TRAPPIST-1 is one of the most fascinating planetary systems discovered so far. Located just 40 light-years away, seven roughly Earth-sized planets are orbiting a red dwarf star, with three of them within the habitable zone. Now, two new studies have looked at the habitability of these planets, with a focus on radiation from the star and tides caused by the other planets. Strangely enough, these factors could help make them more liveable.
One of this system's quirks is that all seven of its planets, dubbed TRAPPIST-1b through -1h, orbit very close to the star, and each other. In fact, all of them could quite comfortably fit between Mercury and the Sun. That might sound like it'd be way too hot for life, but luckily TRAPPIST-1 is a relatively cool red dwarf, so its habitable zone is much closer.
The flipside to that is that it's a very active star, throwing off far more high-energy particles than our own Sun does. This radiation has the potential to break apart complex molecules that are important to life, effectively sterilizing these planets before life can even get started.
Red dwarf radiation
One of the new studies set out to model just how high-energy particles like protons move after they're released by TRAPPIST-1. By simulating their interactions with the star's magnetic field, they found that the protons would bombard the fourth planet, TRAPPIST-1e, in particular. Unfortunately, this is widely considered to be the best candidate for life in the system.
"You expect that the particles would get trapped in these tightly wrapped magnetic field lines, but if you introduce turbulence, they can escape, moving perpendicularly to the average stellar field," says Federico Fraschetti, lead author of the study. "The flux of these particles in the TRAPPIST-1 system can be up to 1 million times more than the particles flux on Earth."
Here on Earth, our magnetic field helps shield us from these potentially dangerous particles, so maybe a similar thing is happening on TRAPPIST-1e. But the team calculated that the planet would need a magnetic field that's hundreds of times stronger than the Earth's to keep the radiation at bay, which doesn't seem plausible.
Interestingly enough though, that doesn't necessarily rule out life. Other studies have suggested that the TRAPPIST planets could be water worlds, and deep oceans could protect also life from this radiation.
The planets are also tidally locked, meaning one side always faces the star while the other never sees the light of day. Normally that would suggest that each face is either too hot or too cold for life, but in this case it could help. Especially with tidal forces at play, as the second study investigates.
Tides of TRAPPIST
It's not enough to consider only the star's influence on the TRAPPIST system – given how close together the seven planets are, they're all jostling each other too. Just like the Moon does to Earth, the planets could be affecting the tides of any water on the surface, as well as molten rock underground.
"It's such a unique process that no one's thought about in detail before, and it's kind of amazing that it's actually a thing that happens," says Hamish Hay, lead author of the study.
The team found that the two innermost TRAPPIST planets orbit close enough together that they can raise powerful tides on each other, which could be strong enough to fuel volcanic activity. That in turn can help sustain an atmosphere.
That kind of tidal heating could also help transfer heat to the cold night-side of the planets, potentially making them more habitable.
The team says that TRAPPIST-1g, the sixth planet in line, is likely feeling the equal tidal forces of the star and the other planets. If this is an ocean world, as has been suggested before, that might be enough to keep its waters warm enough for life.
The TRAPPIST system continues to be one of the most important places in the search for extraterrestrial life.
Both studies were published in the Astrophysical Journal.
Source: University of Arizona
