Updated Kepler catalog contains 219 new exoplanet candidates

NASA’s Kepler space telescope team has identified 219 new planet candidates, 10 of which are near-Earth size and in the habitable zone of their star. Credit: NASA/JPL-Caltech

Scientists have published a catalog of exoplanet discoveries made by NASA’s Kepler space telescope, identifying 219 previously-unknown planet candidates circling stars elsewhere in the galaxy, including 10 would-be worlds that appear to be about the same size of Earth with temperatures potentially hospitable for life.

Culling data collected during the first four years of Kepler’s mission, researchers used computers to pick out and analyse signals from stars that could be have caused by nearby planets. Automated software identified the detections most likely to be real worlds, according to Susan Thompson, a Kepler research scientist at the SETI Institute and NASA’s Ames Research Center who led the cataloging effort.

“This is the last search that we performed, and we used our most improved techniques, and with that we found 4,034 candidates, which include 10 new terrestrial-sized candidates in the habitable zone of their star,” Thompson said.

Follow-up observations have, so far, confirmed 2,335 of the more than 4,000 candidate worlds discovered by Kepler are real. The 10 new Earth-sized exoplanet candidates identified by Kepler scientists bring the mission’s total haul to 49 likely worlds about the same size as our home planet that could have the right temperature to harbour liquid water, Thompson said.

Thirty of the 49 Earth-sized planets have been verified.

“This new result presented today has implications for understanding the frequency of different types of planets in our galaxy, and helps us to advance our knowledge of of how planets are formed,” said Mario Perez, Kepler program scientist at NASA Headquarters.

The four years of data covered in the exoplanet catalog come from Kepler’s observations of around 200,000 stars in the constellation Cygnus. Using a 37-inch (95-centimetre) telescope and a unique wide-angle 95-megapixel camera, Kepler looked for subtle dips in the brightness of stars in a predetermined patch of sky beginning soon after its 2009 launch on a Delta 2 rocket.

NASA’s Kepler space telescope was the first agency mission capable of detecting Earth-size planets using the transit method, a photometric technique that measures the minuscule dimming of starlight as a planet passes in front of its host star. For the first four years of its primary mission, the space telescope observed a set starfield located in the constellation Cygnus (left). New results released from Kepler data June 19, 2017, have implications for understanding the frequency of different types of planets in our galaxy and the way planets are formed. Since 2014, Kepler has been collecting data on its second mission, observing fields on the plane of the ecliptic of our galaxy (right). Credit: NASA/Wendy Stenzel

The brightness fluctuations — if they occur in regular patterns — could be caused by a planet transiting in front of the star, blotting out a tiny fraction of its light. Sophisticated software written specifically for the Kepler mission was tasked with rooting out false positives that could be caused by starquakes or other natural phenomena.

Thompson said scientists injected simulated transits and measured how often Kepler and its data-crunching computers missed a planet. The catalog also accounted for noise in Kepler’s data archive that software could have mistaken for a planet.

That is how scientists arrived at the 4,034 planet candidates from Kepler’s four-year observing campaign in the constellation Cygnus. Subsequent detections from other telescopes, in space or on the ground, have verified 2,335 of them to date.

“These are planets where there is no question at all that that signal is coming from an exoplanet,” Thompson said.

In the case of the exoplanet candidates, “there is still some room for doubt whether that signal is coming from a planet,” she said. “It still could be coming from other astrophysical signals.”

Several of the newly-discovered planet candidates orbit G dwarf stars like our sun. Thompson singled out one exoplanet candidate named KOI-7711, which is about 1.3 times the size of Earth and orbits its star every 302 days.

She said KOI-7711 “gets approximately the same amount of heat that we get from our own star.

“However, there’s a lot we don’t know about this planet, and as a result, it’s hard to say whether it’s really an Earth twin,” Thompson said Monday. “We need to know more about its atmosphere, whether there’s water on the planet.”

Alien astronomers looking into our solar system through a distant telescope could be tricked into assuming more than one planet was hospitable to life.

“I always like to remind people that it looks like there are three planets in our habitable zone — Venus, Earth and Mars – and I’d only really want to live on one of them,” Thompson said.

Kepler’s updated exoplanet listing will help astronomers estimate how common rocky, potentially habitable planets are in our galaxy.

