Blowing in the stellar wind: Scientists reduce the chances of life on exoplanets in so-called habitable zones

November 30, 2017 by John Greenwald, Princeton University
Image of starlight on exoplanet, courtesy of NASA/JPL-Caltech.

Is there life beyond Earth in the cosmos? Astronomers looking for signs have found that our Milky Way galaxy teems with exoplanets, some with conditions that could be right for extraterrestrial life. Such worlds orbit stars in so-called "habitable zones," regions where planets could hold liquid water that is necessary for life as we know it.

However, the question of habitability is highly complex. Researchers led by space physicist Chuanfei Dong of the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL) and Princeton University have recently raised doubts about water on—and thus potential habitability of—frequently cited exoplanets that orbit red dwarfs, the most common stars in the Milky Way.

Impact of stellar wind

In two papers in The Astrophysical Journal Letters, the scientists develop models showing that the —the constant outpouring of charged particles that sweep out into space—could severely deplete the atmosphere of such planets over hundreds of millions of years, rendering them unable to host surface-based life as we know it.

"Traditional definition and climate models of the consider only the surface temperature," Dong said. "But the stellar wind can significantly contribute to the long-term erosion and atmospheric loss of many exoplanets, so the climate models tell only part of the story."

To broaden the picture, the first paper looks at the timescale of atmospheric retention on Proxima Centauri b (PCb), which orbits the nearest star to our solar system, some 4 light years away. The second paper questions how long oceans could survive on "water worlds"—planets thought to have seas that could be hundreds of miles deep.

Two-fold effect

The research simulates the photo-chemical impact of starlight and the electromagnetic erosion of stellar wind on the atmosphere of the exoplanets. These effects are two-fold: The photons in starlight ionize the atoms and molecules in the atmosphere into charged particles, allowing pressure and electromagnetic forces from the stellar wind to sweep them into space. This process could cause severe atmospheric losses that would prevent the water that evaporates from exoplanets from raining back onto them, leaving the surface of the planet to dry up.

On Proxima Centauri b, the model indicates that high stellar wind pressure would cause the atmosphere to escape and prevent atmosphere from lasting long enough to give rise to surface-based life as we know it. "The evolution of life takes billions of years," Dong noted. "Our results indicate that PCb and similar exoplanets are generally not capable of supporting an atmosphere over sufficiently long timescales when the stellar wind pressure is high."

"It is only if the pressure is sufficiently low," he said, "and if the has a reasonably strong magnetic shield like that of the Earth's magnetosphere, that the exoplanet can retain an atmosphere and has the potential for habitability."

Evolution of habitable zone

Complicating matters is the fact that the habitable zone circling red stars could evolve over time. So high stellar wind pressure early on could increase the rate of atmospheric escape. Thus, the atmosphere could have eroded too soon, even if the exoplanet was protected by a strong magnetic field like the magnetosphere surrounding Earth, Dong said. "In addition, such close-in planets could also be tidally locked like our moon, with one side always exposed to the star. The resultant weak global magnetic field and the constant bombardment of stellar wind would serve to intensify losses of atmosphere on the star-facing side."

Turning to water worlds, the researchers explored three different conditions for the stellar wind. These ranged from:

  • Winds that strike the Earth's magnetosphere today.
  • Ancient stellar winds flowing from young, Sun-like stars that were just a toddler-like 0.6 billion years old compared with the 4.6 billion year age of the Sun.
  • The impact on exoplanets of a massive stellar storm like the Carrington event, which knocked out telegraph service and produced auroras around the world in 1859.

The simulations illustrated that ancient stellar wind could cause the rate of atmospheric escape to be far greater than losses produced by the current solar wind that reaches the magnetosphere of Earth. Moreover, the rate of loss for Carrington-type events, which are thought to occur frequently in young Sun-like stars, was found to be greater still.

"Our analysis suggests that such space weather events may prove to be a key driver of atmospheric losses for exoplanets orbiting an active young Sun-like star," the authors write.

High probability of dried-up oceans

Given the increased activity of red stars and the close-in location of planets in habitable zones, these results indicate the high probability of dried-up surfaces on planets that orbit red that might once have held oceans that could give birth to life. The findings could also modify the famed Drake equation, which estimates the number of civilizations in the Milky Way, by lowering the estimate for the average number of planets per star that can support life.

