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Life Beyond Earth? Look to Small Stars

Kerry Hensley

Five billion years ago, our Sun condensed out of a cloud of gas and dust. Shortly after, planets coalesced out of the flattened nebula, and the strong winds of the young Sun blew away the remaining gas and dust. Moons were ejected, moons were captured. Cosmic debris careened through the solar system, causing chaos. After a time, everything settled. The planets cooled. Life blossomed.

Five billion years from now, our Sun will exhaust its supply of hydrogen. Its outer layers will swell and engulf Mercury, Venus, and Earth. The Sun’s core will be exposed and its outer layers will drift away, driven to fluoresce by X-rays emitted by the hot core but later invisible to our eyes. The core will cool until it too fades from view, living out the rest of its days as a black dwarf.

Our Sun is a middle-aged star in the suburbs of the Milky Way, a rather average spiral galaxy. While humans have come to see the Sun as a typical star, that’s really not quite true. Sunlike stars are much more common than the massive stars that end their lives as supernovae, leaving behind rapidly spinning neutron stars or black holes, but they still only make up a small fraction of the stars in the universe. The most common stars in the universe are all around us, but hidden, too dim to be seen with the naked eye.

A false-color near-infrared image of the central region of the Orion Nebula, a star-forming region over a thousand light-years away. The bright stars in the center of the image are extremely massive—about 15-30 times the mass of the Sun. These few massive stars are far outnumbered by the smaller, cooler stars in this star-forming region. Credit: ESO

Invisible Stellar Neighbors

About three quarters of the stars in the universe belong to a class of small, cool stars called M dwarfs. The name is historical, harkening back to a system of classifying stars based on the strength of a spectral line of hydrogen. (The modern classification system scrambles the historical system and instead labels stars according to their temperature, from O, the hottest and most massive stars, through B, A, F, G, and K, to M, the coolest and least massive stars. It’s common to tack L, T, and Y on to the end of the sequence, but it’s up for debate as to whether these ultra-cool objects should be considered stars or not.)

The fact that there are so many M dwarfs compared to other types of stars is a consequence of something called the initial mass function. The initial mass function describes what will happen when a gas cloud fragments and forms stars. I think of it kind of like breaking a glass; when it shatters, there are usually a few large pieces and a bunch of smaller ones (waiting to be found later by unsuspecting bare feet). Star clusters are much the same; a collapsing gas cloud gives birth to a few massive stars and churns out huge numbers of smaller stars.

M dwarfs are tiny, relatively speaking, containing about eight percent to fifty percent of the Sun’s mass in about 0.1 percent to twelve percent the Sun’s volume. For most stars, more mass translates to more energy output, so these tiny stars emit only a few hundredths or thousandths as much energy per second as the Sun. While most of the light from stars in the universe comes from the relatively rare very massive stars, most of the mass comes from these very small, but extremely numerous stars.

A comparison of the sizes of the Sun, an M dwarf, a brown dwarf, and Jupiter. M dwarfs are smaller, cooler, and redder than Sunlike stars. Brown dwarfs are, in a manner of speaking, in between stars and planets; they are too small and cool to fuse hydrogen in their cores, but they are able to fuse deuterium (otherwise known as heavy hydrogen; deuterium has a nucleus made up of a proton and a neutron whereas a hydrogen nucleus is just a proton). Unlike stars, brown dwarfs are thought to have bands of clouds and have aurorae at their poles. Credit: NASA/Goddard Space Flight Center

The Sun’s nearest neighbor is an M dwarf: Proxima Centauri. It’s just over four and a quarter light-years away, but in order for it to be just barely visible to the naked eye it would need to be ten times closer. In order for Proxima to be as bright as Sirius, the brightest star in the night sky, it would have to be more than three hundred times closer—less than a light-year away. This would place the star practically in our backyard, between the Kuiper belt and the Oort cloud.

