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2017-06-19. Earth-Size Planets Among Final Tally of NASA’s Kepler Telescope.

posted Jun 20, 2017, 11:13 PM by Alan Gould

By Dennis Overbye, The New York Times. For GSS A Changing Cosmos chapter 8. Excerpt: ...astronomers released a list on Monday of 4,034 objects they are 90 percent sure are planets orbiting other stars. The new list is the final and most reliable result of a four-year cosmic census of a tiny region of the Milky Way by NASA’s Kepler spacecraft. ... The catalog — the eighth in the endeavor — was released at a meeting of exoplanet astronomers here at the Ames Research Center that represents a last hurrah for the survey mission, which will end on Sept. 30. The space telescope itself is doing fine, and it has embarked on a new program of short-term searches called K2. Among other things, Dr. Batalha said, for the first time there is at least one planet, known as KOI 7711 (for Kepler Object of Interest), that almost matches the Earth, at only 30 percent wider and with an orbit of almost exactly one year. In all, there are 219 new planet candidates in the catalog. Ten of them, moreover, are in the habitable zones of their stars, the so-called Goldilocks realm, where the heat from their stars is neither too cold nor too hot for liquid water. ...A new study, led by Benjamin Fulton of the California Institute of Technology, of 1,305 stars and 2,025 planets that orbit them has found a curious gap in the planet population that seems to mark the boundary between rocky planets, which can be up to one and a half times the size of the Earth, (sometimes called super-Earths) and gaseous planets, so-called mini-Neptunes, which are more than about twice the size of Earth. (Neptune itself is four times the diameter and 17 times the mass of Earth.) ...All planets seem to start out with about the same amount of rock in their cores, he said. How much gas — mostly hydrogen and helium from the primordial cloud that birthed us — adheres to them makes all the difference. While the Earth, which has hardly any atmosphere at all by weight, is a pleasant place, the pressure on a world with just a little more gas would be toxic. ...Presumably, Mr. Fulton said, the planets that are rocky now, like Earth, had their gassy envelopes stripped away or evaporated by radiation from their stars. But nobody really knows how it works. Adding to the mystery is that our own solar system has no example of a mini-Neptune, and yet they are prevalent in alien planet systems....  https://www.nytimes.com/2017/06/19/science/kepler-planets-earth-like-census.html