Can red dwarf stars have Earth-like planets with life?
This is an important question, at least in the long run, because 80% of the stars in the Milky Way are red dwarfs, even though none are visible to the naked eye. 20 of the 30 nearest stars are red dwarfs! It would be nice to know if they can have planets with life.
Also, red dwarf stars live a long time! They’re small—and the smaller a star is, the longer it lives. Calculations show that a red dwarf one-tenth the mass of our Sun should last for 10 trillion years!
So if life is possible on planets orbiting red dwarf stars—or if life could get there—we could someday have very, very old civilizations. That idea excites me. Imagine what a galactic civilization spanning the 80 billion red dwarfs in our galaxy could do in 10 trillion years!
(No: you can’t imagine it. You don’t have time to think of all the amazing things they could do.)
Let’s start close to home. Proxima Centauri, the nearest star to the Sun, is a red dwarf. If we ever explore interstellar space, we may stop by this star. So, it’s worth knowing a bit about it.
We don’t know if it has planets. But it could be part of a triple star system! The closest neighboring stars, Alpha Centauri A and B, orbit each other every 80 years. One is a bit bigger than the Sun, the other a bit smaller. They orbit in a fairly eccentric ellipse. At their closest, their distance is like the distance from Saturn to the Sun. At their farthest, it’s more like the distance from Pluto to the Sun.
Proxima Centauri is fairly far from both: a quarter of a light year away. That’s about 350 times the distance from Pluto to the Sun! We’re not even sure Proxima Centauri is gravitationally bound to the other stars. If it is, its orbital period could easily exceed 500,000 years.
If Proxima Centauri had an Earth-like planet, there’s a bit of a problem: it’s a flare star.
You see, convection stirs up this star’s whole interior, unlike the Sun. Convection of charged plasma makes strong magnetic fields. Magnetic fields get tied in knots, and the energy gets released through enormous flares! They can become as large as the star itself, and get so hot that they radiate lots of X-rays.
This could be bad for life on nearby planets… especially since an Earth-like planet would have to be very close. You see, Proxima Centauri is very faint: just 0.17% the brightness of our Sun!
In fact many red dwarfs are flare stars, for the same reasons. Proxima Centauri is actually fairly tame as red dwarfs go, because it’s 4.9 billion years old. Younger ones are more lively, with bigger flares.
Proxima Centauri is just 4.24 light-years away. If explore interstellar space it may be a good place to visit. It’s actually getting closer: it’ll come within about 3 light-years of us in roughly 27,000 years, and then drift by. We should take advantage of this and go visit it soon, like in a few centuries!
Gliese 667 Cc
Gliese 667C is a red dwarf just 1.4% as bright as our Sun. Unremarkable: such stars are a dime a dozen. But it’s famous, because we know it has at least two planets, one of which is quite Earth-like!
This planet, called Gliese 667 Cc, is one of the most Earth-like ones we know today. But it’s weirdly different from our home in many ways. Its mass is 3.8 times that of Earth. It should be a bit warmer than Earth—but dimly lit as seen by our eyes, since most of the light it gets is in the infrared.
Being close to its dim red dwarf star, its year is just 28 Earth days long. But there’s something even cooler about this planet. You can see it in the NASA artist’s depiction above. The red dwarf Gliese 667C is part of a triple star system!
The largest star in this system, Gliese 667 A, is three-quarters the mass of our Sun, but only 12% as bright. It’s an orange dwarf, intermediate between a red dwarf and our Sun, which is considered a yellow dwarf.
The second largest, Gliese 667 B, is also an orange dwarf, only 5% as bright as our sun.
These two orbit each other every 42 years. The red dwarf Gliese 667 C is considerably farther away, orbiting this pair.
What could the planet Gliese 667 Cc be like?
Tidally locked planets
Since a planet needs to be close to a red dwarf to be warm enough for liquid water, such planets are likely to be be tidally locked, with one side facing their sun all the time.
