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Gone perhaps, but Kepler won't soon be forgotten

Astronomers look forward to building on planet-hunting telescope's discoveries

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When scientists at the Harvard-Smithsonian Center for Astrophysics scheduled a conference called “Exoplanets in the Post-Kepler Era,” they figured that era would still be several years away when the meeting happened. But after last week’s malfunction of a crucial piece of equipment on NASA’s planet-hunting Kepler space telescope, the May 20 gathering of more than 100 astronomers in Cambridge, Mass., proved all too timely.

As astronomers presented new planetary measurements and observing techniques at the conference, Kepler engineers in California were strategizing about how to remotely repair one of two broken reaction wheels that precisely point the telescope. It will take at least several weeks before they beam commands up to the $600-million telescope, and they admit that a fix is a long shot.

Though Kepler is shut down and probably out of service for good, its discoveries have revolutionized scientists’ understanding of planets beyond the solar system and are steering the course of existing and future missions. Astronomers would have liked to get a few more years out of the instrument, but it has already delivered an avalanche of data that has planet hunters more confident than ever that they will detect an Earthlike world with the ingredients and conditions for life.

“This is still an upbeat, exciting field,” says David Latham, a Harvard-Smithsonian astronomer and a member of the Kepler team. “Kepler contributed enormously, and now we’re excited to go on to the next steps.”

Kepler has become so synonymous with exoplanets that it can be hard to remember the state of the science before the telescope launched, on March 6, 2009. When a Delta II rocket carried Kepler into space on that cool, clear Florida night, astronomers knew that the galaxy contained at least 350 exoplanets, nearly all of them the size of Jupiter or larger.

Kepler’s then spent four years staring at stars and detecting the shadows of their orbiting worlds. It has added nearly 3,000 planets to that census. And because of Kepler, astronomers are convinced that the Milky Way contains hundreds of billions of planets, roughly one for every star, with at least 17 billion of them Earth-sized.

Those numbers boosted the case for funding NASA’s next exoplanet-hunting mission, the Transiting Exoplanet Survey Satellite, which is scheduled for a 2017 launch. Whereas Kepler has fixed its gaze on distant stars, TESS will focus on bright, nearby stars so that powerful telescopes like the upcoming James Webb Space Telescope will be able to probe the atmospheres of planets that TESS discovers. Kepler’s planet haul has TESS scientists optimistic that their modest $200-million telescope, while less sensitive than Kepler, will nonetheless uncover plenty of planets in our neighborhood, including a handful of Earth-sized worlds.

Kepler has also exposed an intriguing new class of potentially habitable planets, larger than the Earth-sized realms astronomers have always focused on finding. Kepler’s database includes nearly 700 worlds that are between 25 percent and 100 percent larger in diameter than Earth. Some of them orbit stars far cooler than the sun but sit close enough to them that they could have liquid water on their surfaces. These worlds are not Earthlike, and there is no comparable planet in our solar system, but that doesn’t mean they are inhospitable to life.

A key to understanding these so-called super-Earths, as well as other planets seemingly ripe for life, is to determine their composition. Astronomers hope to pair size measurements of planets observed by telescopes such as TESS with mass readings from ground-based scopes that look for subtle wobbles in stars’ motion caused by the gravitational pull of orbiting planets.

Several years ago this technique, known as radial velocity, could pick out only hulking planets that delivered a hard yank to their stars. But lately the technology has improved so drastically that in October 2012, the High Accuracy Radial velocity Planet Searcher instrument, which is affixed to a 3.6-meter telescope in Chile, spotted what appears to be a planet only slightly heavier than Earth tightly orbiting Alpha Centauri B, a sunlike star a mere 4.4 light-years away.

Other radial velocity instruments are popping up at observatories throughout the United States, Europe and South America. By 2016, the European Southern Observatory plans to install one with unprecedented precision, called ESPRESSO, which will be able to pick out tiny stellar quivers caused by Earth-mass planets in life-friendly orbits around nearby stars.

ESPRESSO will be able to pull off that feat only if there is a healthy population of such planets. Kepler’s main goal was to determine the frequency of Earthlike planets in the galaxy, but achieving that is in jeopardy due to the telescope’s mechanical failure. “Right now we have enough data to make an intelligent extrapolation about what that number is, but that is not the same as actually determining that number,” says Alan Boss, an astronomer and Kepler team member at the Carnegie Institution for Science in Washington.

Other astronomers share in Boss’s frustration, but they are still optimistic. They have a year of data from the telescope left to analyze, which should yield some exciting finds, quite possibly including an Earth-sized planet orbiting a sunlike star at a distance suitable for life. “We can’t feel sad because we have a beautiful dataset that we’re going to work on for years,” Latham says.

Combine those data with upcoming missions, plus 16 bottles of wine uncorked after the meeting presentations, and Latham says astronomers at the post-Kepler conference were upbeat about the future.

“In a way,” he says, “everyone was toasting Kepler.”

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