Authors of the PCb paper note that predicting the habitability of planets located light years from Earth is of course filled with uncertainties. Future missions like the James Webb Space Telescope, which NASA will launch in 2019 to peer into the early history of the universe, will therefore "be essential for getting more information on stellar winds and exoplanet atmospheres," the authors say, "thereby paving the way for more accurate estimations of stellar- induced atmospheric losses."

Scientists spot potentially habitable worlds with regularity. Recently, a newly discovered Earth-sized planet orbiting Ross 128, a red dwarf star that is smaller and cooler than the sun located some 11 light years from Earth, was cited as a water candidate. Scientists noted that the star appears to be quiescent and well-behaved, not throwing off flares and eruptions that could undo conditions favorable to life.

Collaborating with Dong on the PCb paper were physicists from Harvard University, the Harvard-Smithsonian Center for Astrophysics, the University of California, Los Angeles, and the University of Massachusetts.

Explore further: The atmospheres of water worlds

More information: Chuanfei Dong et al. Is Proxima Centauri b Habitable? A Study of Atmospheric Loss, The Astrophysical Journal (2017). DOI: 10.3847/2041-8213/aa6438

Chuanfei Dong et al. The Dehydration of Water Worlds via Atmospheric Losses, The Astrophysical Journal (2017). DOI: 10.3847/2041-8213/aa8a60

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1 / 5 (3) Nov 30, 2017
Ahh, what a pity. More evidence for the hypothesis that alien worlds, capable of sustaining biospheres, will be damn few and far apart in space and time.

Makes one sad that the comicbook believers will be so disappointed.

Guess we're all stuck here on this, the only planet proven habitable to any recognizable lifeforms.

The wanna-be-a-Cortez or a Pizzaro brother are trapped here with the rest of us. No where for their cowardly, fat asses to run.

Best you all make the best of this reality. Cause if you continue your efforts to destroy the Earth's biosphere? Your bones will be bleaching with all the rest of the Humanity you despise.
5 / 5 (2) Dec 01, 2017
Not quite so fast, rrwillsj. The stellar sweet spot for habitability is probably in the K part of the main sequence, where flare activity is relatively less and the habitable zone is farther from the star. Those stars are not so common as red dwarf stars, but are still more common that larger stars like Sol.
not rated yet Dec 01, 2017
This article only mention one side of the story. If live on earth appeared in the sea as we think, a waterworld sounds more probable for live to appear. Also this type of stars last many times longer than the sun, giving more chances for live to start and evolve. And tidally locked world's can in fact expand the habitable zone: if one side is too hot and the other side is too cold, then there must be big zones in between with the right temperature ranges, and a lot of energy moving around to mix the live ingredients in all the possible ways, also with ha lot of stellar wind radiation creating a factory of DNA & DNA mutations just at the edge of the habitable areas, so more combinations are tested . Sure there are also many other factors against live to exist in there, what I say is that this article is not giving us the whole picture
1.5 / 5 (2) Dec 01, 2017
TB, jj, hard for me to decide which of you is more capable at conflating unprovable supposition with fictional speculations.

You are both, each in your own indomitable way. "Laying the egg before the rooster can get to the hen."

When there is any credible evidence with multiple sources of verification? Then, we can revisit the accuracy of my hypothesis versus either of yours.
1 / 5 (1) Dec 01, 2017
For those of you who have not seen the 1,476 photos, 34 films, 125 eye witness statements, multiple photographers, metal samples, sound recordings, fingerprints, footprints, and 200 corroborations of Billy Meier, I suggest you start here:
or here:
not rated yet Dec 03, 2017
rr, try again with something like an actual argument instead of straw man attacks and stupid insults like 'comic book believers'. You had a reasonable statement about the rarity of Earth-like life-bearing worlds, which you then undermined by failing to participate in a mature discussion. Your self-estimate isn't evidence.
Whydening Gyre
5 / 5 (1) Dec 03, 2017
What I find kinda funny...
Talking about life chances of developing on the surface of a water world...:-)
not rated yet Dec 04, 2017
Even a water world has a surface. It's just not a very stable one. Plenty of life floats at the surface of Earth's oceans and lakes. It also depends on your definition of 'water world' as some definitions allow for scattered islands here and there.

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