If exoplanets around M dwarfs host life, it’s probably very different from that on Earth
Our corner of the galaxy teems with alien worlds. In the 25 years since the discovery of the first planets beyond our solar system, astronomers have found more than 3,600 worlds orbiting other stars. A select few have become tantalizing targets in the search for life despite orbiting stars that are much smaller, cooler — and in many ways harsher — than the sun.
Just 39 light-years away, seven planets, all roughly the size of Earth, whirl around a dim red star dubbed TRAPPIST-1, astronomers announced in February (SN: 3/18/17, p. 6). Three are potentially habitable. In April, a team reported the discovery of another world snuggled up to a red sun, LHS 1140b, described by researchers at the European Southern Observatory as perhaps the best candidate in the search for signs of life. And last August, astronomers revealed that not only does a small planet named Proxima b orbit the star closest to the sun, a red neighbor, but it too could support life (SN: 9/17/16, p. 6).
All of these worlds orbit faint ruddy stars known as M dwarfs, the most common type of star in the galaxy. Of the roughly 200 planets that have been spied around M dwarfs, dozens are in the coveted habitable zone. It’s this region around a star where a planet could have temperatures that support liquid water, widely considered an essential ingredient for life.
But M dwarfs are quite different from the sun, and their planets might be rough places to eke out a living — “the low-rent district of the galaxy,” says Victoria Meadows, an astrophysicist at the University of Washington in Seattle. Harsh flares, bright beginnings and a tight gravitational grip on the innermost planets could be disastrous for any liquid water that’s available.
Many more planets are expected to be found in habitable zones around M dwarfs. So researchers want to get a better handle on what these planets are up against. New observations and computer simulations reveal that, while it’s difficult for M dwarf planets to hold on to substantial amounts of water, not all hope is lost.
“There are always ways around these things,” says astrophysicist Rory Barnes, also at the University of Washington. M dwarfs and their planet families are plentiful, and there are many conditions in which these worlds can grow and evolve. What’s becoming clear is that any habitable locales around these stars will probably be quite different from Earth.
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Of the 52 potentially habitable exoplanets identified so far, 51 have a known distance from Earth, and 13 have the greatest chance of being life-friendly. Among those, most that are closest to Earth orbit M dwarfs (red). The rest circle K dwarfs (blue) and sunlike stars (orange).
Explore the interactive graphic below to learn more about individual exoplanets.
An optimistic list
M dwarfs make up about 70 percent of the several hundred billion stars in the galaxy. They are cool and tiny — at least for a star. Proxima Centauri, an M dwarf and the closest star to the sun, is roughly 2,800° Celsius. That’s about 2,700 degrees cooler than the sun, giving Proxima a soft glow. Many M dwarfs aren’t much bigger than Jupiter, which is only about one-tenth as wide as the sun. All this means that none are visible from Earth to the unaided eye. Proxima Centauri is about one one-hundredth as bright as the faintest stars our eyes can see without a telescope.
Because M dwarfs are so lightweight, they don’t burn through their nuclear fuel as fast as their heavier cousins. So they live for an extraordinarily long time. A star that weighs about one-tenth as much as the sun, for example, has a projected lifetime of roughly 12 trillion years — more than 800 times the current age of the universe. That’s plenty of time for life to arise and evolve on any planets orbiting these stars.
M dwarfs appear to be prolific planet producers. The dim stars harbor 3.5 times as many small planets, defined as planets between one and 2.8 times as wide as Earth, as do stars more like our sun. Compared with slightly warmer stars called K dwarfs and with suns like ours, M dwarfs probably have the lead on habitable worlds.
“Habitable” doesn’t mean inhabited, nor does it necessarily mean a pleasant place to live. For most exoplanets, astronomers cannot directly measure anything about the climate or atmosphere. All they know is that the planet receives the right amount of solar energy to conceivably have liquid water on its surface. Though aliens might have very different needs than Earth-based critters, and may not even require water, scientists lean on a go-with-what-works approach in the search for life.
By one conservative estimate, 13 known exoplanets are “habitable,” and 11 are around M dwarfs. That estimate comes from the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo. A precise number is elusive, however, because there are different ways to estimate the boundaries of the habitable zone. It’s also not clear how massive a planet can be and still have a solid surface for life to take hold. Broadening their criteria to include larger planets and a wider habitable zone, the Arecibo researchers identified an additional 39 habitable exoplanets (20 orbiting M dwarfs and six around sunlike stars). That puts the more optimistic list of potential habitables at 52.
M dwarfs feature prominently partly because it’s easier to find habitable planets around these stars. For a planet to stay warm around such a cool star, it has to huddle up close, offering the best chance to get noticed. A close-in planet will have a stronger gravitational tug on its star, making it easier to detect the star’s wobble. And because these planets loop around their star faster than remote worlds, dips in the star’s brightness as well as changes in the star’s speed will be seen more often.
