that come streaking into the inner solar system from afar. A few known distant objects have been hypothesized to belong to the innermost fringe of the cloud, but their true provenance remains unknown.



In the March 1 Astrophysical Journal Letters, astrophysicists Eran Ofek of the California Institute of Technology and Ehud Nakar of Tel Aviv University estimate that Kepler could observe as many as 100 eclipses of its target stars by Oort cloud objects during its 3.5-year exoplanet-hunting mission. If Kepler detects enough eclipses, Ofek and Nakar propose, the data could be used to place observational constraints on the location and size distribution of the Oort cloud.



This effort would not be the first time that a space telescope has been used to look for distant solar system objects in this way. In December Ofek and his colleagues reported finding in archived Hubble Space Telescope data an eclipse caused by an object in the nearer Kuiper belt, the icy debris field where Pluto resides. And a 2004 paper proposed that Kepler might be able to spot Kuiper belt objects, although Ofek says that his and Nakar's calculations indicate that will not happen. The spacecraft's line of sight, angled high above the orbital plane of the solar system's planets, makes it more likely to detect objects in the spherical Oort cloud than in the more planar Kuiper belt.



Matthew Lehner, an astronomer with the Institute of Astronomy and Astrophysics at Academia Sinica in Taipei, Taiwan, says he finds the proposal to be plausible, even though Kepler's exposures are much longer than the duration of an eclipse. As such, each eclipse would show up as a dip in starlight in only one reading, whereas an exoplanet's passage would be spread out across several readings. "The events are short relative to the exposure time," Lehner says, "but Kepler also has a very high photometric [light intensity] accuracy, so it should be sensitive to at least some of the events."



Kepler would have to detect several candidate events in order for astronomers to confidently correlate them with eclipses by Oort cloud objects rather than with, say, detector glitches. Ofek and Nakar propose authenticity tests that would favor viable eclipses over random noise, such as checking to see if the eclipses tend to occur on stars that appear smaller from Kepler's viewpoint or if the eclipse rate is dependent on the spacecraft's motion.



Ofek acknowledges that firmly identifying Oort cloud comets will require a high yield of eclipses from Kepler, something that is hardly guaranteed, given how little is known of the cloud's makeup. Based on various estimates of the Oort cloud's properties, Ofek and Nakar calculate that Kepler may spot anywhere from zero to 100 eclipses in its mission. "The big 'if' here is that to implement this you need to detect many events," Ofek says. He notes that, roughly speaking, an event count in the tens would allow astronomers to say something about the Oort objects Kepler has seen.



"If the estimates of the contents of the Oort cloud are correct, there is a good chance that we will detect more than 10 events," Ofek says. "If you don't detect anything, you still have the strongest constraint yet on the contents of the Oort cloud."