Authors: Kevin France, Girish Duvvuri, Hilary Egan, et al.
First author’s institution: University of Colorado, Boulder
Status: Posted on arXiv (open access); Accepted to AJ (closed access)
There are lots of stars out there in the Universe, and a large chunk of those are M dwarfs. These are the smallest and reddest stars, coming last in the sequence of spectral types (O, B, A, F, G, K, and last but not least: M). Bonus: since they’re so small and dim, it actually makes it easier to find smaller, terrestrial planets around them! Given that they’re so plentiful and we have a good shot at peering into their habitable zones, it makes sense that we’d want to think about what life on a planet around a M dwarf would be like.
But, there’s a catch. M dwarfs are also known to be very active stars, flaring and giving off a lot of ultraviolet light and X-rays that are bad news for biological life. This stellar activity is so strong that it drives atmospheric escape, stripping these rocky planets of their atmospheres, which are critical for habitability. Extreme ultraviolet light (known as EUV or XUV) is particularly good at stripping away an atmosphere, and young M dwarfs give off more of this since they spend a longer time in their pre-main sequence evolution phase. So, the beginning of these stars’ lives are extreme, ruining chances for a planet to be habitable. What about older M dwarfs? Planets around M dwarfs could have a do-over on their atmosphere, gaining a “secondary atmosphere” created by gases released through impacts or volcanos. Do they mellow with age, quieting down all that radiation, making it possible for this secondary atmosphere to stick around long enough for life to arise?
Today’s paper seeks to answer these questions by observing a nearby old M dwarf for it’s UV and X-ray activity, then computing what would happen to the atmosphere of an Earth-like planet in its habitable zone.
The Search for the Atmosphere Killers
The authors used the Hubble Space Telescope (for UV observations) and the Chandra X-ray Observatory to observe Barnard’s Star, a nearby old M star. Barnard’s Star is only about 6 light-years away, making it one of our closest neighbors in space. It’s only 16% the size of the Sun, but about twice as old. It’s also known to host a cold (around -300°F!) super-Earth about 3 times the size of our planet, discovered by the radial velocity method.
The average UV luminosity of Barnard’s star is among the lowest ever measured for an M dwarf, but it still emits more XUV than the Sun, as shown in Figure 1. They also measured a weak (but non-zero) X-ray flux, also among the lowest observed on an M dwarf. Barnard’s Star still flared just about as frequently as younger M dwarfs, but the flares on the older star were lower intensity (still more intense than a star like our Sun though!). Another atmosphere-harming event is the CME, or “coronal mass ejection”, which releases high energy particles from the star; the authors found that these events have similar energies to solar flares, but are much more frequent. There is a caveat on this though: M dwarfs have been theorized to have stronger magnetic fields, which may keep CMEs from traveling far from the star and impacting planets, so there’s a bit of uncertainty on the effect of CMEs on an atmosphere discussed here.
The Verdict on the Atmosphere
Now that we know a bit more about the environment around an old M dwarf, what would happen to a planet’s atmosphere? The authors estimated the atmospheric escape from a hypothetical Earth-like planet in the habitable zone of Barnard’s Star that encounters this observed high-energy radiation.
First, to make sure their models made sense, they tested them on the Sun/Earth system to see if they could reproduce what we observe in our own solar system. Then, they moved on to look at the thermal and ion escape from our hypothetical planet. Thermal escape happens when particles are hot enough, and therefore moving fast enough, to exceed the escape velocity of the planet. Around Barnard’s Star, our hypothetical planet would lose its atmosphere in about 11 million years. Or, you can think about it as losing 87 times the Earth’s atmosphere in a billion years (for context, Earth is over 4 billion years old!).
They also looked at ion escape, which is actually the main way Earth loses atmosphere. This is a bit more complicated, since it requires a plasma interaction model. Their simulations showed that in a normal, quiescent (not flaring) state, Barnard’s Star only slightly increases atmospheric escape compared to Earth. However, when a flare happens, there is much more atmosphere loss, as seen in Figure 2. One thing to note is that the hypothetical planet here is unmagnetized; magnetism could make a difference, as it does on Earth, shielding from some of these high energy particles. The big takeaway here though is that atmospheric loss around old M dwarfs will be dominated by the flare periods.
Can Life Find a Way?
Flares might actually have a positive effect on life in a different way. Other work has shown that near-UV (NUV) photons might drive the formation of precursor molecules to RNA; Barnard’s Star has a little less NUV radiation than is needed for this in its quiet state, but flaring could be enough to support these prebiotic pathways. Also, now that we know flares might be an issue for keeping an atmosphere, we might want to extend our search for habitable planets further from the star; there’s a possibility of an “extended habitable zone” further out from the star where the radiation is less extreme!
Although they’re less active, this paper has shown that even old M dwarfs can lose a lot of atmosphere, particularly due to flares. We still need to learn more about the flare cycles, since that seems to be a key parameter in atmospheric retention and M dwarf habitability!