An artist's conception of Earth-like planet orbiting evolved star that has formed a stunning 'planetary nebula'.
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Two of them have been touted as most Earth-like — Kepler-438b and Kepler-442b — which are orbiting red dwarf stars that are smaller and cooler than our Sun. Kepler-438b circles its star every 35 days while Kepler-442b completes one orbit every 112 days.
With a diameter just 12% bigger than Earth, Kepler-438b has a 70% chance of being rocky, according to scientists. Kepler-442b is about one-third larger than Earth but still has a 60% chance of being rocky.
Kepler-438b receives about 40% more light than Earth. In comparison, Venus gets twice as much solar radiation as Earth. As a result, the team calculates that it has a 70% likelihood of being in the habitable zone of its star. Kepler-442b receives about two-thirds as much light as Earth. Scientists give it a 97% chance of being in the habitable zone. This doubles the number of small planets (less than twice the diameter of Earth) believed to be in the habitable zone of their parent stars."Most of these planets have a good chance of being rocky like Earth," said lead author Guillermo Torres of the Harvard-Smithsonian Center for Astrophysics (CfA). To be in the habitable zone, an exoplanet must receive about as much sunlight as Earth. Too much sunlight, and any water would boil away as steam. Too little, and water will freeze solid. "We don't know for sure whether any of the planets in our sample are truly habitable," said second author David Kipping of the CfA. "All we can say is that they're promising candidates."
Prior to this, the two most Earth-like planets known were Kepler-186f, which is 1.1 times the size of Earth and receives 32% as much light and Kepler-62f, which is 1.4 times the size of Earth and gets 41% as much light. The newly found planets are distant enough to make additional observations challenging. Kepler-438b is located 470 light years from Earth while the more distant Kepler-442b is 1,100 light-years away.
The team studied planetary candidates first identified by Nasa's Kepler mission. All of the planets were too small to confirm by measuring their masses. Instead, the team validated them by using a computer program called BLENDER to determine that they are statistically likely to be planets. After the BLENDER analysis, the team spent another year gathering follow-up observations in the form of high-resolution spectroscopy, adaptive optics imaging and speckle interferometry to thoroughly characterize the systems. This revealed that four of the new finds are in multiple-star systems. But the companion stars are distant and don't significantly influence them.
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