Among the almost-4,000 exoplanets found so far, none really capture the imagination like TRAPPIST-1. This system consists of seven roughly Earth-sized planets orbiting a red dwarf star, and with three of them within the habitable zone there's an exciting possibility of life. But there's more to the equation than just distance from a star, and now a team of astronomers led by the University of Washington has simulated the climates that could be found on each TRAPPIST world.

When we look for life out in space, we tend to start with places reasonably similar to Earth – after all, this is the only place we know where it's cropped up. But our home planet seems to be the exception not the rule, and considering red dwarfs are the most common stars in the universe, it might pay to check whether they could also create the right conditions for life.

And that's just what the new research set out to do. The team examined the evolution of the TRAPPIST-1 system by modeling the physics, light and radiation coming off the star and how that might interact with the atmospheres of its seven planets, and determined what their different climates might look like.

"We are modeling unfamiliar atmospheres, not just assuming that the things we see in the solar system will look the same way around another star," says Andrew Lincowski, lead author of the study. "We conducted this research to show what these different types of atmospheres could look like. This is a whole sequence of planets that can give us insight into the evolution of planets, in particular around a star that's very different from ours, with different light coming off of it. It's just a gold mine."

Compared to the Sun, TRAPPIST-1 is tiny and cool, so its planets need to bunch up close in order to stay warm enough for liquid water to potentially pool on the surface. Unfortunately, red dwarfs are often more active than Sun-like stars, so the extra radiation that hits them could prevent life from taking hold.

According to the team's climate models, TRAPPIST-1 b, the closest planet to the star, would be a boiling, utterly inhospitable world. Moving down the list, TRAPPIST-1 c and d would be slightly more temperate, although still pretty hot and cloaked in a thick atmosphere, making them unlikely places to look for life. That said, scientists have suggested that Venus – which is the closest solar system equivalent to these two planets – may be able to support microbial life in its high-altitude clouds.

An artistic render of the seven planets in the TRAPPIST-1 system NASA/JPL-Caltech

Next in line, TRAPPIST-1 e is right in the sweet spot and is widely believed to be the best bet for finding life in the system. Orbiting in the middle of the habitable zone, the new models indicate this planet may be covered in a global ocean, giving it a relatively Earth-like climate. And finally, out on the fringes of the system, TRAPPIST-1 f, g and h are most likely frozen, desolate planets.

But, the team explains, there's a chance that all seven planets are Venus-like worlds with hot, dense atmospheres. The star probably went through a rowdy phase in its younger years, burning hotter and brighter and potentially ruining the chances of life. If any TRAPPIST planets had liquid water oceans they would have evaporated. Then, rather than remaining in the atmosphere to fall back down as rain, the extreme UV light from the star would break the molecules into hydrogen and oxygen. The hydrogen could escape into space, leaving behind a thick atmosphere of almost pure oxygen, which hasn't been seen before.

The team says the research could help future planet-studying telescopes, such as James Webb, get a better understanding of the types of signatures they should be looking for as markers of habitability and maybe even life.

"The processes that shape the evolution of a terrestrial planet are critical to whether or not it can be habitable, as well as our ability to interpret possible signs of life," says Victoria Meadows, co-author of the study. "This paper suggests that we may soon be able to search for potentially detectable signs of these processes on alien worlds."

The research was published in the Astrophysical Journal.

Source: University of Washington