Most likely Earth-like exoplanets cataloged
The research, detailed in an article to be published in the Astrophysical Journal, outlines 216 Kepler planets located within the "habitable zone" - the area around a star in which a planet's surface could hold liquid water.
Of those, they list 20 that are the best candidates to be habitable rocky planets like Earth.
"This is the complete catalog of all of the Kepler discoveries that are in the habitable zone of their host stars," said Stephen Kane, an associate professor of physics and astronomy at San Francisco State University and lead author of the study.
"That means we can focus in on the planets in this paper and perform follow-up studies to learn more about them, including if they are indeed habitable."
The research also confirms that the distribution of Kepler planets within the habitable zone is the same as the distribution of those outside of it -- additional evidence that the universe is teeming with planets and moons where life could potentially exist.
The boundaries of the habitable zone are critical. If a planet is too close to its star, it will experience a runaway greenhouse gas effect, like Venus. But if it's too far, any water will freeze, as is seen on Mars.
Kane and his colleagues sorted the planets by whether they were in a conservative or a more optimistic interpretation of the habitable zone. Then they further sorted them by planet size: smaller, rocky planets versus larger gas giants.
The four categories are aimed at helping astronomers focus their research. Those looking for moons that could potentially hold life can study exoplanets in the gas giant categories, for example.
The 20 planets in the most restrictive category - rocky surface and a conservative habitable zone - are the most likely to be similar to Earth. Kane has already begun gathering additional data on these planets, as well as those in the other categories.
Studying and cataloging the more than 4,000 exoplanets took more than three years and involved researchers at NASA, Arizona State University, Caltech, University of Hawaii-Manoa, the University of Bordeaux, Cornell University and the Harvard-Smithsonian Center for Astrophysics.
The team included Natalie Hinkel, a former SF State post-doctoral scholar now with Arizona State, and Michelle Hill, an undergraduate Australian student studying abroad at SF State. ■