Editorial: Kepler – Down but not out

The failure of the second of the four gyroscope-like reaction wheels of the Kepler space telescope in May this year brings down the curtain on its primary mission of identifying planets outside our solar system, namely exoplanets. A spare wheel failed in July last year. Three wheels are necessary for maintaining precision steering for the telescope to identify exoplanets that are orbiting Sun-like stars at distances where the surface temperature of earth-sized planets might be suitable for liquid water to exist. Till recently, the 0.95 metre-aperture telescope was fixedly gazing at some 150,000 stars in the constellation Cygnus to look for any dip in a star’s brightness when a planet crossed it. At least, three such transits are required to confirm the signal as an orbiting planet. Based on this approach, the telescope has successfully been able to confirm 135 exoplanets and identify over 3,500 candidates during the four-year period from May 2009 to May 2013. More importantly, the space telescope, which completed its prime mission in November last year, has returned voluminous data. As about two years’ worth of data remains to be analysed, the treasure trove may, in all probability, spring some more surprises and answer the very question that inspired the Kepler mission — “are earths in the habitable zone of stars like our Sun common or rare?” Aside from using the transit method to hunt for exoplanets, Kepler has used the asteroseismology technique to decipher, among others, the size and mass of about 500 stars. This technique looks for subtle periodic variations in a star’s brightness.

Though the telescope can no longer hunt for exoplanets using the transit technique, as an August 22, 2013 study in Nature reveals, the data collected by the telescope would help in estimating a star’s size and mass by studying the “minute variations in total solar brightness” using a different technique — the flicker method. Calculating the size of a star would help in confirming if the candidate exoplanets are indeed earth-like. The technique, first tested on 500 stars whose size and mass are known, has already been used to determine the size of 1,000 stars; Kepler has collected the flicker data of about 50,000 stars. Even in its crippled state, Kepler can still be used for collecting information about extrasolar planets by using yet another technique — microlensing. This method looks for magnification of a star’s light when two stars align themselves with respect to the telescope. Any star with orbiting planets could even double the brightness of the other star’s light. Even as Kepler’s fate is being decided, there can be no denying that it has revolutionised our understanding of stars and earth-twins.

Published in The Hindu on August 28, 2013