by Scientists have discovered that a potentially habitable planet is being stripped of its atmosphere, a process that could leave the world, Trappist-1e, inhospitable to life. The stripping appears to be caused by electrical currents created as the planet orbits its red dwarf host star.
It is a significant discovery because the Trappist-1 system, in which this exoplanet orbits a small red star, is one of the main targets in the search for alien life. Of the seven rocky worlds in the Earth-like system, at least 3 are located in the habitable zone, a region around a star that is neither too hot nor too cold to allow a planet to sustain 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 indicates that while Trappist-1e may be in the habitable zone of the red dwarf Trappist-1, located 40 light-years from Earth, it may be quite habitable.
The same phenomenon affecting Trappist-1e’s atmosphere could also be affecting the atmospheres of other planets in this habitable zone, which is bad news for the possibility of life in this system.
Related: This exoplanet TRAPPIST-1 appears to have no atmosphere – the truth may be hidden in its star, James Webb Space Telescope reveals
Ways to remove an exoplanet’s atmosphere
Trappist-1e is about the size of Earth, but has about 0.7 times the mass of our planet. It is the fourth planet from its star, orbiting at exactly 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 its surface compared to our star’s habitable zone. To that end, it appears that TRAPPIST-1e’s atmosphere is not being affected by radiation from this red dwarf, but by a wind of charged particles blown from the star known as the “stellar wind”.
“We looked at how space weather changes through the planet’s orbit, with TRAPPIST-1e transitioning very quickly between very different wind conditions and pressures, resulting in pulsing compression and relaxation of the planetary magnetic field ,” Cecilia Garraffo, staff. member and astrophysicist at Harvard & Smithsonian, said Space.com. “This drives strong electric currents in the upper atmosphere – the ionosphere – which heats the atmosphere just like an electric heater.”
Garraffo explained that the solar wind also varies on Earth, causing our atmosphere to be similarly heated. The difference is that the heat felt by TRAPPIST-1e is up to 100,000 times stronger than what Earth experiences with the sun’s solar winds. That’s how Trappist-1e moves rapidly around its star, and the motion drives powerful ionospheric currents that spread and create extreme heating, which the team calls “voltage-driven Joule heating”.
Although the team predicted this effect back in 2017, the researchers were surprised by how powerful it is now.
“It could be so strong for TRAPPIST-1e that the heat basically disappears in the upper atmosphere,” Garraffo said. “Over millions of years, the planet could completely lose its atmosphere with this phenomenon.”
The team’s research shows that there are more than a few ways for a planet to lose its atmosphere.
Lowell Center for Space Science & Technology staff member and researcher Ofer Cohen told Space.com that the loss of an exoplanet’s atmosphere is generally believed to be triggered by some external process. This includes strong radiation from the star, which may cause the atmosphere to heat up and escape, or charged particles in the planets stellar wind, which creates a strong stripping effect.
“In this case, the heating of the atmosphere, and its loss as a result, is driven by the fast planetary motion. Therefore, the planet itself dooms to lose its atmosphere just by moving around,” said Cohen. “It’s like when we are too lazy to clean the roof of our car from the snow, and we start driving, hoping that the air moving around the car will do the work for us and we will remove the snow – at least we do in the Boston area.
“I think it’s really cool that planets can do this with their atmosphere.”
What about the other Trappist-1 planets?
On Earth, our magnetosphere protects our atmosphere by redirecting charged particles down magnetic field lines and out behind our planet. Mars, without a strong magnetic field, has its atmosphere destroyed by solar winds and solar radiation. In fact, the Red Planet probably lost its water to space as a result.
Trappist-1e is also believed to have a magnetosphere, but these results indicate 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 compromised. The planet’s magnetic field connects with the star, creating paths that allow particles from the star to hit the planet directly, ” said Garraffo. “This not only destroys the atmosphere but also adds to it, leaving TRAPPIST-1e and its neighbors at risk of losing their atmosphere entirely.”
Trappist-1e is the fourth planet from the red dwarf star at the heart of this legendary planetary system of rocky life. Astronomers previously discovered that Trappist-1b, the closest exoplanet to the star, appears to have already lost its atmosphere.
The team thinks that voltage-driven Joule heating could also affect Trappist-1f and Trappist-1g, removing their atmospheres as well, although to a lesser extent than they see happening with Trappist-1e. That’s because, at 0.038 and 0.04683 times the distance between the Earth and the sun from their star respectively, these planets are moving more slowly through the great winds of the red dwarf than Trappist-1e.
“Planets closer to Trappist-1 will have an even more extreme fate, and others a little milder,” Garraffo said. “I would imagine that all Trappist-1 planets will be difficult to hold any atmosphere.”
Related Stories:
— An exoplanet in the Trappist-1 system is more likely to be habitable than scientists once thought, according to a study
— TRAPPIST-1 solar system not bombarded by space rocks like early Earth, study finds
— The James Webb Space Telescope could help search for habitable alien life
The team’s findings could have implications beyond the Trappist-1 system as well as the search for habitable exoplanets and life outside the solar system. They suggest that exoplanets close to their stars are likely to have lost their atmospheres even if they are well within that star’s habitable zone.
The results could further help to know which stars could host planets with molecules that indicate the presence of life: Biomarkers.
“Our research suggests that these low-mass host stars are unlikely to be the most promising stars for hosting planets with atmospheres,” Garraffo said. “Identifying which host stars can support 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 on February 16 in the Astrophysical Journal.
