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Astrotalk: Behind the News Headlines of May 2013

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Astrotalk: Behind the News Headlines of May 2013 AstroTalk: Behind the news headlines of May 2013 Richard de Grijs (何锐思) (Kavli Institute for Astronomy and Astrophysics, Peking University) Dramatic developments in the hunt for Earth-like planets Tuesday 14 May 2013 will likely long be remembered by professional “planet hunters” as the day when their dreams were put on hold. Until 1995, the entire known population of planets in the Universe consisted of the nine planets we had discovered in our own solar system, from Mercury close to the Sun to Pluto in the outer regions. In the mean time, in 2006, that number was reduced to eight, when the International Astronomical Union decided to downgrade Pluto to a “dwarf planet”, along with a handful of other rocky bodies in our immediate solar system neighbourhood. Then, when in 1995 Michel Mayor and Didier Queloz of Geneva Observatory (Switzerland) discovered the first planet orbiting a star other than our Sun, they had little idea that their achievement would open up the now immensely popular field of extrasolar planetary science. Exoplanet discovery and follow-up studies are a mainstay of current professional astronomy (it’s fashionable!), most prominently facilitated by the 2009 launch of the Kepler space telescope, whose key aim was a search for Earth-like planets. So far, Kepler has discovered 132 genuine planets—whose nature has been verified by later observations with other telescopes and follow-up scientific analysis— and spotted 2,740 potential planets. About 230 are the size of the Earth, and 820 others are probably rocky worlds similar to our own but roughly twice as big. The satellite’s mission was supposed to have ended by now, but in 2012 the US space agency NASA agreed to keep Kepler running through 2016, at a cost of some US$20 million a year. That is, until that fateful date of 14 May 2013. News headlines the following few days attempted to outdo each other to cover the unfolding drama of the telescope’s failure; “Breakdown Imperils NASA’s Hunt for Other Earths” claimed The New York Times, “NASA’s Kepler Spacecraft May Be Finished” said Top News Today, and many other news outlets led with “NASA’s Kepler spacecraft is critically damaged”. You get the point: NASA’s planet-hunting Kepler telescope is broken, potentially jeopardizing the search for other worlds where life could exist outside our solar system. In January, one of the reaction wheels that keep the spacecraft oriented in space (similar to a flywheel) experienced too much friction. Kepler’s engineers shut down the telescope for a few weeks to give it a rest, in the hope that the wheel’s lubricant might spread out and solve the problem. but it didn’t, although this didn’t really affect the observations either. However, on 14 May, the spacecraft went into a “safe mode”, because the reaction wheel had stopped operating completely. Kepler is equipped with four reaction wheels, but one already failed last year, after showing signs of erratic friction. Three operational wheels are required to keep the spacecraft aimed properly and precisely, but at present only two are available. With only two working wheels left, the spacecraft can’t point at stars with the precision required to search for Earth-like planets. Project managers hope to remedy the situation by rocking the wheel that failed this month back and forth, finding a workaround, or perhaps by resurrecting the wheel that failed last year. This will probably take several months, according to Kepler’s deputy project manager, Charles Sobeck. If engineers can’t find a fix, the failure could mean an end to the US$600 million mission, although NASA remains optimistic. “I wouldn’t call Kepler down-and-out just yet,” said John Grunsfeld, associate administrator of the Science Mission Directorate at NASA Headquarters in Washington, DC (USA). In orbit around the sun, 65 million kilometers from Earth, Kepler is too far away to send astronauts on a repair mission like the expedition that fixed a mirror on the Hubble Space Telescope in the mid-1990s. For John Grunsfeld, who acted as mechanic to the Hubble Space Telescope during several lengthy spacewalks, the Kepler malfunction is particularly frustrating. “Unfortunately, it’s not in a place where I can go and fix it,” he said. The telescope could be used for other purposes, even if it can no longer track down planets. Figuring out how to continue Kepler’s operations in this scenario will also take some time to sort out. While ground-based telescopes can hunt for planets outside our