M dwarf planets will probably continue to dominate lists of habitable worlds. But when it comes to a climate that’s suitable for liquid water, M dwarf planets have plenty of hurdles, says James Kasting, a geophysicist at Penn State.
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In our solar system, the habitable zone (green) extends more than 160 million kilometers, from just beyond Venus (at about 110 million kilometers from the sun) to beyond Mars. Around the relatively cool star TRAPPIST-1, a mere 11 percent as wide as the sun, the habitable zone is much closer and narrower. Three of its planets are in the habitable zone. (Planets are not to scale.)
Sources: NASA Space Science Data Coordinated Archive; Planetary Habitability Lab/Univ. of Puerto Rico at Arecibo; M. Gillon et al/Nature 2017
One of the biggest challenges — uncovered only recently — is how to survive the star’s early years. M dwarfs are faint, but they don’t start off that way. When they first begin to shine, M dwarfs can be roughly as bright as our sun, up to about 100 times as bright as they’ll eventually become, Barnes says. It can take several hundred million years for an M dwarf to settle down to the low-level luminosity it will maintain for the rest of its life. Stars like the sun also start brighter than they end up, but they fade much faster, needing only about one-tenth as much time as M dwarfs.
A small world that today sits in the habitable zone of an M dwarf spent hundreds of millions of its early years blasted with more intense light. Using computer simulations, Barnes and Washington graduate student Rodrigo Luger showed that prolonged exposure to bright starlight could strip a planet’s atmosphere of its water, leaving behind a barren world. The amount of water lost depends on the planet’s mass, proximity to its star and initial inventory of water, the team reported in 2015 in Astrobiology. A “habitable” M dwarf world such as Gliese 667Cc, roughly 3.7 times as massive as Earth and about one-twelfth the distance from its star as Earth is from the sun, could have lost as much as 10 times the amount of water as is currently found in Earth’s oceans.
“Once you’ve lost all the water, you’re sunk,” Barnes says. A once-promising planet could “potentially turn into Venus, and Venus is not a good place to live,” he adds. While Venus might have once had oceans and a more temperate climate (SN Online: 8/26/16), today it is home to a crushing carbon dioxide atmosphere and surface temperatures exceeding 460° C — hot enough to melt lead.
Ultraviolet radiation could strip not only the water vapor from a habitable M dwarf planet, but also the oxygen and nitrogen in just tens of millions of years, astrophysicist Vladimir Airapetian of NASA’s Goddard Space Flight Center in Greenbelt, Md., and colleagues suggested in the February 10 Astrophysical Journal Letters. And Proxima b could have lost significant amounts of water during its formative years, astronomer Ignasi Ribas of the Institute of Space Sciences in Barcelona and colleagues reported in 2016 in Astronomy & Astrophysics.
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Surface temperatures on Proxima b, a small planet orbiting the dim red star nearest to Earth, depend on the planet’s spin and the makeup of its atmosphere. In six of many possible scenarios (shown below), solid lines mark areas where liquid water could endure year-round. Orange dots mark zones with seasonal water potential.
But there’s room for optimism. Ribas and collaborators came up with equally likely scenarios in which Proxima b loses less water than the volume of Earth’s oceans. Astrophysicist Emeline Bolmont of the Saclay Nuclear Research Centre in France and colleagues took a similar look at the three innermost planets of TRAPPIST-1 (before the other four planets were discovered). While the two closest planets could have lost 15 times as much water as is in all of Earth’s oceans, the third planet — still closer to the star than the habitable zone — might have lost less than one ocean, they reported in the January Monthly Notices of the Royal Astronomical Society.
There may also be ways to sequester and protect some of a planet’s water — probably delivered by icy asteroids (SN: 5/16/15, p. 18) — during those first billion years. As proof that a planet can take a beating and hold on to its water, one need look no farther than Earth.
“We had a moon-forming impact that pretty much destroyed everything, and we still retained water and an atmosphere,” Meadows says. While researchers debate the origin of the moon, the prevailing story is that Earth had a run-in with a protoplanet the size of Mars (SN: 4/15/17, p. 18), which probably blew away most of Earth’s atmosphere. Water and other gases trapped deep in the planet’s mantle could have trickled up and built a second atmosphere. As long as the mantle is not desiccated, there’s an option to vent water and carbon dioxide over the lifetime of a planet, Meadows says. “We call them zombie planets.”
Starting with a world that’s akin to a miniature Neptune — one that’s between one and 10 times as massive as Earth with a thick atmosphere of up to 50 percent hydrogen and helium — might be another way to make a habitable M dwarf planet, Luger and colleagues suggested in 2015 in Astrobiology. Ultraviolet radiation from the star, coupled with some movement of the planet toward its sun, could evaporate much of that primordial atmosphere and leave behind a rocky, possibly water-rich world.