Because M dwarfs are too faint to be seen by stargazers, it makes sense that a lot of early SF took place around stars that were visible in the night sky. The allure is understandable; peering toward Aldebaran and imagining a swarm of Taurans preparing to invade the Moon is far more captivating than conjuring up an image of an impossible-to-see star with a hard-to-remember name. Despite being hundreds of light-years away, Aldebaran looks like it’s just an arm’s reach away. Aldebaran sounds like an exotic place, with exotic stories following suit. An M dwarf named GJ1214, on the other hand, is indistinguishable from the thousands of other stars that fill out the catalog from which it gets its name.

Slowly, though, nearby M dwarfs became the setting for SF adventures. Some of the first novels involving interstellar travel took humanity to the nearest stars—M dwarfs like Proxima Centauri and Barnard’s Star. Many novels simply transported Earth (in all but name only) into another star system, which, to an astronomer, always seemed like a missed opportunity.

Planets are intimately linked to their parent stars since they form from the same cloud of gas and dust. Earth certainly wouldn’t be the same around a different star! On short timescales, the Sun’s eleven-year cycle of activity, which manifests as rising and falling rates of solar storms, directly affects life on Earth. On long timescales, the lifetime of the Sun sets an absolute upper limit on how long humanity (or whatever might take our place) can remain on Earth. We’ve got five billion years to leave the solar system—better start learning what else is out there. What kind of planets might we find around our neighboring stars? Should we go looking for Earth twins around M dwarfs?

The Good

There are several reasons why M dwarfs should be the first stars we look to in our search for Earthlike planets. We’ve already touched upon a couple: there are a lot of M dwarfs out there in the universe, and many of them are close to us—several thousand of them within a hundred light-years or so. What else makes them good candidates for hosting discoverable Earthlike planets (and possibly Earthlike life)?

Short Years

M dwarfs are much cooler than the Sun, so in order for their planets to have the same average temperature as Earth, they need to be much closer to their star than Earth is to the Sun. This has some downsides, which we’ll explore later, but for now this simply means that a year on these planets will be much shorter than a year on Earth. For example, a year on Proxima Centauri b is about 11 Earth days. (Exoplanets are named after their parent star, with b assigned to either the first planet discovered or the planet nearest the star, and additional letters in alphabetical order indicating planets discovered later or farther away from the star.) This means that a stargazer on this planet would see a markedly different sky each night!

The short length of year is good news for astronomers searching for exoplanets; in order to confirm the presence of an exoplanet, a good standard is that you have to observe the planet crossing in front of the star at least three times. (Assuming you’re searching for exoplanets using the transit method, that is. Check out William Ledbetter’s "The Exoplanet Hunters" to learn more about different methods for discovering exoplanets.) This lowers the chance that what you think is a planet is actually just a starspot masquerading as a planet. If you’re looking for a planet with a period of an Earth year, that means waiting at least three years to confirm that the planet exists—and waiting even longer to learn about its atmosphere and whether it might be habitable.

Venus passing in between Earth and the Sun as seen by Helioseismic and Magnetic Imager onboard Solar Dynamics Observatory. While exoplanet hunters are looking for the dimming of starlight as planets cross their disks, cool, dark starspots can mimic this behavior as the star rotates. Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio

Long Lives

One of the most intriguing things about these stars is their extraordinary longevity; every M dwarf that has ever formed is still roaming the universe today, and many will outlast the Sun by billions of years. The least massive M dwarfs have expected lifetimes in the trillions of years. When I say lifetime, I’m referring to the main sequence lifetime—the amount of time a star spends converting hydrogen into helium in its core. When most stars run out of hydrogen for nuclear fusion, they evolve off the main sequence and become giants or supergiants, before eventually ending their days as white dwarfs, neutron stars, or black holes. However, this may not be the case for all stars; because M dwarfs evolve so slowly, the universe is too young for us to know what happens to them after they leave the main sequence!

During a star’s main sequence lifetime, its power output tends not to change too much, with the exception of isolated events, like stellar flares. M dwarfs provide steady energy to their planets hundreds to thousands of times as long as the Sun will. This has big implications for life; unlike extremely hot and massive stars which are born, burn through their hydrogen, and explode as supernovae within a few million years—likely too quickly for life to develop—M dwarfs persist and are stable long enough for life to develop and evolve. M-dwarf lifetimes are so long that there’s time for life to blossom and fade many times over, with civilizations arising from the millennia-old ashes of the last.

