Red herrings in the search for extraterrestrial life?

Are planets orbiting red dwarf stars the abodes for life that we hope they are? New research suggests that we shouldn’t visit without a radiation suit.

We think of our Sun as an average star, but it is far from average. Of more than 350 stars within 33 light years of the Solar System, only 19 are like the Sun in terms of their mass, temperature and luminosity. Our life-giving Sun is of a rare breed indeed. 

On the flip-side, ubiquitous are the red dwarfs – the smallest, coolest, least massive stellar objects on the block, yet amounting to three-quarters of all stars in the Universe. If there are 200 billion stars in our Milky Way Galaxy alone, then about 150 billion of them are red dwarfs. And they have orbiting planets. Given the sheer number of red dwarfs, most of the planets in our Milky Way Galaxy will inevitably be found around a red dwarf star.

An artist’s impression of an active red dwarf star. Image: NASA/ESA/G. Bacon (STScI).

This is tremendously exciting to astronomers. Because red dwarfs are so small, some with as little as ten per cent of the mass of our Sun, their planetary systems are drastically scaled down compared to our own Solar System. Whereas our innermost planet, Mercury, has an orbit that varies in distance from the Sun by between 46 million and 69.8 million kilometres, the planets that orbit red dwarfs might do so at distances of just a few million kilometres. Their entire planetary system could easily fit inside the orbit of Mercury.

This makes things so much easier for exoplanet hunters, particularly those watching for planetary transits, i.e. when an exoplanet crosses in front of its star, temporarily blocking some of the star’s light. Consider this: Earth takes 365 days to orbit the Sun, so alien astronomers watching for our planet to make a transit of our star would have to wait 365 days from one transit to the next. A planet orbiting close to a red dwarf, however, might complete one orbit in a matter of days, or in the most extreme cases, a handful hours. And in the transit game, it’s all about the number of transits that you can observe: the more periodic transits that you detect, the stronger the signal-to-noise ratio and the more confident astronomers can be that the planet is real.

Living up to their potential?

Planets have been found orbiting many of the nearest red dwarf stars, and some of them even reside in their star’s habitable zone – the distance from the star at which liquid water, which is essential to all life that we know of, could exist on the surface of a planet that has a suitable atmosphere. 

One of these habitable zone planets is found a little over four light years away, orbiting Proxima Centauri, which is the closest star to our Solar System. Eleven light years away is a similar world, around a similar red dwarf star called Ross 128. Twelve light years away, the red dwarf known as Teegarden’s Star has two orbiting planets that skirt the inner and outer edges of its habitable zone. Thirty-nine light years distant, the red dwarf called TRAPPIST-1 has not one or two, but seven orbiting planets, including several of which that could potentially be habitable.

It seems that everywhere we look there are potentially habitable planets to be found orbiting red dwarf stars.

The seven worlds of the TRAPPIST-1 system in order of distance (not to scale) from their star. The innermost planets, b and c, are likely too hot for life as we know it. The outermost planets, g and h, are probably too cold. Planets d, e and f, however, seem to reside within the star’s habitable zone. Image: NASA/R. Hurt/T. Pyle.

There’s that word though: ‘potentially’. Now, sure, as with all exoplanets regardless of what type of star they orbit, we don’t know if any of these planets are truly habitable, only that they exist at a distance suitable for liquid water. Yet despite astronomers’ enthusiasm, there are some big caveats when it comes to the habitability of worlds around red dwarf stars, which lead us to question whether we should really treat them as serious candidates in the search for life.

Two-faced worlds

A quirk of orbiting so close to their star is that most of these worlds will be tidally locked, meaning that gravitational tidal forces from the red dwarf act to bring the planet’s rotation period (i.e. the length of its day) into sync with its orbital period (i.e. the length of its year) so that as the planet goes around the star, it turns on its axis at a rate that leaves it always showing the same face to its star, much like the Moon does to Earth. 

We can imagine what the scenario might be like for one of these tidally-locked worlds. One hemisphere, the one facing the star, will be in constant daylight. At the sub-stellar point at the equator in this hemisphere, it will always be high noon. Meanwhile, on the other hemisphere, which is the far-side facing out into the depths of space, there is permanent night. On the terminator, the boundary between daylight and night on the limb of the planet, the star will sit on the horizon, the land stuck in eternal twilight.

An artist’s impression of the surface of a planet orbiting in the habitable zone of a red dwarf star. The location is near the unmoving terminator between night and day, where the red dwarf appears low on the horizon of the tidally-locked world, and conditions are suitable for liquid water. Image: M. Weiss/CfA.

One of the most fundamental things about Earth is that there is always change. The Sun rises and sets as the world turns. On a planet around a red dwarf there would be no sunrise or sunset, no diurnal cycle. 

The night-side, never seeing the sun, need not be a frozen wasteland. If the world has an atmosphere, then winds blowing from the dayside to the nightside will transport heat around the globe. If not, then the night-side’s temperatures could plunge close to absolute zero. 

