Surprisingly, TRAPPIST-1's violent flares could make life even more likely
Thanks to a cool physics trick, flares from a nearby star can heat a planet from the inside, powering plate tectonics.
Habitability is more complicated than you think. Stellar flares are usually considered a major hazard for life, especially around red dwarf stars, where the planets likeliest to host life are also close to the star and right in the line of fire for destructive flares. But a recent study suggests that stellar flares may also heat the interior of rocky planets, powering geological activity that, in turn, provides an energy source for life.
What’s new — Grayver and his colleagues used computer models to simulate how stellar flares affect the insides of rocky planets that orbit close to red dwarfs, especially the innermost worlds of the TRAPPIST-1 system.
They started with data from NASA’s now-retired Kepler Space Telescope, which had tracked TRAPPIST-1’s flares over 10 weeks. Most of those 39 flares were relatively low in energy, not the kind of big, devastating event that can totally wipe out life on a planet. Assuming that 10-week period wasn’t unusually slow or unusually busy for the star, Grayver and his colleagues modeled the same level of activity over a period of about 50,000 years.
Over that 50,000 year period, their simulation predicted that about 8 percent of TRAPPIST-1’s flares — or about 16 per Earth year — should actually hit any given planet. And in the long run, the simulated planets’ interiors absorbed enough energy from those 16 stellar flares a year to heat their upper mantles.
Our home planet is still holding onto some of the heat from its formation, but that alone wouldn’t be enough to keep Earth’s mantle flowing and power our planet’s very active geology. Earth’s innards are also heated by radioactive elements, which release energy as they decay. Grayver and his colleagues say that according to their simulations, “The heat generated inside the TRAPPIST-1 planets could be comparable to the energy released by Earth’s radionuclides at present day.”
But there’s a caveat there.
How it works — Stellar flares can heat a planet from the inside, without raising the temperature of the atmosphere, thanks to a cool physics trick called Ohmic dissipation.
When a star flares, it blasts waves of radiation — radio waves, ultraviolet light, and X-rays — out into space. That energy can strip or push away the upper layers of a planet’s atmosphere, but it doesn’t heat the atmosphere in the process. But when a stellar flare passes through the atmosphere and into the planet itself, the electromagnetic energy in the flare ends up spreading through the mantle in a different form: heat.
If the rocky planet bombarded by stellar flares happens to have a magnetic field, two things happen. First, its atmosphere has some protection against getting blown away, and any life forms on its surface has a shield against the most harmful radiation. Second, the magnetic field can strengthen the effects of Ohmic dissipation, so the flare heats up the inside of the planet even more.
In Grayver and his colleagues’ simulations, stellar flares could produce as much heat inside TRAPPIST-1 planets as radioactive decay produces in the bowls of our Earth — but only when the simulated planet had a magnetic field as strong as Earth’s. And we don’t actually know whether that’s the case.
“Presently, there is no information on the existence or strength of an intrinsic magnetic field for any TRAPPIST-1 planets,” write Grayver and his colleagues.
But even without a magnetic field, Grayver and his colleagues say Ohmic dissipation of stellar flares could produce enough heat to drive convection in the mantle. Convection happens when hotter material rises and cooler material sinks, and that process is what powers plate tectonics here on Earth.
“Ultimately, it is this energy that powers geological processes,” write Grayver and his colleagues.
Why it matters — Active geological processes — plate tectonics, volcanoes, hydrothermal systems, and so on — provide an energy source for life and also make important nutrients accessible to life forms. Most astrobiologists have a hard time imagining life evolving on a geologically dead planet. So evidence that potentially habitable worlds may also have active interiors could be good news for alien hunters.
And rocky exoplanets orbiting close to red dwarf stars like TRAPPIST-1 may be some of the best places to look for alien life. In part, that’s because red dwarfs make up 75 percent of the known stars in our galaxy. They’re also easier to study. A close-in planet passing in front of a small, relatively dim star makes a much easier target for astronomers who want to measure an alien atmosphere than, say, a planet about the same size orbiting farther away from a bigger, brighter star.
The TRAPPIST-1 system, in particular, has three rocky, roughly Earth-sized planets in its habitable zone (the area around a star where the temperature is right for liquid water to exist). But some red dwarfs, including TRAPPIST-1, are restless and flare often — which can be a problem for life. After all, a planet whose atmosphere has been dragged away by stellar flares isn’t very hospitable, and getting blasted with UV and x-rays isn’t ideal, either.
But Grayver and his colleagues say that if most of a star’s flares are small ones, there’s less to worry about.
“Although individual extreme energy flares pose a significant threat to life and can deprive a planet of its atmosphere, their occurrence rate is low,” Grayver and his colleagues write.
They suggest that there could be a balancing act between the destructive force of stellar flares bombarding a planet’s atmosphere and surface, and the benefits of heating a planet and powering active geology. To know which factor outweighs the other, we’ll need a lot more information about the TRAPPIST-1 planets and other rocky worlds orbiting small, cool red stars.
What’s next — In June 2023, one team of JWST astronomers will observe TRAPPIST-1e, one of the planets in the star’s habitable zone, looking for carbon dioxide and evidence of an atmosphere. Another will focus on TRAPPIST-1c, which is almost certainly too close to the star to be habitable. And yet another will survey nine small rocky planets around other red dwarf stars, searching for atmospheres.
Together, these upcoming studies may shed some light on whether the good outweighs the bad when it comes to stellar flares and the prospect of life around other stars.