Distant planet looks like Earth? Could be a mirage

Planets orbiting close to low-mass stars—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 UW 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 dwarfs, are smaller than the sun, and also much less luminous, 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 astronomers to find than their siblings farther out. Astronomers discover and measure these worlds by studying the slight reduction in light when they 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 velocity method. But in a paper published in the journal Astrobiology, doctoral student Rodrigo Luger and co-author Rory Barnes, a UW research assistant professor, find that some planets close to low-mass stars likely had their water and atmospheres burned away when they were still forming.

Luger says the working title of their paper was “Mirage Earths.” “Because of the oxygen they build up, they could look a lot like Earth from afar—but if you look more closely you’ll find that they’re really a mirage; there’s just no water there.”