'Mirage Earth' exoplanets may have burned away chances for life

December 2, 2014 by Peter Kelley
Illustration of a low-mass, M dwarf star, seen from an orbiting rocky planet. Credit: NASA / JPL

(Phys.org) —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 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 transit, 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 velocity 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 collapse 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 fully 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 initially be very hot, with surface temperatures in excess of a thousand degrees. When this happens, your oceans boil and your entire atmosphere becomes steam."

Also boding ill for the atmospheres of these worlds is the fact that M dwarf emit a lot of X-ray and ultraviolet light, which heats the upper atmosphere to thousands of degrees and causes gas to expand so quickly it leaves the planet and is lost to space, Luger said.

"So, many of the planets in the of M dwarfs could have been dried up by this process early on, severely decreasing their chance of actually being habitable."

A side effect of this process, Luger and Barnes write, is that ultraviolet radiation can split up water into its component hydrogen and atoms. The lighter hydrogen escapes the atmosphere more easily, leaving the heavier oxygen atoms behind. While some oxygen is clearly good for life, as on Earth, too much oxygen can be a negative factor for the origin of life.

"Rodrigo has shown that this prolonged runaway greenhouse phase can produce huge atmospheres full of oxygen—like, 10 times denser than that of Venus and all oxygen," said Barnes. "Searches for life often rely on oxygen as a tracer of —so the abiological production of such huge quantities of oxygen could confound our search for life on exoplanets."

Luger said 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."

Explore further: Can life emerge on planets around cooling stars?

More information: Astrobiology, arxiv.org/abs/1411.7412

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1.7 / 5 (6) Dec 02, 2014
Another useless what if paper. Why do we pay people to produce this nonsense. It is simply a computer model of a bunch of assumptions. Earth is supposed to have got its water from comets so who cares how long the star formation process lasted, we only need some handy comets from the outer cloud to hit the planet and we have water. The other tiny problem is you do not need a lot of water, you only need some.
1.8 / 5 (5) Dec 02, 2014
As a check on this model we have stars we know are only a few million years old so the computer model is most likely GIGO
5 / 5 (3) Dec 03, 2014
Why do we pay people to produce this nonsense.

I dunno. How much did we pay for this work? I see a grad student's work here, to be published in a journal to which I don't subscribe.

On early Earth and comets: assuming it's correct and perfectly understood, does that model apply to all solar systems, all types and sizes of stars? I don't recall being struck by a comet recently. Maybe it only happens at a certain time during formation? Maybe the drawn out process experienced by planets in M dwarf solar systems still allows all that water to be baked off.

Maybe the point of the paper was to provide an opportunity for people to vent caustically.
5 / 5 (4) Dec 03, 2014
@mbee: Just because you don't understand the science here, or the use of science in general, doesn't make them 'useless'. Personal incredulity is a fallacy, not a hypothesis.

This is a nice paper, since comets (one of the main hypotheses for water delivery) has gotten into trouble. When they started to measure D/H ratios, at most 40 % of water could have been comet delivered. If the recent result that half the water is presolar (from the molecular cloud originating the system), that decrease to at most 20 %.

And lately people have discovered that you shouldn't look at D/H, because they overlap with asteroids, but N isotope ratios where comets are a class of their own. Then the comet water delivery is effectively 0, the water is there from the beginning, precisely as chondrites, martian meteorites and the oldest zircons tell us.

Science nets high ROI (see NASA research on that), and while you can't predict exactly what Is will give the R, it is mutually reinforcing.

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