We’re About to Know a Lot More

On April 18, 2018, NASA’s long-awaited Transiting Exoplanet Survey Satellite (TESS) successfully launched. TESS is predicted to discover about a thousand new exoplanets, seventy-five percent of which will orbit M dwarfs.i Because of the way the TESS mission is designed (most of the stars observed will be looked at continuously for twenty-seven days, although some will be observed for as many as 351 days), most of the planets discovered in their stars’ habitable zones will orbit M dwarfs. This is really exciting—while the exoplanet-hunting Kepler space telescope helped us get a sense of the broad range of planets in one region of the galaxy, TESS will help us understand what planets in our neighborhood are like.

The Bad

Now, as they say, if it’s too good to be true, it probably is. Nearby stars with habitable planets just waiting to be discovered? Well, maybe. There are two big reasons that M dwarfs might not be the habitable-planet-hosting stars of our dreams.

Tidal Locking

Earthlike planets around M dwarfs will have short orbital periods. This made the list of best things about planets around M dwarfs, too, because short orbital periods mean more transits and greater likelihood of discovery. However, short orbital periods mean that these planets are very close to their parent stars, and they are likely to be tidally locked—one side of the planet permanently facing the star, with the other side permanently facing away.

Without considering any other factors, this means that the dayside will be very hot while the nightside will be very cold—cold enough that without some way for warmth to be transferred from the dayside to the nightside, the atmosphere will freeze out at night. As daunting as this seems, it doesn’t spell certain doom for Earthlike planets around M dwarfs. Since M dwarfs are otherwise good candidates for potentially habitable (and discoverable) planets, a lot of research has explored the details of these planetary systems and tried to find ways for M dwarf exoplanets to be friendlier to life.

Astronomers have found that if a planet around an M dwarf can cling to even a little bit of its atmosphere, heat can flow from the dayside to the nightside, preventing the dayside from becoming unbearably hot and the nightside atmosphere from freezing out.ii Other models have shown that if there’s a substantial amount of water in the atmosphere, persistent clouds will form at the substellar point (where it’s always high noon with the star directly overhead), deflecting some starlight and further lowering the temperature there.iii

Another weird facet of tidally-locked planets is the dramatic change in lighting from the substellar point to the terminator (where it’s permanently dawn or dusk). Think Ursa Minor Beta (from Douglas Adams’s The Restaurant at the End of the Universe), where it’s always perpetual afternoon just before the bars close—tidal locking is a way to achieve that time-standing-still feeling. (No word on discoveries of exoplanet bars yet.) If you want to take in a romantic starset or starrise with a loved one, you’d have to travel to the terminator. To get a glimpse of the stars, you’d have to cross over into the perpetual night.

In Stephen Baxter’s Proxima, this fact is exploited to great effect. (Mild setting-related spoilers in the rest of this paragraph.) At the substellar point, storms rage continuously, turning the hottest region of the planet into a dense jungle. Away from the substellar point, as Proxima inches toward the horizon, the landscape transitions to deserts patterned with shifting oases. Nearer to the terminator, the desert gives way to forests. The clash of warm air from the dayside and cool air from the nightside creates a permanent storm system in a ring around the planet. Life on Proxima c is entwined so closely to its star, which looms, far larger than the Sun appears to us, in the sky. Seasons are caused not by the tilt of the planet relative to its orbit, as on Earth, but instead come from Proxima’s dramatic stellar cycle, to which the planet’s unwilling inhabitants scramble to adapt.

While the fictional Proxima c is habitable enough, it’s not yet clear whether or not the planet that actually orbits Proxima Centauri (Proxima b, discovered in 2016) could be. We don’t yet know whether or not the planet is tidally locked, although it’s certainly possible. Whether or not Proxima b is tidally locked, it will have to contend with another serious problem.