Locked in space
If a planet manages to get through its first billion years with some water remaining, it faces another potential problem: gravity. Habitable M dwarf planets huddle much closer to their stars than any of the planets in our solar system do. Mercury orbits the sun once every 88 days; all of the potentially habitable worlds at TRAPPIST-1 whip around their star in about six to 12 days.
At such close proximity, the star’s gravity can force one side of the planet to always face the star, resigning the other half to eternal darkness. The climate on such a world would be quite different from Earth’s. Our planet’s relatively brisk spin helps circulate the atmosphere and spread out heat from the sun. The dayside of a planet close to an M dwarf, however, might become so hot that water escapes to space; on the frigid nightside, the atmosphere could freeze to the surface.
But locked rotation isn’t the deal-breaker it was once thought to be. “There are plenty of ways around it,” says astronomer Jacob Haqq-Misra of the Blue Marble Space Institute of Science in Seattle. A little bit of CO2 in the atmosphere, for example, can help store heat and distribute it around the planet.
A thick, permanent cloud deck might also form on the side facing the star, astrophysicist Jun Yang of Peking University in Beijing and colleagues reported in 2014 in Astrophysical Journal Letters. Rising parcels of hot air could trigger cloud formation, which would reflect lots of sunlight and prevent the dayside from becoming too hot. The researchers also found that this might widen the habitable zone around a star. With a protective cloud shield, a planet could be much closer to the star and remain temperate.
As long as a locked planet holds on to a relatively tiny amount of water (as little as 0.001 percent of that found in Earth’s oceans in some cases), there’s still a chance for liquid water to endure somewhere on the surface, a recent simulation suggests. Martin Turbet, a graduate student at the Laboratoire Météorologie Dynamique in Paris, and colleagues looked at possible climates for Proxima b. They reported last year in Astronomy & Astrophysics that for a range of conditions — different rotation rates, abundances of atmospheric gases and initial amounts of water — the planet could hold on to some surface water, enough to maintain at least a few habitable niches.
“Since it’s such a different configuration from what we have on Earth, there’s a lot of climate science still to be explored,” Haqq-Misra says. Ruling out M dwarf planets as totally uninhabitable is premature. “There are lots of ways to keep liquid water on the surface.”
Look to the skies
There are other potential pitfalls too, though. Strong solar flares, which some M dwarfs are known for, could rip away a planetary atmosphere. And computer simulations disagree on how easy it is to get water to a rocky planet around an M dwarf in the first place.
To test these ideas, astronomers need data. The best source now comes from planets that periodically pass in front of, or transit, their star (SN: 4/30/16, p. 32). During a transit, some starlight passes through the planet’s atmosphere, and molecules in the atmosphere absorb specific frequencies of that light. Careful analysis of what frequencies are absorbed, and by how much, can directly reveal the presence of water vapor and other compounds, and can divulge climate parameters, such as temperature and pressure, that determine if liquid water is sustainable.
When it comes to habitable M dwarf planets, “we don’t have any data yet,” Barnes says. “Every data point is going to be huge and open up a new window into these worlds.” One recent detection is telling: A planet orbiting a roughly 5-billion-year-old M dwarf, named Gliese 1132, appears to have an atmosphere that might contain either water vapor or methane, John Southworth, an astrophysicist at Keele University in England, and colleagues reported in the April Astronomical Journal. The planet, which is about 1.6 times as massive as Earth, is too close to its star to be considered habitable. But it shows that 5 billion years after its formation, a planet snuggled up to an M dwarf can retain an atmosphere.
Atmospheres have been reported on only three other small worlds: one around an M dwarf, one around a K dwarf and one orbiting a star similar to the sun. The first two, about 2.5 times as wide as Earth, give few clues about their atmospheres. Only the planet orbiting the sunlike star gave hints of methane in its atmosphere.
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These measurements are exceedingly difficult for existing observatories. But NASA’s James Webb Space Telescope, scheduled to launch in October 2018, will investigate hundreds of transiting exoplanets, many of them around M dwarfs. The TRAPPIST-1 system has jumped to the top of the list. It has three planets in the habitable zone, three too close to the M dwarf and one too far out (see Page 16). All of them are transiting, which makes TRAPPIST-1 an ideal test for all sorts of ideas about how M dwarf planets and their climates evolve, Meadows says.
With planets orbiting M dwarfs quickly becoming the darlings in the search for life beyond our solar system, a new generation of observatories are poised to discover hundreds of worlds around these stars. Climate simulations hint at diverse alien environments that could be harsh but also potentially habitable. “I just think it’s really exciting that finally we’ll be able to look at some of these planets,” Kasting says. “There have been lots of surprises in the exoplanet business, so I’m prepared to be surprised again.”
This story appears in the June 24, 2017, issue of Science News with the headline, "The opportunity zone: Exoplanets found in a narrow band around M dwarf stars could host a very different kind of life."
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