“For M dwarfs, which are small stars that make up 75 percent of the stars, in the galaxy, we know that one out of every four of them has a planet that is small and is in the habitable zone,” said Courtney Dressing, a NASA Sagan Fellow at the California Institute of Technology.

Dressing said scientists still trying to determine the ubiquity of Earth-sized planets around sun-like stars, one of the chief goals of the Kepler mission. But the catalog released this week will arm scientists with better data to answer that question.

“One thing that’s important for us is are we alone?” Perez said Monday. “And maybe Kepler today has told us indirectly — although we don’t have confirmation — that we are probably not alone.”

Statistics from the Kepler planet catalog also suggest small planets fall into two families, said Benjamin Fulton, a doctoral candidate at the University of Hawaii in Manoa.

Researchers using data from the W. M. Keck Observatory and NASA’s Kepler mission have discovered a gap in the distribution of planet sizes, indicating that most planets discovered by Kepler so far fall into two distinct size classes: the rocky Earth-size and super-Earth-size (similar to Kepler-452b), and the mini-Neptune-size (similar to Kepler-22b). This histogram shows the number of planets per 100 stars as a function of planet size relative to Earth. Credit: NASA/Ames Research Center/CalTech/University of Hawaii/B.J. Fulton

One grouping of planets ranges from smaller than Earth to less than twice the size of Earth, and another set of planets found by Kepler measure up to four times Earth’s diameter. There are relatively few worlds in between, Fulton said.

“Most of the planets in the first group may be akin to the Earth with rocky surfaces and little to no atmospheres,” Fulton said. “Planets in the second group are probably more like cousins of Neptunes with thick atmospheres and no surface to speak of.”

Astronomers turned to the W. M. Keck Observatory in Hawaii to measure the sizes of approximately 1,300 stars known to have planets. The data yielded better estimates of the planets’ dimensions.

Scientists believe the delineation between “super-Earths” and “mini-Neptunes” stems from the way the planets form. Some worlds suck in more hydrogen and helium, growing thick, deep atmospheres, while others develop comparatively thin atmospheres that can be blown away by stellar winds and heat.

“This result has significant implications for the search for life,” Fulton said. “Approximately half of the planets that we know are so common have no solid surface, or a surface deep beneath the crushing weight of a thick atmosphere, and these would not be nice places to live.

“Our result sharpens up the dividing line between potentially habitable planets, and those that are inhospitable to life as we know it,” he said.

Kepler’s mission has been plagued by the failure of two of the observatory’s four reaction wheels, spinning gyro-like mechanisms that kept the telescope steadily pointed at the mission’s star field in the constellation Cygnus.

With the loss of Kepler’s second reaction wheel in 2013, the telescope could no longer meet the mission’s original pointing requirements.

Engineers found a way to harness the pressure of photons of sunlight by positioning the spacecraft to prevent solar radiation from slowly pointing the telescope away from its astronomical targets. Although solar pressure exerts very low forces on spacecraft, the constant bombardment of solar photons can alter the orientation of satellites.

Controllers can eliminate the effects of solar pressure by balancing Kepler against the stream of sunlight, similar to balancing a pencil on a finger. The telescope cannot detect the faint signatures of planets without stable pointing.

Kepler orbits the sun at roughly the same distance as Earth. NASA calls telescope’s current observing program the K2 mission.

Keeping Kepler balanced means it must be pointed in the ecliptic plane, or the plane where all the solar system’s planets orbit the sun. The mitigation against solar pressure means Kepler can only look at a narrow band of stars, shifting its 100-square-degree field-of-view every two or three months to avoid pointing its sensitive camera at bright sunlight.

The new observing method means Kepler is now best-suited to finding exoplanets located very close to their host stars.

Kepler continues searching for planets, but officials expect it to run out of fuel some time next year.

The spacecraft has about 10 percent of its hydrazine fuel supply remaining, according to Jessie Dotson, the K2 mission’s project scientist at Ames.

“We think the limiting factor is probably going to be the fuel,” Dotson said.

NASA’s next planet-hunting mission, the Transiting Exoplanet Survey Satellite, is scheduled to launch from Cape Canaveral in March 2018 on a SpaceX Falcon 9 rocket. TESS will survey the entire sky to look for exoplanet signatures around nearby, bright stars.

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