New research reveals that planets in the habitable zone of low-mass stars may have long since lost their chance at hosting life because of intense heat during their formative years.
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 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 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.”
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.”
Reference: “Extreme Water Loss and Abiotic O2 Buildup on Planets Throughout the Habitable Zones of M Dwarfs” by Rodrigo Luger and Rory Barnes, 14 February 2015, Astrobiology.
The research was funded by NASA’s Astrobiology Institute, through the Virtual Planetary Laboratory, headquartered at the UW.
I don’t buy a lot of what is written in this article. First, as for the premise that the water gets all boiled away from close-in rocky planets during the star-system’s formation…so what? There are cosmologists who claim that the same thing happened here in the inner solar system as well, leaving water a relative rarity within the “freeze line” which is about the distance from the sun to the orbit of Jupiter. The earth, too is believed to have completely lost all of its water that it originally had during its formation, but the water we have in our abundant oceans today is only there because comets delivered it to the inner solar system, and the earth had sufficient atmospheric pressure and low enough temperature to hang onto it the second time around. As for the “class M stars (i.e., cool red dwarfs) emit a lot of X-ray and ultraviolet light…I don’t buy that, either Red dwarfs are far less energetic and cooler at the visible surface than main sequence yellow stars (K-class) such as the Sun, and as such they (1) emit considerably less UV and X-rays than the Sun (read up on “blackbody radiation” and how the spectrum relates to visible surface temperature), and (2) they are extremely stable, and in fact it is said that in the 13.8 billion year history of the known universe, no red dwarf has burned itself out yet.
Other: Another “knock,” if you will, on the possibility of habitable-zone rocky planets around red dwarfs harboring life, is that because they are close-in, they tend to be “tidally locked” to the star, that is, they present only one face to it as the moon does to the earth. Supposedly this means two things: (1) one side is a hot ever-lit desert, and (2)the rotation, once per “year” (i.e., revolution around the star) is too slow to cause a magnetic field to occur without which the planet cannot hold onto its atmosphere. I refute this also: (1) there can exist a termperate “terminator” strip, which can be quite wide as one goes up from the equator due to any polar tilt of the planet. (2) Because the planet is so close in, its “year may be as little as a few days, and that may be enough rotation to create a sufficient magnetic field, particularly in the presence of the red dwarfs lessened atmosphere-stripping UV and X-ray radiation compared to our Sun.
Finally, as alluded to above, Red dwarfs are MUCH more stable, and long-lived, than our sun, and I speculate that many of them could therefore be exceedingly good candidates for harboring carbon-based life as we know it. In fact, were I a “little green man” on a planet orbiting a small red star, I would look at stars like the Sun and wonder how in hell anything could develop in such a violent and unstable radiation-filled neighborhood. “There cannot possibly be life as we know it around stars like the Sun.”
I totally agree with Scott. In fact I’d go further. Tidally locked planets might be extremely good places for life, especially orbiting low-mass stars – there is nothing to say that the zone facing the cool star need be a desert. Imagine the zone around the Arctic without a long winter. I would imagine this to be highly habitable environment which would benefit from all-year-round UV for photosynthesis.
Additionally, the authors are not taking into account recent findings connecting H/D isotopic ratios between Earth and some comets, and particularly the asteroid 4-Vesta. It seems like the water on Earth is of a constituency that has parallels elsewhere in the solar system. This provides mechanisms for water to pitch up later in the day onto a potentially habitable world, or even for migration patterns of outer watery planets to shift into habitable zones after the irritable teenage years of the developing planetary system.
If the study of real exo-planets tells us anything it’s that the universe is a varied and unpredictable place, and that theoretical computer studies like this tend to ignore such diversity at their peril.