Exoplanet scientists are eagerly awaiting the discovery of an atmosphere around a terrestrial exoplanet. Not a thin, tenuous, barely perceptible collection of molecules, but a thick, robust, potentially life-supporting atmosphere. Due to the way we detect exoplanets, most of the terrestrial planets we find are orbiting red dwarfs (M dwarfs).
This presents a problem for finding an atmosphere, because red dwarfs are known for violent flaring. Since red dwarfs are so dim, their habitable zone is very close to them. That means that exoplanets in the habitable zone are so close to the stars they’re exposed to the flaring, and it’s expected to destroy any atmospheres these planets may have. And without an atmosphere, the prospects for habitability are extremely weak.
Since they’re so close, exoplanets in red dwarf habitable zones are also likely tidally locked to their stars. This means one side of the planet is constantly lit up (dayside) and the other is constantly dark (nightside). So while the dayside is extremely hot, the nightside is very cold.
That could lead to a very unusual situation, according to new research. It’s titled “Atmospheric collapse and re-inflation through impacts for terrestrial planets around M dwarfs,” and the lead author is Prune August. August is a Ph.D. student in the Department of Space Research and Technology at the Technical University of Denmark. The research has been submitted to The Astrophysical Journal Letters and is available online on the arXiv preprint server.
As the title makes clear, the work concerns terrestrial exoplanets orbiting M dwarfs. “The atmospheres of these planets are vulnerable to atmospheric erosion and collapse due to condensation of volatiles on the nightside,” the authors write. They’re saying that not only are these atmospheres prone to destruction by flaring, but that some of the volatiles freeze and collapse onto the surface on the cold darkside. “However, these collapsed volatiles accumulated as nightside ice constitute a stable reservoir that could be re-vaporized by meteorite impacts and re-establish the atmospheres.”
This is an unusual idea. If red dwarf flaring is most destructive early in the star’s life, then once the flaring dies down, the heat from impacts could reconstitute volatiles from the nightside into a new atmosphere. “Through a simple energy balance model applied to atmospheric evolution simulations with stochastic impacts, we assess the viability and importance of this mechanism for CO atmospheres,” the authors write.
In their work they considered exoplanets from the JWST DDT Rocky Worlds program, an observational effort to find atmospheres on exoplanets orbiting small red dwarfs. As a first step, they ran simulations for random impacts on an Earth-sized, Earth-mass exoplanet orbiting a red dwarf at three different orbital distances. They also gave the planet a fixed CO offgassing rate the same as modern Earth’s.
Overall, they found that moderately sized impactors around 10km in diameter striking a planet about every 100 million years could maintain an atmosphere that’s detectable.
From there, they applied the resulting model to three planets from Rocky Worlds: LTT 1445 Ab, LTT 1445 Ac, and GJ 3929 b. “Instead of focusing on a static, final state of the evolution, we compute the fraction of time each planet spends with an inflated atmosphere,” the researchers explain. “This approach accounts for the presence of transient atmospheres, such as the ones generated by impacts.”
The researchers ran 50,000 Monte Carlo situations with a variety of impact rates and CO2 outgassing rates. The simulations began when the planets are 2.2 billion years old and 12 billion years old. Together, they determined what the optimal range of impact rates are for atmospheric regeneration.
