Earth-size Planets Are Common, Kepler Retrospective Finds

NASA’s Kepler telescope was retired a few years ago, but ongoing analyses of its data, both by professional astronomers and citizen scientists, are still producing new results.

The mission's primary goal was to try and estimate the prevalence of Earth-size planets on Earth-like orbits around Sun-like stars. But understanding the occurrence of such planets has proven difficult, even though Kepler has found more than 2,600 exoplanets (and counting). Now, an international collaboration led by Steve Bryson, a researcher at NASA Ames, has announced a refined estimate.

The team, including NASA scientists, SETI researchers, academics, former-Keplerites, and other planet hunters, performed a statistical analysis that combined Kepler’s planet catalog and stellar data from the European Space Agency’s Gaia observatory. They found that about half of the Sun-like stars in our galaxy could have a rocky planet in their habitable zones.

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Making up for Missing Data

Team member Michelle Kunimoto (MIT) says this estimate is more reliable than previous ones: “Most previous estimates did not take into account that planets are more or less common around stars with different temperatures,” she says.

The Kepler mission used the transit method, detecting planets by the slight dimming of a host star when its planet passes in front of it. This technique revealed planets by the thousands, but it’s easier to find gas giants orbiting close to their host star rather than Earth-size planets on farther-out orbits. Kepler is sure to have missed lots of rocky worlds.

To account for this effect, Bryson’s team worked with Kepler’s planet-detection “pipeline,” which has become well-versed in both authenticating and debunking planet candidates, as well as finding ones that were previously missed. Experience has taught them which ‘dips’ in the light are caused by interference from objects like eclipsing binaries, stellar companions, or planets from other systems. Isolated blips also sometimes turn out to be planets with longer orbits.

“We figured a way to measure how many planets we were missing. It’s a huge number,” Bryson explains. “And then we had to figure how many were typically false positives. That's also a huge number.” Taking all this information, the team estimated the number of rocky planets with ½ to 1½ times Earth’s mass in the habitable zone around Sun-like stars. The team analyzed all their stellar and planetary data via two different techniques and compared the results. They matched.

 “It was a relief when the answer was reasonable, not a million planets, or zero,” Bryson says.

At Least 300 Million

The study, soon to be published in The Astronomical Journal, predicts that there are at least 300 million habitable-zone rocky worlds in the Milky Way. A handful of these are within a few light-years of Earth. This result assumes that the section of the sky Kepler monitored for four years is representative of the whole galaxy.

It is important to note that this new estimate does not tell us where to find an exoEarth, or what fraction of those above-mentioned 300 million worlds actually has life. But the result does suggest, based on an analysis of a large amount of astronomical data and with a high degree of confidence, that potentially habitable Earth-size planets around Sun-like stars are common.

“We have absolutely no information about whether planets in the habitable zone are in fact inhabited,” Bryson said. “That will be the purpose of future missions. And this prediction helps the endeavor by making it hopeful; because it’s much more likely that a direct imaging telescope will actually be able to succeed in imaging a rocky planet in the habitable zone.”

Bryson sees this project as a kind of ode to the Kepler mission. Indeed, most of the original team members, including the mission’s principal investigator Bill Borucki (NASA), were involved in the study.

“We're not claiming this is the final answer,” Bryson says. “But it is the best answer the Kepler team knows how to do.”