Nasty Space Weather

Space isn’t a perfect vacuum, especially not near a star. Our solar system is filled with a mix of plasma and magnetic fields called the solar wind. The solar wind flows out from the Sun and forms a bubble that shields the planets as the Sun travels through interstellar space, which is similarly not totally empty; the interstellar medium is mostly hydrogen but can also include molecules like buckyballs, too.

Space weather happens when the solar wind and other solar outputs interact with Earth and the other planets, sometimes with disastrous results. The Sun is a writhing mass of plasma, laced with tangled magnetic fields. When the Sun’s magnetic field becomes too tightly wound, it can snap into a more comfortable configuration, unleashing solar storms and coronal mass ejections—torrents of hot plasma and magnetic fields that lash against Earth’s protective magnetic field.

Space weather can be both beautiful and destructive; solar storms cause the Northern and Southern Lights, and the more powerful the storm, the more widespread the auroras become. A classic example is the 1859 Carrington event—a solar storm so violent that the aurora was visible as far south as the Caribbean. The flipside of this is that the Carrington event threw telegraph systems (the height of technology at the time!) into disarray. If the Carrington event were to happen today, it could cause serious damage—to Earth-orbiting spacecraft, power grids, and oil pipelines—to the tune of a trillion dollars in damages in the U.S. alone.iv (We’ve had near misses since 1859, but eventually one of these storms is going to smack us right in the magnetosphere. I hope not to be around when one does.)

As devastating as solar storms can be to life on Earth, the Sun is a relatively calm star. Unfortunately, M dwarfs are exactly the opposite. These stars, though tiny, unleash powerful stellar storms far more often than the Sun does. Remember Proxima Centauri, our stellar neighbor that would need to move ten times closer just for us to be able to catch a glimpse of it? Two years ago, Proxima let loose a stellar flare so powerful that the star was briefly visible to the naked eye, and these immense flares are apparently common. Bad news for Proxima b…

If planets around M dwarfs don't have strong magnetic fields, their atmospheres will be stripped away by the stellar wind well before life has a chance to develop. Strong stellar flares and coronal mass ejections will only speed up the process; while all planets experience atmospheric loss, including Earth, atmospheric stripping by the solar wind seems to be enhanced for planets without magnetic fields. NASA’s Mars Atmosphere and Volatile EvolutioN (MAVEN) mission has observed the slow removal of Mars’s atmosphere by the solar wind.

An artist's rendition of ions being torn from Mars's atmosphere by the solar wind. NASA’s MAVEN spacecraft measures the rate at which Mars loses hydrogen and oxygen to space. Over millions of years, the interaction of the Sun with Mars’s atmosphere has caused our neighboring planet to lose most of its atmosphere (sixty-six percent, as estimated by MAVEN Principal Investigator Bruce Jakosky and collaborators.v

The space weather issue is addressed loosely in Proxima as well, with speculation about Proxima c’s massive iron core. This could imply that the planet has a stronger magnetic field than Earth’s, although the absolute magnetic field strength is never touched upon. One studyvi found that an Earthlike exoplanet orbiting an M dwarf would need a magnetic field 10 to 10,000 times stronger than Earth’s, depending on how violent the star’s stellar storms are. It’s not yet clear how a small, tidally-locked (and therefore slowly rotating) planet could generate such a strong magnetic field. Perhaps a showstopper, perhaps not.

If all of this seems dire, cling to one simple fact: The universe is a very big place, and humans have explored only a tiny corner of it. Sure, physics and chemistry are the same no matter where you go, but tweaking the temperature, pressure, and composition of a planet might open up some interesting possibilities. Finding Earthlike life on an Earthlike planet orbiting a Sunlike star would be exciting, a long-sought validation that we aren’t alone in the universe. But finding truly strange life around a far-off star… That would change everything. It could change our whole definition of life, and we might not even recognize it when we find it. Where should we look for life as we don’t know it?

