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Planet diversity
A graphic shows the diversity of planets. (Credit: NASA)

The scientists behind NASA’s Kepler mission are using statistics to put their campaign to identify new planets into overdrive: New software that automates the process has verified 1,284 candidates as genuine planets rather than celestial “impostors,” more than doubling its database of confirmed worlds.

“This is the most exoplanets that have ever been announced at one time,” Princeton University researcher Timothy Morton said today during a teleconference revealing the latest counts.

Kepler’s official tally of potentially habitable planets close to Earth’s size took a jump as well, from 12 to 21.

NASA chief scientist Ellen Stofan hailed the rapid progress. “This gives us hope that somewhere out there, around a star much like ours, we can eventually discover another Earth,” she said in a statement.

The dramatic acceleration in the planet hunt is due to a statistical method pioneered by Morton and his colleagues, and described in a paper published in the Astrophysical Journal.

Since its launch in 2009, the Kepler space telescope has gathered data about 150,000 stars in a patch of sky straddling the constellations Lyra and Cygnus. The spacecraft keeps watch for the faint dimming of starlight that occurs when a planet makes a transit across the star’s disk.

The mission’s most recent catalog, compiled in July 2015, listed 4,302 planetary candidates based on the patterns of dimming. But astronomers know that some of those detections are “impostors.” For example, a dim star or brown dwarf passing in front of a brighter star could produce a similar pattern.

In the past, astronomers had to rely on other types of follow-up data to sort out the genuine planets from the impostors. For example, they checked ground-based observations for gravitational wobbles, or looked for discrepancies in the timing of transits that had to have been caused by other planets in the system.

The newly developed method doesn’t rely on follow-up observations. Instead, it compares the precise pattern of dimming with models for planetary transits, based on previous observations of different stars. It also accounts for the probabilities for other phenomena, such as eclipsing binary stars.

The software package, which is called Vespa, automatically assigned a “reliability index” to each of the candidates. Morton and his colleagues considered a candidate to be verified as a planet if the reliability index was higher than 99 percent. In addition to the 984 candidates that were previously confirmed and the 1,284 that were newly verified, 1,327 were considered more likely than not to be planets, and 707 were considered more likely to be impostors.

Kepler candidates
Here’s the breakdown for the statistical analysis conducted on a batch of 4,302 potential planets from last July’s batch of Kepler detections. Click on the chart for a larger version. (Credit: W. Stenzel / NASA Ames / T. Morton / Princeton)

The verification rate was higher than some expected, but when the researchers checked a sampling of planets that were confirmed by other means, they found that the new method was in close agreement with the previous findings.

Kepler has now discovered 2,325 verified planets, and detections by other planet-hunting missions bring the total tally of alien worlds to more than 3,200. Before Kepler’s launch, that figure was around 320.

The current list includes more than 500 planets that could be rocky like Earth, based on their size. Twenty-one of those orbit in the “habitable zone” of their parent stars, where water could conceivably exist in liquid form. Astrobiologists see that as a key requirement for life as we know it.

Nine of those potentially habitable, roughly Earth-sized planets were added to the list today. The new statistical method should make the job of identifying life-friendly planets easier, said Natalie Batalha, a co-author of the paper and the Kepler mission scientist at NASA’s Ames Research Center.

Batalha said she was particularly intrigued by two of the brand-new prospects. One is Kepler-1638b, a world that’s about 60 percent larger than Earth and orbits a star that’s a little hotter than our sun, at a distance that would put it between Earth and Venus in our own solar system. The other is Kepler-1229b, which is only a little bigger than Earth, and orbits well within the habitable zone of its cool parent star.

Kepler small habitable zone planets
This chart shows how 21 verified planets rank, based on the surface temperature of the stars they orbit and the energy they received (with Earth serving as the standard). The green region represents the habitable zone for the star systems. Click on the chart for a larger version. (Credit: N. Batalha and W. Stenzel / NASA Ames)

Those stellar systems are more than 700 light-years from Earth, so there’s no prospect of visiting the planets anytime soon. But getting a better sense of what kinds of worlds are out there, and in what proportions, will smooth the way for future missions that are designed to study closer planetary systems.

One such mission – known as the Transiting Exoplanet Survey Satellite, or TESS – is due for launch next year, and will scan the sky looking for planets orbiting nearby stars. NASA’s James Webb Space Telescope, set for launch in 2018, should be capable of analyzing the atmospheres of alien planets. And the Wide-Field Infrared Survey Telescope, which is expected to go into service in the 2020s, is designed to take pictures of Neptune-sized planets orbiting their parent stars as closely as Jupiter orbits our sun.

In the meantime, Kepler is continuing to operate in what’s known as its “K2” mission, with capabilities that have been reduced due to failures experienced by its navigation system. Mission manager Charlie Sobeck estimated that Kepler could keep going for two more years before the spacecraft’s fuel runs out.

Why is the planet search so important? Batalha handled that question, which was passed along from a class of fifth-graders on Twitter.

“I would say to the kids, we’re going to change the way you see the universe,” she said. “When you look up in the sky, you’re not just going to see pinpoints of light and see them as stars, you’re going to see pinpoints of light and see them as planetary systems.

“It’s part of that larger strategic goal of finding evidence of life beyond Earth. To know if we’re alone or not, to know how life manifests itself in the galaxy … Being able to look up and point to a point of light and say, ‘That star has a living world orbiting it.’ I think that’s very profound, and answers questions about why we’re here and how we got here.”

Kepler planet size distribution
This chart shows the distribution of planet sizes recorded by the Kepler mission. The dearth of “sub-Jupiter-size” planets may reflect something about the physics of planet formation. Click on the chart for a larger version. (Credit: W. Stenzel / NASA Ames)

In addition to Morton and Batalha, the authors of the Astrophysical Journal paper, “False Positive Probabilities for All Kepler Objects of Interest: 1,284 Newly Validated Planets and 428 Likely False Positives,” include Stephen Bryson, Jeffrey Coughlin, Jason Rowe, Ganesh Ravichandran, Erik Petigura and Michael Haas.

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