A billion things had to go just right for Earth to become the life-supporting world it is today. Our planet is just the right distance from the right type of star. It has a hot, convective core that generates its protective shield. And early in its history, it was the recipient of organic molecules and water that shepherded Earth towards habitability. Without impacts from both rocky and icy Solar System bodies, Earth wouldn’t have received these materials, and it wouldn’t be the planet it is today.
The same must be true of exoplanets.
The search for habitable worlds focuses on Earth-like worlds orbiting low-mass stars. These M-dwarf stars are prevalent and are known to host many rocky worlds. Since they’re so dim, their habitable zones are smaller. Many planets in M-dwarf habitable zones are so close to their stars that the planets are tidally-locked. From this perspective, Earth is an outlier.
Tidally-locked worlds are desirable targets for modelling planetary climates precisely because they don’t rotate. Exoplanets are a long ways away, and astronomers don’t know what their spin rates are or what their obliquities are. The spin rate determines length of day and obliquity determines seasons. Without those crucial parameters, it’s impossible to understand the details of an exoplanet’s atmosphere and its potential habitability. By focusing on tidally-locked exoplanets, scientists can sidestep that restriction.
Researchers from the School of Physics and Astronomy at the University of Leeds in the UK wanted to understand how the atmospheres of these types of planets are affected by impacts from icy bodies. They used comet impact models and a 3D Earth system model to determine how cometary impacts can change an exoplanet’s atmosphere. It’s titled “The Response of Planetary Atmospheres to the Impact of Icy Comets. I. Tidally Locked Exo-Earths,” and it’s published in The Astrophysical Journal. The lead author is Felix Sainsbury-Martinez, a Research Fellow in Astrophysics at the University of Leeds.
” … individual cometary impacts, particularly massive impacts, have the potential to drive long-lasting changes in the climate.” – from “The Response of Planetary Atmospheres to the Impact of Icy Comets. I. Tidally Locked Exo-Earths.”
“Impacts by rocky and icy bodies are thought to have played a key role in shaping the composition of solar system objects, including the Earth’s habitability. Hence, it is likely that they play a similar role in exoplanetary systems,” the authors write. “We investigate how an icy cometary impact affects the atmospheric chemistry, climate, and composition of an Earth-like, tidally locked, terrestrial exoplanet, a prime target in the search for a habitable exoplanet beyond our solar system.”
Scientists know that material delivered by comets has played an important role in shaping not only Earth but other bodies in the Solar System. Earth, Mercury, Venus and Mars all received water, while Jupiter’s metallicity was raised by impacts. Even the movement of dust in the Solar System could’ve played a role in delivering organic molecules and volatiles like water.
“Under the assumption that exoplanetary systems form in a similar manner to our own solar system, we can infer that planetary bombardment and cometary/asteroidal impacts should also have played a significant role in shaping the composition, atmosphere, and hence habitability of exoplanets,” the researchers write.
The team used a comet impact model and a climate model to understand how a pure water ice comet with a radius of 2.5 km and a density of 1 g cm−3 would affect a planet similar to TRAPPIST-1e. This exoplanet is often used in studies because it’s in its star’s habitable zone, and is also easily observed. Scientists have measured its density, gravity, and composition with more certainty than other exoplanets. It’s both a little smaller and a little less massive than Earth. In their simulations, they gave TRAPPIST-1e a “preindustrial atmospheric composition and an Earth-like land–ocean distribution, including orography (mountains) and a dynamic ocean.”
Their work showed that even a single icy comet can impact an exoplanet’s climate for years by delivering both water and heat. Initially, much of the delivered water is held in the upper atmosphere due to thermal ablation. Atmospheric pressure plays a large role, allowing the water to persist in the upper atmosphere where the pressure is lower. The higher atmospheric pressure near the planet’s surface resists the infiltration of water. “As such, the same mass of water has a much smaller effect on the atmospheric composition near the surface compared to low pressures,” the researchers explain. This could aid in the detection of comet-delivered water since transmission spectra typically probe low-pressure regions. This is fortunate, because water in the lower-pressure upper and mid-atmospheres has the greatest effect on climate.