As unusual as this environment would be, it is feasible that life could still form and evolve in it, although its biology could not depend on the circadian rhythms that imprint themselves on Earthly life. Indeed, it turns out that the unnervingly unchanging environment would be the least of life’s concerns.

Stellar temper tantrums

Stars are rarely completely placid. Even our Sun rages from time to time, unleashing vast plumes of plasma called coronal mass ejections, and blasting out high-energy flares of ultraviolet and X-ray radiation. This activity is the result of the Sun’s taught, tangled and twisted magnetic field, which can snap and reconnect, releasing torrents of energy in the process.

An artist’s impression of a violent, magnetically active red dwarf star, the radiation from which is eroding the atmosphere of an orbiting planet. Image: NASA/ESA/D. Player (STScI).

Red dwarf stars ratchet up this magnetic violence to the next level, particularly when they are young. A Hubble Space Telescope survey of 12 juvenile red dwarfs, each about 40 million years old, found that in a ten-hour period, the dozen stars collectively released 18 ultraviolet flares, ten of which had energies in excess of 1030 erg (1023 joules), which would be considered a large flare on our Sun. Furthermore, one of these flares was what astronomers call a super-flare, with an energy of 1033 erg (1026 joules), greater than any ultraviolet flare ever seen on our Sun. Remember, this was in just a ten-hour period of observations – these stars must be emitting powerful flares every day, possibly sometimes even more powerful than those seen by Hubble.

Imagine what conditions must be like for any orbiting planets just a few million kilometres away. A harsh stellar wind of radiation complemented by those ferocious gusts from flares and super-flares will sweep away the atmospheres of nearby planets, leaving their surfaces unprotected from the radiation.

Still, in the long run, would this matter? As red dwarfs age, they should grow calmer, and when they do an orbiting planet could outgas a new atmosphere through volcanic eruptions, gain fresh water from comet and asteroid impacts, and in general revitalise itself once the radiation storm dies down.

It does die down, doesn’t it?

Grumpy older stars

Until 2018, the main claim to fame of Barnard’s Star, which is a red dwarf 5.9 light years away, is that it is the star with the highest proper motion in the sky. In other words, it appears to be moving across the sky faster than any other star – 10.3 arcseconds per year, which is partly illusory, a result of both its proximity to us and its direction of motion through space, relative to the background stars.

Then, in November 2018, astronomers announced the discovery of a planet orbiting Barnard’s Star. The planet is what we call a ‘super-earth’, a rocky world larger and more massive than our own. At a similar distance from its star as Mercury is from the Sun, in the scaled-down system of Barnard’s Star the planet lies far beyond the habitable zone, in the frozen realm beyond the snow line, where ice is as abundant as rock, if not more so.

That’s not to say there couldn’t be other planets, closer in, which remain undetected as they orbit Barnard’s Star. And Barnard’s Star is ancient, ten billion years old – more than twice the age of our Solar System. Based on our limited understanding of how red dwarfs age, the star should be peaceful now, its quiescence allowing planetary atmospheres to grow and develop without being scoured away by stellar radiation.

So it was all the more shocking when the Hubble Space Telescope saw Barnard’s Star unleash two powerful ultraviolet flares in March 2019, followed by NASA’s Chandra X-ray Observatory detecting an X-ray flare later that June. Based on the rate of flare activity relative to the time Hubble and Chandra spent observing Barnard’s Star, it is estimated that the star spends at least a quarter of its time releasing flares, with those observed by Hubble and Chandra having energies of approximately 1029 erg (1022 joules). Under such a constant barrage of radiation, life on a planet close by would stand no chance. 

Would a planet orbiting a red dwarf be radiation blasted, and losing both its atmosphere and its water? Image: ESO/M. Kornmesser.

“If these snapshots are representative of how active Barnard’s Star is, then it is pumping out a lot of harmful radiation,” said Girish Duvvuri of the University of Colorado, who was involved in the detection of the flares. More pertinently, he adds, “This amount of activity is surprising for an old red dwarf.”

We shouldn’t have been surprised though. In 1998 a similarly powerful flare was detected on Barnard’s Star. At the time it was thought to be a one-off. Apparently not.

Even more powerful flares have been observed on other red dwarfs. In 2019 Japanese astronomers detected multiple violent flares coming from the star AD Leonis, which is 16 light years away. One of these flares was a super-flare, with an immense energy of 2 x 1033 ergs (2 x 1026 joules).

Warding-off the radiation

Does this radiation danger mean that life cannot exist in red dwarf systems? It certainly doesn’t look good, but there are two defence mechanisms that a planet might employ to give it, and any burgeoning life it possesses, some protection from stellar outbursts.

The first line of defence is a magnetic field. While a planetary magnetosphere is no barrier to ultraviolet or X-ray radiation, it can deflect charged particles riding the stellar wind or those belched out by a coronal mass ejection. Earth has a magnetic field. Mars does not, and for four-and-a-half billion years the solar wind has been gradually eroding the red planet’s atmosphere. Venus doesn’t have a magnetic field either, but what it does have, by virtue of its larger mass, is enough gravity to hang on to its dense atmospheric shroud of carbon dioxide. That said, Venus has lost what water it once had, the water molecules evaporated by heat, blasted apart by solar ultraviolet, and the hydrogen atoms from the water molecules swept away into space on the solar wind.

