Daniel Fabrycky, an astronomer at the University of Chicago in Illinois, has an alternative follow-up study in mind. He and his colleagues have proposed looking at planetary systems in which densely packed planets are affected by one another’s gravitational pulls — creating periodic cycles in which the timing of transits are first advanced and then delayed. The light dip during a transit reveals only the size of the eclipsing planet, but knowledge of transit-time variation yields the planet’s mass, which is crucial for working out the density and composition of the bodies. Like Welsh, Fabrycky wants Kepler to zero in on planetary systems with long orbits, for which the full cycle of these transit-timing variations has not yet been seen.
But Andrew Gould, an astronomer at Ohio State University in Columbus, says that he is skeptical about using the craft to simply follow up on its original tasks when its pointing precision has been degraded by a factor of as much as 1,000. “People really have to break out and come up with new ideas,” he says.
Perhaps taking such advice to heart, Fabrycky’s team has a second proposal: putting Kepler to work not as a planet hunter, but as a sentinel for near-Earth objects, including asteroids several hundred meters in diameter that might be on a collision course with Earth. A survey of space rocks would take advantage of Kepler’s large field of view. And at least part of the study could be completed with Kepler looking for targets within its orbital plane, so as to optimize its pointing.
Gould has proposed another scheme, in which Kepler would survey stars towards the Milky Way’s central bulge for signs of planets, using a technique known as microlensing.
Microlensing relies on a prediction of Einstein’s theory of general relativity: the gravity of any massive object bends light. Like a magnifying lens, a foreground star bends and brightens light from stars behind it. A single foreground star, or microlens, produces a characteristic brightening curve, but if that lensing star has a planet, the curve will have an additional wiggle.
Researchers have already used microlensing to reveal some 40 planets towards the center of the Galaxy, but the observations typically do not reveal masses. By observing microlens planets using Kepler and ground-based telescopes at the same time, differences in transit duration and brightness emerge that can yield the planets’ mass. However, the survey could be performed for only about five weeks of the year because of limited chances to view the Galactic Centre without interference from the Sun.
If any of the proposals recommended by the Kepler team seems worthwhile to NASA, they will be examined early next year by a review panel of external scientists. At that stage, a repurposed Kepler would face its biggest hurdle — a competition for the limited pot of funds against nine other astrophysics missions, including the Hubble Space Telescope and the Fermi Gamma-ray Space Telescope. On receiving recommendations from the review panel, NASA will make its final funding decisions next June.
Not everyone is rooting for Kepler. Doug Finkbeiner, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, wants NASA to support missions that are still healthy. He has used Fermi to discover two galaxy-sized bubbles of ionized gas blowing from the center of the Milky Way, and is counting on continued funding for the γ-ray telescope. “My very biased and self-interested perspective is that I hope we let Kepler die,” he says.