A closer look at the TRAPPIST-1 system

The planets around TRAPPIST-1 are being bombarded by tremendously strong stellar winds. This partially strips them of their atmospheres and poses a real threat to any potential life within the system.

This artist’s concept shows what the surface of the planet TRAPPIST-1f may look like. According to a study published last summer, conditions on this planet would be harsh, as strong stellar winds have plowed a hole through the planet’s protective magnetic field. //NASA/JPL-Caltech

The discovery of the TRAPPIST-1 system has brought to light a new avenue for discovering Earth-like planets — and maybe even life — around otherwise unexplored ultracool dwarf stars. The existence of life around these stars, however, depends greatly on the relationship between the planets and their host stars. This means that distance, stellar activity, and atmospheric composition all play vital roles in a planet’s ability to support life.

According to a study led by Cecilia Garraffo, and published in The Astrophysical Journal Letters, the tightly packed planets around TRAPPIST-1 are unlikely to hang on to their atmospheres. For example, extreme stellar wind pressures bore through one planet’s protective magnetic field all the way down to its surface, creating an “open field region.” This strips away TRAPPIST-1f’s atmosphere, allowing stellar particles to flow directly to the planet's surface. A similar scenario occurs in our own solar system, where Uranus’ magnetic field seems to open and shut daily.

For life to exist on any planet, it cannot be too close to its host star, nor too far away. This is because, in addition to emitting light, a star will emit a stream of high-energy charged particles known as the stellar wind. The farther a planet is from its host star, the more diffuse the starlight and wind becomes. In other words, the farther away the planet, the less energy it receives.

“This is something like a campfire in the night, the closer you are to it, the more of its light and heat you experience. Move far enough away, and that same campfire will provide you with very little light or heat,” explains Jonathan Mound, a professor at the University of Leeds in the UK. Thus, the closer planets will tend to be hotter and more exposed to the stellar wind.

The TRAPPIST-1 system has seven known exoplanets, with all of them residing closer to their host star than Mercury is to the Sun. The planet TRAPPIST-1b sits 100 times closer to TRAPPIST-1 than Earth does to the Sun. //NASA

Another important aspect to consider while discussing the possibility of extraterrestrial life on a TRAPPIST-1 planet is the geodynamo. Our planet has a self-sustaining dynamo, thanks to Earth’s molten iron core and its rotation. However, the TRAPPIST planets may not be so lucky. They have rotation periods similar to that of Earth — from 1.5 days (TRAPPIST-1b) to about 20 days (TRAPPIST-1h). This means that the innermost planets are capable of generating their own magnetic fields through a geodynamo much like Earth does. But this does not necessarily shield them from TRAPPIST-1’s persistent stellar winds.

“Even if [the planets] generate magnetic fields, they may not be strong enough to protect their planet surfaces from charged particles. They are so close to their host star that they are likely moving through that star’s magnetosphere directly, much like the jovian moons move through Jupiter’s magnetic field,” said Adam Burgasser, a professor at the University of California, San Diego.

In order to understand more of TRAPPIST-1’s behavior, Cecilia Garraffo and her team used 3-D computer models to simulate the stellar wind around the TRAPPIST-1 system. They then studied the expected conditions each planet faces.

What did they find?

• Each TRAPPIST-1 planet spends a large fraction of its orbital period in the sub-Alfvenic regime, where stellar winds are extremely powerful. The plasma that these planets go through is around 10,000 times the density of the solar wind.

• The planets’ magnetospheres are greatly compressed and undergo much more dynamic changes than the magnetosphere of Earth. This means that the TRAPPIST-1 planets do not have the comfort of a stable, protective magnetic barrier like the one that shields Earth from the solar wind.

• The extreme stellar wind pressure drills through the planetary magnetic field all the way down to the surface of TRAPPIST-1f, creating an open field region. Through these holes in the magnetosphere, stellar wind particles flow directly onto the planet’s surface, resulting in strong atmospheric stripping. This is mainly because stellar wind particles carry a lot of kinetic energy, so when they collide with molecules in the atmosphere, they transfer energy to the atmospheric particles, providing them with enough momentum to escape the planet.

Artist’s concept of TRAPPIST-1 system. //NASA/JPL

So can a planet in the TRAPPIST-1 system achieve complete immunity to massive solar flares, intense stellar winds, and unpredictable space weather? Probably not. But thanks to studies like this, we are now one tiny step closer to discovering the truth.