An atmosphere is the second line of defence against stellar radiation, absorbing a flare’s energy before it can cause harm on the ground. It’s why we should all be concerned about holes in Earth’s ozone layer, which acts to block hazardous ultraviolet light.

If a planet has a strong magnetic field, far stronger than Earth’s, and enough gravity to hold onto its atmosphere securely – perhaps the extra gravity belonging to a super-Earth – then it’s feasible that it could resist even a red dwarf’s radiation. However, it seems likely that this would be the exception to the rule – most planets probably won’t have a strong enough magnetic field or enough gravity to be able to hold onto their atmosphere. We’ve got to face up to the strong possibility that most worlds around red dwarf stars are radiation blasted, airless and barren.

Could life survive underground on a planet orbiting Barnard’s Star? Image: NASA/CXC/M. Weiss.

Perhaps life could still survive in such conditions, but deep underground where the radiation cannot touch it. A trio of scientists, – Edward Guinan and Scott Engle of Villanova University in Pennsylvania, USA, and Ignasi Ribas of the Institut d’Estudis Espacials de Catalunya (the Institute of Space Studies of Catalonia) – have proposed that if the cold planet orbiting Barnard’s Star has a large, hot iron-nickel core and enhanced geothermal activity, then it would be warm enough underground to support pockets of liquid water and possibly microbial life, similar to how sub-surface lakes exist beneath the ice sheets of Antarctica, or the ocean beneath the ice on Jupiter’s moon Europa. Crucially, such life would be shielded from the radiation.

Of course, it’s unlikely we’d be able to detect such hidden life remotely, and microbes would be a far cry from the complex and intelligent life that astrobiologists hope to one day find on another world. SETI searches of red dwarf systems would turn up nothing if they are populated by only underground microbial life forms. Consequently, maybe we shouldn’t be paying as much attention to red dwarf systems as we currently are in our search for life elsewhere.

“It may turn out that most red dwarfs are hostile to life,” says Tommi Koskinen of the University of Arizona, Tucson, who was one of the astronomers who made the Hubble and Chandra observations of Barnard’s Star’s flares. “In that case, the conclusion might be that the planets around more massive stars, like our own Sun, might be the optimal location to search for inhabited worlds with the next generation of telescopes.”

A future home

It’s a disturbing thought that three-quarters of the stellar real estate in our Galaxy could be a wasteland. And yet, there’s something enticing about red dwarfs. Can they really play no part in the story of life?

Stars like our Sun – warm, yellow, steady – might be the cradles of life, but to all things there comes an end. In about a billion years’ time the maturing Sun will have warmed to such a degree that Earth’s oceans will begin to evaporate and the surface will roast. If, somehow, humankind, or our descendants far-removed from what we are today, have persevered into that far future, then we would have no option but to migrate, taking what other life remains on Earth with us into the outer Solar System. The moons of the outer planets will also be feeling the effects of the warming, causing a thawing of the ice and perhaps turning frozen bodies such as Saturn’s moon Titan into more hospitable refuges, bolstered by whatever advanced technology our future selves might bring to bear to help them adapt to these new worlds.

That migration will buy us time, but eventually we will be forced out of the Solar System. Our Sun is currently 4.6 billion years old, and is estimated to still have five billion years’ of its lifespan left. However, at some point in that distant future, our Sun will start to exhaust its supply of hydrogen fuel in its core. Nuclear reactions will move into its outer layers, and it distend, swelling to become, for a short time, a red giant that will engulf Mercury, Venus and quite probably Earth too, before casting off its outer layers to form a gaseous planetary nebula in one final grand gesture before stellar death.

Life cannot survive without the Sun, and its fatal bout of gigantism will be our cue to leave the Solar System forever. But where could we go that won’t suffer a similar fate too soon? Where could we find a home for the long term?

Because their mass is so small compared to stars like the Sun, red dwarfs don’t have to generate energy at the same rate as other stars do in order to hold themselves up against the pull of gravity. The stores of hydrogen for nuclear fusion inside red dwarfs will last hundreds of billions, perhaps trillions, of years before the red dwarf stars grow old and are extinguished. Red dwarfs will inherit the Universe when every other kind of star has gone, and thus technologically-advanced life might choose to settle on worlds around red dwarfs, knowing that their longevity will allow civilisations to survive through deep time.

It’s not the only type of location that space-faring life might flee to – I’ve written previously about how technologically-advanced life might prefer to settle around black holes instead, which might happen anyway in trillions of years’ time when even the red dwarfs expire – but red dwarfs would likely be on their radar. Sun-like stars that formed ten billion years ago are reaching the end of their lives now, expanding into red giants and then blossoming into beautiful planetary nebulae. Maybe refugees from these dying systems are already settling on planets around red dwarfs, digging in for the long term, their technology able to protect them from the radiation.

As such, maybe we’re being a tad hasty in suggesting red dwarfs are not viable places to look for life – perhaps they are the perfect SETI target after all.

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