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 are 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 PhD 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 at arxiv.org.
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 atmosphere 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-vaporised 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 programme, 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.
Of course, our knowledge of impact rates on exoplanets is far from certain. “Estimating impact rates for exoplanetary systems remains highly uncertain, depending on factors like the presence and structure of debris belts, and the planetary system architecture,” the authors write.
There are many other uncertainties, like the extent of nightside ice sheets compared to polar caps. There has to be a lot of ice and the impactors need to strike it. “An impactor has a higher probability of striking ice for nightside-wide ice sheets compared to polar caps,” the researchers explain.
All those uncertainties aside, this work paints a different picture than we’re used to. Instead of terrestrial exoplanet atmospheres evolving from an initial state to an evolutionarily final state, they may be transient.
Rather than being governed purely by bulk properties, episodic regeneration may be at work. “This dynamic view is observationally important, as it suggests detection rates may reflect atmospheric persistence rather than evolutionary endpoints,” the authors write.
This has some implications for how we observe exoplanets and search for atmospheres. “If a planet spends 1 − 10 % of its time with an atmosphere, we should expect a corresponding success rate in detecting it,” the researchers write. One of the three planets, LT 1445 Ab, may have an atmosphere for more than 50% of the time. That means that impact driven atmospheres are a “viable pathway for maintaining detectable atmospheres around rocky exoplanets.”
These results are counterintuitive. In their conclusion, the authors point out that having a frigid nightside may be what protects terrestrial exoplanet atmospheres from being destroyed and stripped away by the flaring from red dwarf stars. The atmosphere basically waits in a frozen state until impacts regenerate it.
“Atmospheric collapse, though typically seen as detrimental to the survival of atmospheres around tidally locked rocky exoplanets, plays a protective role for volatiles by shielding them from atmospheric escape,” the authors write in their conclusion.
Though the nightside can act as a sizable reservoir for volatiles that can reconstitute the atmosphere, too many impacts may be detrimental.
There’s a sweet spot for the impact rate, and for the impactor size, too. If their diameters range from 5 to 10 km, and between 1 and 100 of them strike a single planet in one billion years, a rocky exoplanet could reconstitute its atmosphere.
“Under this metric, rocky planets around M-dwarfs could retain detectable CO2 atmosphere for about 1−45 % of their lifetime under plausible conditions,” the researchers conclude.
Written by Evan Gough/Universe Today.