solar system, Kepler is much more advanced. For the past four years, Kepler has focused its telescope on a distant patch of the Milky Way containing more than 150,000 stars, recording slight dips in brightness—a sign of a planet passing in front of the star. When a planet candidate transits, or passes in front of the star from the spacecraft’s vantage point, a percentage of light from the star is blocked. The resulting dip in the brightness of the starlight reveals the transiting planet’s size relative to its star. Now “we can’t point where we need to point. We can’t gather data,” Charles Sobeck said. Scientists who have no role in the Kepler mission also mourned the news. They said that the latest loss means the spacecraft may not be able to determine how many Earth-size planets are located in the so-called “Goldilocks zone” (or “habitable zone”), where it is not too hot or too cold for water to exist in liquid form on the surface. And while they praised the data collected by Kepler so far, they said several more years of observations are needed to nail down that number. “This is one of the saddest days in my life. A crippled Kepler may be able to do other things, but it cannot do the one thing it was designed to do,” according to Alan boss of the Carnegie Institution of Washington. Scientists said there is a backlog of data that they still need to analyse even if Kepler stopped looking for planets. “I think the most interesting, exciting discoveries are coming in the next two years. The mission is not over,” said chief scientist and Kepler’s principal investigator William borucki at the NASA Ames Research Center in California (USA), which manages the mission. Scientists have only begun to dig through its vast trove of data, where proof of another Earth- like planet may be hiding. “The signals are there, in the data we have now—we have to search for them,” said Borucki. The Kepler project has been a tremendous success, changing our view of the Universe. Before Kepler, we knew little about other solar systems in our own galaxy, the Milky Way. Now we have a huge inventory. The mission has taught us that planets orbit virtually every star in the sky. Early in the mission, the Kepler telescope mostly found large, gaseous giants in very close orbits to their stars. Known as “hot Jupiters,” these are easier to detect owing to their size and very short orbital periods. Earth would take three years to accomplish the three transits required to be accepted as a planet candidate. So far, Kepler’s first 22 months of data have revealed planets that are quite obvious, but most are in the wrong neighbourhoods to sustain life—either too close, or too far away, from their stars. Or they are very big and gaseous, like Jupiter or Saturn. The telescope has found the largest tally of planets credited to a single telescope yet, but only two so far are the best candidates for habitable planets. Just last month, Kepler scientists announced the discovery of a distant pair of planets that seem like ideal places for some sort of life to have evolved. Most recently, it revealed some planets circling in the "habitable zone around stars Kepler 62 and 69. The Kepler-62 system has five planets; Kepler-62b, 62c, 62d, 62e, and 62f. The Kepler-69 system has two planets; Kepler-69b and 69c. Kepler-62e, 62f, and 69c are super-Earth-sized planets. Two of the newly discovered planets orbit a star that is smaller and cooler than our Sun. Kepler-62f is only 40% larger than Earth: it is the exoplanet closest to the size of our planet known in the habitable zone of another star. Kepler-62f is likely to have a rocky composition. Kepler-62e orbits on the inner edge of the habitable zone and is roughly 60% larger than Earth. The third planet, Kepler-69c, is 70% larger than the Earth, and orbits in the habitable zone of a star similar to our Sun. Scientists are uncertain about the composition of Kepler-69c, but its orbit of 242 days around a Sun-like star resembles that of Venus in our own solar system. Scientists do not know whether life could exist on the newfound planets, but their discovery signals that we are another step closer to finding a world similar to the Earth orbiting a star like our Sun. “The Kepler spacecraft has certainly turned out to be a rock star of science,” said John Grunsfeld. “The discovery of these rocky planets in the habitable zone brings us a bit closer to finding a place like home. It is only a matter of time before we know if the galaxy is home to a multitude of planets like Earth, or if we are a rarity.” Orbiting its star every 122 days, Kepler-62e was the first of the habitable zone planets identified.
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