What can exoplanets orbiting M-dwarf stars teach scientists about planetary formation and evolution? This is what a recent study submitted to the American Astronomical Society journals hopes to address as a team of researchers investigated the possibility of exo-Titans, exoplanets with atmospheres comprised of nitrogen and methane like Saturn’s moon Titan, orbiting M-dwarf stars, which are smaller and cooler than our Sun. this study has the potential to help scientists better understand the formation and evolution of exoplanets orbiting M-dwarf stars and whether they could possess life as we know it.
For the study, the researchers used a series of computer models called Photocem to simulate photochemistry on exo-Titans, specifically estimating the lifetimes of atmospheric methane. Along with methane, the models examined the presence of hydrogen, nitrogen, oxygen, and carbon, which are part of the Photocem network. Additionally, the team used the exoplanet TRAPPIST-1e, which orbits an M-dwarf star, as part of the model by treating it as an exo-Titan. In the end, the researchers found that methane on an exo-Titan TRAPPIST-1e would have very short lifetimes, along with estimating between a 1 to 10 percent chance of astronomers detecting a warm exo-Titan.
The study notes, “This finding is consistent with recent JWST nondetections of CH4-dominated atmospheres on warm terrestrial exoplanets. The low prior probability means that the standard of proof required to claim a warm exo-Titan detection is high, and we offer specific suggestions towards such a standard of proof. Observation of oxidized carbon species would corroborate a putative warm exo-Titan detection. Confirmed detection of warm exo-Titans would signal the need to fundamentally rethink our understanding of the structure, dynamics, and photochemistry of Titan-like worlds.”
As noted, TRAPPIST-1e was chosen for this study since it orbits an M-dwarf star, the latter of which are increasingly getting attention for their exoplanets potentially being habitable. While TRAPPIST-1e takes only 6.1 days to complete one orbit (for context, the planet Mercury orbits in 88 days), it is orbiting squarely in the middle of its star’s habitable zone, making it an ideal target for astrobiology and the search for life beyond Earth. Additionally, M-dwarf stars have much longer lifetimes than our Sun potentially reaching trillions of years compared to the 10-billion-year lifetime of our Sun. This much longer lifetime could enable their exoplanets to develop the necessary components for life as we know it, or even as we don’t know it.
To complement using TRAPPIST-1e as an analog, the researchers chose Titan as an analog due to its potential biosignatures like nitrogen and methane, with methane being the dominant atmospheric composition on Saturn’s largest moon. Titan has long been hailed as a target for astrobiology due to these biosignatures, with past studies even suggesting it could mimic conditions that were present on ancient Earth. While Titan orbits much farther beyond our Sun’s habitable zone, a potential exo-Titan orbiting within its star’s habitable zone could provide a unique opportunity for astronomers to find life orbiting an M-dwarf star. However, if the findings prove to be correct regarding the low probability of finding a warm exo-Titan, finding life there might not be the case.
This study comes as the number of confirmed exoplanets recently reached 6,000, along with NASA preparing to send its Dragonfly quadcopter to Saturn’s largest moon, which is currently scheduled to be launched in July 2028 and an estimated arrival date at Titan of 2034. Therefore, studies like this can help astronomers better understand what types of exoplanets exist throughout the universe, and whether they could be habitable for life.
What new discoveries about exo-Titans will researchers make in the coming years and decades? Only time will tell, and this is why we science!
As always, keep doing science & keep looking up!