The reflection of sunlight off liquid in Titan’s north polar lakes. Credit: NASA/JPL/University of Arizona/DLR

Life on Titan

Saturn’s largest moon, Titan, may be the coolest object in the solar system. It has been slowly unveiled, first by Pioneer 11 and later by Voyagers 1 and 2 and Cassini. Cassini returned one of the most haunting images I’ve ever seen: the narrow crescent of Titan, tinted orange with photochemical haze, almost obscuring the glint of sunlight off an alien sea.

In some ways, Titan is Earthlike: It’s rocky, with a thick atmosphere and persistent liquid on its surface. It even has a complex weather system and distinct geographical regions. However, the similarities end there. Titan dips in and out of the protection of Saturn’s magnetic field, spending part of its orbit bracing against the solar wind and part of it shielded against it. It’s tidally locked to Saturn, so only one side of the moon faces the giant planet. (To a human standing on Titan’s surface, Saturn would be only dimly visible because of absorption of optical light by methane in its atmosphere. Titanic lifeforms would need infrared vision to see Saturn through the haze.) It’s bitterly cold, with a surface temperature of -290°F (94 K). The liquid in those seas is a blend of methane and ethane rather than water.

As a recent graduate working at the Jet Propulsion Lab, I was tasked with finding a way to figure out the proportion of liquid methane and ethane in the lakes of Titan. This turned out to be pretty tricky; the methane in Titan’s atmosphere absorbs most of the visible and infrared light reflected off the surface, except in a few narrow regions.

True-color visible light (left) and false-color infrared (right) images of Titan. The moon’s atmosphere is essentially opaque to optical wavelengths, but the surface can be seen through several windows in the infrared. Credit: NASA/JPL-Caltech/Space Science Institute (left) and NASA/JPL-Caltech/University of Arizona/University of Idaho (right).

The best part of my job was sifting through the images taken by Cassini (may it rest in peace!). I spent hours making maps of Titan’s north pole and found every excuse to pore over the RADAR maps of the lakes, mentally tracing the alien shorelines. Since RADAR tells us about the smoothness and elevation of a surface but nothing about the color, the maps were colored in a pretty Earth-centric way: golden-brown plains punctuated by deep blue-black seas.

As tantalizingly Earthlike as Titan may appear, there is no evidence that it currently hosts life. Life as we know it is almost certainly out of the question; Earth life needs liquid water as a solvent, and at Titan’s temperature, water exists as ice that’s as hard as rock. This opens up another interesting possibility: silicon-based life.

We shouldn’t expect to find silicon-based life on Earth. As my college organic chemistry professor said, all life on Earth depends on Schnapps. Well, okay, he didn’t say that, but that was what the college freshmen in the lecture hall heard. What he really said is that life as we know it, from tail-wagging puppies to flagella-wiggling bacteria to the as-yet-undiscovered but inevitably terrifying creatures of the ocean deeps, is built from and subsists on CHNOPS: carbon, hydrogen, nitrogen, oxygen, and just a little bit of phosphorus and sulfur.

One of carbon’s chemical cousins is silicon. Silicon is about ten times less prevalent in the universe than carbon, but the two elements share an affinity for forming four chemical bonds. Silicon is choosier about the elements it bonds with; it doesn’t go around making compounds with just anything. However, when you put silicon and oxygen (or water) together, it forms silicates—one of the major components of a typical Earth rock. This is one reason that carbon lifeforms dominate Earth: there’s just too much water and oxygen around for silicon to form anything other than inert materials. (Although it should be noted that silicates do have a place in Earth life: diatoms and marine sponges incorporate SiO2 into their bodies.)

RADAR image of Titan’s north pole from Cassini. The largest sea, Kraken Mare, is larger than Lake Superior. Credit: International Astronomical Union Working Group for Planetary System Nomenclature. "Gazetteer of Planetary Nomenclature." 2018 March 18.
http://planetarynames.wr.usgs.gov/.

