Scientists have discovered that a potentially habitable planet is being stripped of its atmosphere, a process that could ultimately make the world, Trappist-1e, inhospitable to life. The stripping appears to be caused by electrical currents created as the planet races around its red dwarf host star.
It is an important discovery because the Trappist-1 system, in which this exoplanet orbits a small red dwarf star, has been one of the main targets in the hunt for extraterrestrial life. Of the seven rocky, Earth-like worlds in the system, at least three are in the habitable zone, an area around a star that is neither too hot nor too cold for a planet to contain liquid water.
However, a planet without an atmosphere cannot hold liquid water, even if it is in the habitable zone, also known as the ‘Goldilocks zone’. This shows that although Trappist-1e may be in the habitable zone of the red dwarf Trappist-1, 40 light-years from Earth, its habitability may be transient.
The same phenomenon that affects the atmosphere of Trappist-1e could also affect the atmospheres of the other planets in this habitable zone, which is bad news for the possibility of finding life in this system.
Related: This exoplanet TRAPPIST-1 appears to have no atmosphere – the truth may lie in its star, James Webb Space Telescope reveals
Ways to strip the atmosphere of an exoplanet
Trappist-1e is about the size of Earth, yet has about 0.7 times the mass of our planet. It is the fourth planet from its star, orbiting just 0.028 times the distance between Earth and the Sun, completing one orbit in just 6.1 Earth days.
Despite this proximity, because Trappist-1 is much smaller and cooler than the Sun, its habitable zone is much closer to the surface compared to our star’s habitable zone. Therefore, it is not radiation from this red dwarf that appears to be destroying TRAPPIST-1e’s atmosphere, but rather a wind of charged particles blown from the star called the “stellar wind.”
“We looked at how space weather changes through the planet’s orbit, with TRAPPIST-1e transitioning very quickly between very different stellar wind conditions and pressures, leading to a kind of pulsating compression and relaxation of the planetary magnetic field,” says Cecilia Garraffo, team member and astrophysicist at Harvard & Smithsonian, told Space.com. “This causes strong electrical currents in the upper atmosphere – the ionosphere – which warms the atmosphere, just like an electric heater.”
Garraffo explained that Earth also experiences variations in the solar wind, which cause similar heating of our atmosphere. The difference is that the heating felt by TRAPPIST-1e is up to 100,000 times stronger than what the Earth experiences from the sun’s solar winds. That’s because Trappist-1e is moving quickly around its star and the motion creates powerful ionospheric currents that dissipate and cause extreme heat, what the team calls “voltage-driven Joule heating.”
Although the team had predicted this effect back in 2017, the researchers were surprised by how powerful they have now discovered it.
“It could be so strong for TRAPPIST-1e that the heat actually evaporates the upper atmosphere,” Garraffo said. “Over millions of years, the planet could completely lose its atmosphere due to this phenomenon.”
The team’s research shows that there are more than a few ways a planet can lose its atmosphere.
Team member and Lowell Center for Space Science & Technology researcher Ofer Cohen told Space.com that the loss of exoplanet atmospheres is typically caused by an external process. This includes strong radiation from the star, which can cause the atmosphere to heat up and escape, or charged particles in the stellar wind pelting planets, causing a strong stripping effect.
“In this case, the warming of the atmosphere, and the resulting loss of it, is caused only by the rapid motion of the planets. So the planet is doomed to lose its atmosphere simply by moving around,” Cohen said. “It’s like when we are too lazy to clear the roof of our car from snow, and we just start driving, hoping that the air moving around the car will do the work for us and take away the snow – At least that’s what we do.” we do that in the Boston area.
“I think it’s really cool that planets can do this with their atmospheres.”
What about the other Trappist-1 planets?
On Earth, our magnetosphere protects our atmosphere by guiding charged particles along magnetic field lines behind our planet. Mars, which does not have a strong magnetic field, has had its atmosphere stripped of solar winds and harsh solar radiation. In fact, the Red Planet probably lost its water in space as a result.
It is believed that Trappist-1e also has a magnetosphere, but these findings show that it may not be sufficient to prevent atmospheric stripping.
“Normally, a planet’s magnetic field acts as a protective bubble, but around TRAPPIST-1e this bubble is eroded. The planet’s magnetic field connects with that of the star, creating pathways through which the star’s particles can hit the planet directly,” Garraffo said. “This not only strips away the atmosphere, but also significantly warms it, leaving TRAPPIST-1e and its neighbors vulnerable to losing their atmosphere completely.”
Trappist-1e is the fourth planet from the red dwarf star at the heart of this fascinating planetary system of rocky worlds. Astronomers have previously discovered that Trappist-1b, the closest exoplanet to the star, appears to have already lost its atmosphere.
The team believes that voltage-controlled Joule heating could also impact Trappist-1f and Trappist-1g, stripping them of their atmosphere as well, albeit to a lesser extent than they see happening with Trappist-1e. That’s because these planets, at 0.038 and 0.04683 times the Earth-Sun distance to their star, respectively, move through the red dwarf’s stellar winds more slowly than Trappist-1e.
“The closer planets of Trappist-1 will suffer an even more extreme fate, and the planets further away a little milder,” Garraffo said. “I imagine that all Trappist-1 planets will have a hard time holding on to any atmosphere.”
Related stories:
– Research shows that exoplanets in the Trappist-1 system are more likely to be habitable than scientists once thought
– The TRAPPIST-1 solar system isn’t bombarded by space rocks like early Earth, research suggests
– James Webb Space Telescope could help in the hunt for habitable alien worlds
The team’s findings could have implications beyond the Trappist-1 system, as well as in the search for habitable exoplanets and life beyond the solar system. They suggest that exoplanets close to their stars have likely lost their atmospheres, even though they are well within that star’s habitable zone.
The results could further help suggest which stars could host planets with molecules that indicate the presence of life: biomarkers.
“Our research suggests that such low-mass host stars are unlikely to be the most promising for hosting planets with atmospheres,” Garraffo concluded. “Identifying which host stars could be conducive to habitable planets and observing those atmospheric transits with the James Webb Space Telescope and future observatories, but also building the technology to interpret those results in terms of biomarkers.”
The team’s research was published Feb. 16 in the Astrophysical Journal.