For a long time, this made scientists believe the day side of such a planet would be hot and dry, with all the water locked in ice on the night side, as shown above. People call this a water-trapped world. Perhaps not so good for life!
But a new paper argues that other kinds of worlds are likely too!
In a thin ice waterworld, an ocean covers most of the planet. It’s covered with ice on the night side, maybe 10 meters thick. The day side has open ocean. Ice melts near the edge of the ice, pours into the ocean on the day side… while on the night side, water freezes onto the bottom of the ice layer.
In an ice sheet-ocean world, there’s a big ocean on the day side and a big continent on the night side. As in the water-trapped world, a lot of ice forms on the night side, up to a kilometer thick. But if there’s enough geothermal heat, and enough water, not all the water gets frozen on the night side: enough melts to form an ocean on the day side.
Needless to say, these new scenarios are exciting because they could be more conducive to life!
Read more here:
• Jun Yang, Yonggang Liu, Yongyun Hu and Dorian S. Abbot, Water trapping on tidally locked terrestrial planets requires special conditions.
Abstract: Surface liquid water is essential for standard planetary habitability. Calculations of atmospheric circulation on tidally locked planets around M stars suggest that this peculiar orbital configuration lends itself to the trapping of large amounts of water in kilometers-thick ice on the night side, potentially removing all liquid water from the day side where photosynthesis is possible. We study this problem using a global climate model including coupled atmosphere, ocean, land, and sea-ice components as well as a continental ice sheet model driven by the climate model output.
For a waterworld we find that surface winds transport sea ice toward the day side and the ocean carries heat toward the night side. As a result, night-side sea ice remains about 10 meters thick and night-side water trapping is insignificant. If a planet has large continents on its night side, they can grow ice sheets about a kilometer thick if the geothermal heat flux is similar to Earth’s or smaller. Planets with a water complement similar to Earth’s would therefore experience a large decrease in sea level when plate tectonics drives their continents onto the night side, but would not experience complete day-side dessication. Only planets with a geothermal heat flux lower than Earth’s, much of their surface covered by continents, and a surface water reservoir about 10% of Earth’s would be susceptible to complete water trapping.
From a technical viewpoint, what’s fun about this new paper is that it uses detailed climate models that have been radically hacked to deal with a red dwarf star. Paraphrasing:
We perform climate simulations with the Community Climate System Model version 3.0 (CCSM3) which was originally developed by the National Center for Atmospheric Research to study the climate of Earth. The model contains four coupled components: atmosphere, ocean, sea ice, and land. The atmosphere component calculates atmospheric circulation and parameterizes sub-grid processes such as convection, precipitation, clouds, and boundary- layer mixing. The ocean component computes ocean circulation using the hydrostatic and Boussinesq approximations. The sea-ice component predicts ice fraction, ice thickness, ice velocity, and energy exchanges between the ice and the atmosphere/ ocean. The land component calculates surface temperature, soil water content, and evaporation.
We modify CCSM3 to simulate the climate of habitable planets around M stars following Rosenbloom et al., Liu et al., and Hu & Yang. The stellar spectrum we use is a blackbody with an effective temperature of 3400 K. We employ planetary parameters typical of a super-Earth: a radius of 1.5 R⊕, gravity of 1.38 g⊕, and an orbital period of 37 Earth-days. The orbital period of habitable zone planets around M stars is roughly 10–100 days. We set the insolation to 866 watts per square meter and both the obliquity and eccentricity to zero. The atmospheric surface pressure is 1.0 bar, including N2, H2O, and 355 parts per million CO2.
And so on. Way cool! They consider a variety of different kinds of continents and oceans… including one where they’re just like those here on Earth—just because the data for that is easy to get!
Here’s a question I don’t know the answer to. To what extent can models like Community Climate System Model version 3.0 be tweaked to handle different planets? And what are the main things we should worry about: ways Earth-like planets can be different enough to seriously throw off the models?
We live in exciting times, where just as we’re making huge progress trying to understand the Earth’s climate in time to make wise decisions, we’re discovering hundreds of new planets with their own very different climates.