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Home , Planetary science

AstroTalk: Behind the news headlines of May 2013

Richard de Grijs (何锐思) (Kavli Institute for and , Peking University)

Dramatic developments in the hunt for -like

Tuesday 14 May 2013 will likely long be remembered by professional “ 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 , from close to the to 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 “”, 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 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 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 ” claimed The New York Times, “NASA’s Kepler 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. 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 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 and Kepler’s principal investigator William Borucki at the NASA 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 ,” 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 or . 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 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 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 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. Kepler-62f, with an orbital period of 267 days, was later found by Eric Agol from the University of Washington in Seattle (USA). The size of Kepler-62f has now been measured, but its mass and composition are as yet unknown. However, based on previous studies of rocky similar in size, scientists estimate its mass by association. “The detection and confirmation of planets is an enormously collaborative effort of talent and resources, and requires expertise from across the scientific community to produce these tremendous results,” said William Borucki. “Kepler has brought a resurgence of astronomical discoveries and we are making excellent progress toward determining if planets like ours are the exception or the rule.”

The two habitable zone worlds orbiting Kepler-62 have three companions in orbits closer to their star, two larger than the size of Earth and one about the size of . Kepler-62b, Kepler-62c, and Kepler-62d, orbit every five, 12, and 18 days, respectively, making them very hot and inhospitable for life as we know it. A companion to Kepler-69c, known as Kepler-69b, is more than twice the size of Earth and revolves around its star every 13 days. The Kepler-69 planets’ host star belongs to the same class as our Sun. It is 93% the size of the Sun and 80% as luminous. “We only know of one star that hosts a planet with life, the Sun. Finding a planet in the habitable zone around a star like our Sun is a significant milestone towards finding truly Earth-like planets,” said Thomas Barclay, Kepler scientist at the Bay Area Environmental Research Institute in Sonoma, California.

Two years of data await study. “We haven’t seen the most interesting data,” said Natalie Batalha, deputy science team leader for the Kepler mission and a professor at San Jose State University in California. “The most important discoveries are yet to come. They’re in our back pocket,” she said. “If true Earth– Sun analogues exist, they’re lurking there, just waiting to be pulled out.”

The spacecraft performed well as long as it was supposed to, but scientists had hoped that it would survive far longer. That is because planet hunting proved more difficult than anticipated. For instance, other astrophysical signals can masquerade as transiting planets, and stars naturally fluctuate in brightness. If the spacecraft and programme funding had lasted until it ran out of fuel—in 2019—it would have collected so much data that signs of an Earth-like planet would have been easily confirmed, the researchers said.

More years of study would have given scientists the chance to see more transits, confirming an orbit like the Earth’s. They also could have better estimated the planets’ mass, size and other properties—and ruled out errors. “It’s so frustrating. It’s such a nail-biter," said Laurance R. Doyle of the SETI Institute (“SETI” is short for the Search for ExtraTerrestrial Intelligence). “We are almost there. We really wanted to go the next mile to nail down an Earth and to tell if there are ,” he said. “The extended mission, with extra transits, would have told us that.” Added Alan Gould, co-investigator at the University of California, Berkeley (USA): “The longer you go ... the more certain you are that it is a planet.”

While frustrated by the downed prospects of what could have been, the scientists said they are excited by what remains. “It could have all ended on the launchpad,” Gould said. “We are delighted to have been a part of such a beautiful and almost perfect mission. We’ll work with what we have. We need to make the most of the data that it has already gotten and is still in the pipeline.” The Kepler programme has paved the way for two more planet-seeking missions. In 2017, NASA plans to launch TESS—Transiting Exoplanet Survey Satellite—designed to search for planets around nearby stars. It appears, therefore, that there is renewed hope for planet hunters just beyond the horizon!

Figure 1: The newly discovered planets named Kepler-62e and -f are super-Earths in the habitable zone of a distant Sun-like star. The largest planet in the image, Kepler-62f, is farthest from its star and covered by ice. Kepler-62e, in the foreground, is nearer to its star and covered by dense clouds. Closer in orbits a -sized with another small planet transiting its star. Both habitable-zone planets may be capable of supporting life. (Credit: David A. Aguilar, CfA)

Figure 2: The diagram compares the planets of the inner solar system to Kepler-69, a two-planet system about 2,700 light-years from Earth in the constellation Cygnus. The two planets of Kepler- 69 orbit a star that belongs to the same class as our Sun. Kepler-69c, is 70% larger than the size of Earth, and is the smallest yet found to orbit in the habitable zone of a Sun-like star. are uncertain about the composition of Kepler-69c, but its orbit of 242 days around a Sun-like star resembles that of our neighbouring planet Venus. The companion planet, Kepler- 69b, is just over twice the size of Earth and whizzes around its star once every 13 days. The artistic concepts of the Kepler-69 planets are the result of scientists and artists collaborating to help imagine the appearance of these distant worlds. (Credit: NASA Ames/JPL-Caltech)

Figure 3: The diagram compares the planets of the inner solar system to Kepler-62, a five-planet system about 1,200 light-years from Earth in the constellation Lyra. The five planets of Kepler-62 orbit a star classified as a K2 dwarf, measuring just two thirds the size of the Sun and only one fifth as bright. At seven billion years old, the star is somewhat older than the Sun. Much like our solar system, Kepler-62 is home to two habitable zone worlds, Kepler-62f and Kepler-62e. Kepler-62f orbits every 267 days and is only 40% larger than Earth, making it the smallest exoplanet known in the habitable zone of another star. The other habitable zone planet, Kepler- 62e, orbits every 122 days and is roughly 60% larger than Earth. The size of Kepler-62f is known, but its mass and composition are not. However, based on previous exoplanet discoveries of similar size that are rocky, scientists are able to determine its mass by association. The two habitable zone worlds orbiting Kepler-62 have three interior companions, two larger than the size of Earth and one about the size of Mars. Kepler-62b, Kepler-62c and Kepler-62d, orbit every five, 12, and 18 days, respectively, making them very hot and inhospitable for life as we know it. The artistic concepts of the Kepler-62 planets are the result of scientists and artists collaborating to help imagine the appearance of these distant worlds. (Credit: NASA Ames/JPL-Caltech)

Figure 4: This artist’s conception depicts Kepler-62e, a super-Earth-size planet in the habitable zone of a star smaller and cooler than the Sun, located about 1,200 light-years from Earth in the constellation Lyra. Kepler-62e orbits its host star every 122 days and is roughly 60% larger than Earth in size. Scientists do not know if Kepler-62e is a waterworld or if it has a solid surface, but its discovery signals another step closer to finding a world similar to Earth. (Credit: NASA/Ames/JPL-Caltech)

Figure 5: This artist’s rendering shows the Kepler space telescope. (AP Photo/NASA)

Figure 6: Artist concept of Kepler in space. (Image credit: NASA/JPL)