围绕低质量恒星运行的行星上不太可能存在生命

Planets orbiting close to low-mass stars -- easily the most common stars in the universe -- are prime targets in the search for extraterrestrial life. But new research led by an astronomy graduate student at the University of Washington indicates some such planets may have long since lost their chance at hosting life because of intense heat during their formative years.   Low-mass stars, also called M dwarfs¹, are smaller than the Sun, and also much less luminous², so their habitable zone tends to be fairly close in. The habitable zone is that swath of space that is just right to allow liquid water on an orbiting planet's surface, thus giving life a chance.   Planets close to their host stars are easier for astronomers³ to find than their siblings⁴ farther out. Astronomers discover and measure these worlds by studying the slight reduction in light when they transit⁵, or pass in front of their host star; or by measuring the star's slight "wobble" in response to the planet's gravity, called the radial velocity⁶ method.   But in a paper to be published in the journal Astrobiology, doctoral student Rodrigo Luger and co-author Rory Barnes, a UW research assistant professor, find through computer simulations that some planets close to low-mass stars likely had their water and atmospheres burned away when they were still forming.   "All stars form in the collapse⁷ of a giant cloud of interstellar gas, which releases energy in the form of light as it shrinks," Luger said. "But because of their lower masses, and therefore lower gravities, M dwarfs take longer to fully⁸ collapse -- on the order of many hundreds of millions of years."   "Planets around these stars can form within 10 million years, so they are around when the stars are still extremely bright. And that's not good for habitability, since these planets are going to initially⁹ be very hot, with surface temperatures in excess of a thousand degrees. When this happens, your oceans boil and your entire atmosphere becomes steam."