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The discoveries, described at the American Astronomical Society meeting in Seattle, come as astronomers announce a fresh influx of 554 candidate planets, bringing the total to 4,175 possible planets – at least 1,000 of which have now been confirmed.
"Despite the fact that there are over 4,000 candidates from Kepler, there are still very few small, less-than-two-Earth-radius, habitable-zone planets," said coauthor Doug Caldwell, SETI Institute Kepler scientist at NASA's Ames Research Center. "They really make up a special population that is of interest for understanding the prevalence of life in the universe."
These two planets, described in research led by the Harvard-Smithsonian Center for Astrophysics, are named Kepler-438b and Kepler-442b. Both orbit red dwarf stars, which are much cooler and dimmer than our sun. That means that their habitable zone – the area around the star that's the right temperature for liquid water to exist without getting boiled or frozen – is much closer to the star.
That's good news for these two planets, since Kepler-438b orbits its star every 35.2 days, and Kepler-442b circles its star once every 112 days – compare that with Earth, whose orbit takes 365 days. Kepler-438b gets about 40% more light than Earth (giving it a 70% chance of being in its star's habitable zone), and Kepler-442b receives roughly two-thirds as much light as Earth (giving it a 97% chance of being in its star's habitable zone).
Before Tuesday's announcement, the two most Earth-like planets were Kepler-186f (1.1 times Earth's size, with 32% a much light) and Kepler-62f (1.4 times Earth's size, with 41% as much light).
The scientists think the two new planets are around the right size to be rocky like Earth, rather than gassy like a mini-Neptune. But they can't say for sure nor can they say whether there is water present. Aside from size and estimated temperature, there aren't a whole lot of other qualities the astronomers can home in on. Still, it's a start, the researchers said.
"We're not only closing in on an Earth twin, but we're better understanding the diversity in this special neighborhood of planets," Caldwell said.