In the Milky Way Galaxy, about three-fourths of the stars are M dwarfs. But there’s a deadly catch. New research indicates that M dwarfs which are prime targets for astronomers in the search for extraterrestrial life, may have lost their window of opportunity at hosting life because of intense heat during their formative years. M dwarfs, also known as red dwarfs, are the smallest type of hydrogen-burning stars and are the most common stars and the coolest main-sequence stars in the universe.
Planets Billions of Years Old
The possibility of an ancient technological civilization billions of years old is what intrigues astronomers –the heaviest red dwarfs have lifetimes of tens of billions of years, while the smallest have lifetimes of trillions of years into the future. By comparison, the universe is only 13.8 billion years old. These dim red dwarfs will be the last stars shining in the universe.
The image above is an artist’s impression of the temperate planet Ross 128 b only 11 light-years from Earth, with its red dwarf parent star in the background. This planet was found by a team using ESO’s planet-hunting HARPS instrument. The new world is now the second-closest temperate planet to be detected after Proxima b. It is also the closest planet to be discovered orbiting an inactive red dwarf star, which may increase the likelihood that this planet could potentially sustain life. Ross 128 b will be a prime target for ESO’s Extremely Large Telescope, which will be able to search for biomarkers in the planet’s atmosphere. (M. Kornmesser).
Planets close to their host stars, reports Rory Barnes at the University of Washington, “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.”
Barnes is a theorist in the University of Washington’s Virtual Planetary Laboratory and primarily interested in the formation and evolution of habitable planets, focusing on planets in and around the “habitable zones” of low-mass stars, showing how their composition, orbital oscillations, and tidal processes affect planetary habitability.
But in a paper published in the journal Astrobiology, Rodrigo Luger now a postdoctoral fellow at the Flatiron Institute in New York and co-author Barnes, found through 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,” said 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.”
Early Heat Death?
“Planets around these stars can form within 10 million years” adds Luger, “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 stars 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 habitable zones 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 oxygen 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 extraterrestrial life — so the abiological production of such huge quantities of oxygen could confound our search for life on exoplanets.”
“Because of the oxygen they build up,” says Luger, “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.”
image credit: Image shows a red dwarf star orbited by an exoplanet. © G. Bacon (STScI)/NASA/ESA)
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