Titan, however, has almost no water or oxygen in its atmosphere, and it’s far too cold for water to be a liquid on its surface. Titan also has a substantial reservoir of stable liquid hydrocarbons in those methane-ethane lakes—a good candidate solvent for silicon-based chemistry. As exciting as this seems, Titan is flush with carbon and probably doesn’t have enough silicon for silicon-based life to dominate. It’s more likely that carbon and silicon might bond to each other, as researchers have convinced them to in Earth bacteria.vii

All of this is speculative, of course, but I like to imagine a not-too-distant future in which a probe splashes down in Titan’s seas to discover carbon–silicon lifeforms that stare with infrared-seeing eyes at Saturn, rings and all, looming above. It may be a while until we get a chance to explore Titan’s seas—the proposed Titan Mare Explorer (TiMe) mission was not funded, and the launch window for this type of mission has closed until 2023—but we can look elsewhere in the universe for silicon-based life.

Hazy, Titanlike planets are expected to be common in the universe,viii so there are plenty of places to look. One good place to look is—you guessed it!—around M dwarfs. Titanlike planets would orbit a Sunlike star in thirty years, but would take only one year to orbit an M dwarf. Since they’re at a greater orbital distance than Earthlike planets around M dwarfs, this means they’re unlikely to be tidally locked and less likely to be struck by stellar flares. (This also means they’ll take longer to discover and confirm—astronomy is all about tradeoffs.)

Looking Ahead

The drive to find life, whether it’s like us or not, has expanded the boundaries of astrophysics. In the next few decades, our understanding of alien worlds will be advanced by proposed space missions like the Habitable Exoplanet Imaging Mission (HabEx) and the Large UV/Optical/IR Surveyor (LUVOIR), both undergoing conceptual development now.

Artists’ depictions of two proposed space telescopes that would greatly advance our understanding of exoplanetary systems and the universe as a whole: HabEx (left; not to scale—the star-shaped shade, which is tens of meters in diameter, would be separated from the telescope by tens of thousands of kilometers) and LUVOIR (right; the primary mirror is proposed to be as large as 15 meters in diameter). Credit: NASA/ JPL-Caltech and NASA/GSFC

These missions will be capable of detecting the subtle chemical fingerprints of oxygen, methane, and ozone, and taking pictures—pictures!—of planets light-years away. Imagine—the first glimpse of the blue waters of an extrasolar sea, the first spectroscopic whiff of ozone. Will it be enough for us, just knowing that we share the universe? We’ll never know until we make that first detection. So look to the nearby stars so small that they can’t be seen. Turn your telescopes there. Discoveries await…

i Sullivan, P. W. et al. (2015) The Transiting Exoplanet Survey Satellite: Simulations of Planet Detections and Astrophysical False Positives. The Astrophysical Journal, 809 (77).

ii Several references exist. A good one is Carone, L., Keppens, R., and Decin, L. (2015) Connecting the Dots – II. Phase Changes in the Climate Dynamics of Tidally Locked Terrestrial Exoplanets. Monthly Notices of the Royal Astronomical Society, 453 (3). pp. 2412–2437.

iii Yang, J., Cowan, N. B., and Abbot, D. S. (2013) Stabilizing Cloud Feedback Dramatically Expands the Habitable Zone of Tidally Locked Planets. The Astrophysical Journal Letters, 771 (L45).

iv Damage estimate from https://news.nationalgeographic.com/news/2011/03/110302-solar-flares-sun-storms-earth-danger-carrington-event-science/.

v Jakosky, B. M. et al. (2017) Mars’ Atmospheric History Derived from Upper-Atmosphere Measurements of 38Ar/36Ar. Science, 355 (6332). pp. 1408–1410.

vi Kay, C., Opher, M. and Kornbleuth, M. (2016) Probability of CME Impacts on Exoplanets Orbiting M Dwarfs and Solar-like Stars. The Astrophysical Journal, 826 (195).

vii Kan, S. B. J. and Lewis, R. D. and Chen, K. and Arnold, F. H. (2016) Directed Evolution of Cytochrome C for Carbon–Silicon Bond Formation: Bringing Silicon to Life. Science, 354 (6315). pp. 1048–1051.

viii Lora, J. M., Kataria, T. and Gao, P. (2018) Atmospheric Circulation, Chemistry, and Infrared Spectra of Titan-like Exoplanets Around Different Stellar Types. The Astrophysical Journal, 853 (58).

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