New research suggests planets closely orbiting low-mass stars, considered prime candidates in the search for extraterrestrial life because they seem to be Earth-like, may in fact be "mirages" that are devoid of life.
Although such planets, the most common in the universe, look like candidates for harboring life they may have lost any chance of hosting it early on because of the intense heat they experienced from their parent stars as they formed, new research suggests.
Those parent low-mass stars, known as M dwarfs, have habitable zones -- where liquid water might exist on a planet's surface -- fairly close to them, in a region that makes it easy for astronomers to spot planets as they transit, or move in front of, the stars.
However, despite many of those planets looking somewhat Earth-like, their proximity to their stars also suggest there's a good chance any water or atmosphere they have possessed while forming have long since been burned away.
That's the finding provided by computer simulations crated by University of Washington astronomers.
"All stars form in the collapse of a giant cloud of interstellar gas, which releases energy in the form of light as it shrinks," says researcher Rodrigo Luger. "But because of their lower masses, and therefore lower gravities, M dwarfs take longer to fully collapse -- on the order of many hundreds of millions of years."
Because planets can form around such stars in as little as 10 million years, their formation takes place as the stars are still excessively bright -- and hot.
"And that's not good for habitability, since these planets are going to initially be very hot, with surface temperatures in excess of a thousand degrees," Luger says. "When this happens, your oceans boil and your entire atmosphere becomes steam."
The stars also emit ultraviolet radiation that can split any water on the planet into its separate atoms of oxygen and hydrogen. The lighter hydrogen can escape the atmosphere more easily, creating an atmosphere rich in oxygen -- possibly too rich, the researchers note, since although oxygen might be necessary for life, too much of it can suppress it, especially in its earliest formation stages.
Next-generation telescopes, because they'll have enhanced abilities to determine an exoplanet's atmospheric composition, might yield false hopes about a planet's ability to harbor life in a low-mass star's habitable zone since they'll exhibit oxygen-rich atmospheres, Luger says.
"They could look a lot like Earth from afar," he says, "but if you look more closely you'll find that they're really a mirage; there's just no water there."