This figure shows the difference in response to the cometary water deposition of different layers of the atmosphere. It shows the time evolution of the annual mean (solid lines) and monthly mean (faint lines) fractional water abundance (top row) and temperature (bottom) in the outer atmosphere, mid-atmosphere, and near the surface of the coupled model (i.e., both water and thermal deposition—green) and our nonimpacted reference state (gray). “These pressure regions were chosen in order to emphasize the pressure dependence of the atmospheric response to a cometary impact,” the authors write. Image Credit: F. Sainsbury-Martinez et al 2025 ApJ 982 29
A comet impact also has an overall effect of warming the atmosphere. But since the warming also expands the atmosphere, the effect near the surface is minimal. The warming effect is most pronounced in the mid-atmosphere. In any case, the changes to the global mean atmospheric temperature from a single comet are more short-lived than the effect of the delivered water.
Water vapour on the dayside increases the atmosphere’s opacity, but as the vapour mixes in the atmosphere, it gets deposited on the cooler night-side of the tidally-locked planet. On that side, it has a limited effect on the atmosphere as a greenhouse gas.
This figure shows the fractional water abundance at nine different points in time, ranging from preimpact (top left), to near steady state 19 yr postimpact (bottom right). Initially water is deposited by the comet at and around the substellar point (top middle); however, almost immediately the strong zonal winds at this pressure level drive eastward advection. As such, a little over 2 months post-impact, the researchers found that the impact-delivered water is almost completely longitudinally homogenized. Image Credit: F. Sainsbury-Martinez et al 2025 ApJ 982 29
Water changes a planet’s atmosphere in several ways. It acts as a heat source in the mid-atmosphere, and also affects the atmospheric chemistry, composition, and climate. UV from the star can break water molecules apart, and the oxygen and hydrogen atoms can foster chemical reactions that produce oxygen- and hydrogen-rich molecules like methane and nitrous oxide, both of which are greenhouse gases. Water vapour can also condense into rain, snow, or ice crystals, which can form clouds and scatter incoming radiation.
Much of the effect of a cometary impact is short-lived. The authors explain that it might be possible to detect these changes in an exoplanet atmosphere. “We finish by investigating if the changes in atmospheric chemistry, composition, and climate that result from the impact of a single pure water ice comet with a tidally locked Earth-like exoplanet might be observable,” they write.
This figure shows example transmission spectra from an icy comet impact for different time-frames post-impact. Some spectroscopic features of interest are shown, including the increase in methane and nitrous-oxide due to water photolysis. Image Credit: F. Sainsbury-Martinez et al 2025 ApJ 982 29
While short-lived effects are easily anticipated, it’s the longer outlook that is more critical when it comes to habitability. “However, some long-lasting changes to the temperature structure of the atmosphere are present,” the authors note.
The simulations showed that cometary impacts can shift “multiyear oscillations in atmospheric temperature (and water vapor content).” These oscillations are driven by the exoplanet’s Earth-like orography and it’s tidally-locked insolation. “This suggests that even individual cometary impacts, particularly massive impacts, have the potential to drive long-lasting changes in the climate,” the authors write.
The results show that the water delivered by a comet enriches the atmosphere at mid and upper levels for at least 10 years post-impact, while the effect from heating is less pronounced. The water increases the atmospheric opacity on the dayside, where the surface temperature can drop by as much as 2 degrees Kelvin.
As photolysis breaks H2O molecules apart, it produces molecules including NO2, which leads to ozone destruction. Winds influence this, since they distribute the water vapour globally, and no photolysis is possible on the nightside.
Overall, the authors say that the effects of a singly icy comet impact on a tidally-locked, Earth-like planet are most visible in the first year. “However, as the atmosphere settles back toward the nonimpacted reference state, we find that the long-lasting changes associated with a single cometary impact fall below the noise floor of modern near-infrared telescopes, making them unlikely to be observed,” the authors write.
We would have to be extremely lucky to spot this. The researchers explain that “such an event is rather unlikely for a terrestrial planet with a potentially habitable secondary atmosphere.” Some researchers say that this type of impact likely only happens on average of every 190,000 years for any given planet. But since we’re discovering more and more exoplanets, there’s at least a chance of detecting one in the future.
However, young solar systems are chaotic places. It’s possible that there are far more icy comet impacts during an exoplanet’s formative years, and that could shape a planet’s destiny. “It is possible that repeated or ongoing bombardment might drive large-scale and long-term changes, which might even play a role in shaping planetary habitability,” the authors write in their conclusion. “This is particularly true for young planets, where we expect the bombardment rate to be significantly higher.”
It’s likely that Earth endured repeated impacts during its early years, and scientists are still working to understand how that shaped our planet. The researchers will address that scenario in future work by modelling icy impacts on a true exo-Earth-analog planet with day/night cycles.