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Comets that have hit Earth have been a mixed bag. Early in Earth's history, during the solar system's chaotic beginning, they were likely the source of our planet's water, ultimately making up about 0.02% of the planet's mass. (Mars and Venus a similar fraction.)

Comets brought and , but later posed a threat to the same in cometary collisions. A comet (or asteroid) likely caused the Tunguska Event in 1908 in Russia, and a comet fragment likely triggered the rapid climate shift of the Younger Dryas 12,800 years ago, with its widespread extinctions.

If such collisions happen here, they likely take place in other solar systems as well. Now three scientists in the United Kingdom have modeled the impacts of an icy cometary collision with an Earth-like, tidally locked terrestrial planet. Such objects are prime candidates in the search for habitable exoplanets outside our solar system.

They found even relatively small cometary impacts can significantly disrupt the climate of a terrestrial (Earth-like) tidally locked planet, as well as deliver oxygen to the atmosphere and be a source of an exoplanet's oceans. In fact, we may even be able to observe them with today's space-based telescopes.

Their first of two papers on the topic was in The Astrophysical Journal.

In fact, tidally locked exoplanets, which always show the same side to their star, may have an enhanced rate of cometary impacts compared to Earth. That's because many of them orbit in the habitable zone of M dwarfs, a region quite close to these cool stars.

At such small orbital distances, exoplanets would have orbital speeds higher than Earth's (Kepler's second law), and this, when combined with the star's focusing effect of cometary intruders, would create higher impact rates.

For example, the TRAPPIST-1 planetary system about 40 light-years from Earth has a cool, red M dwarf star ("red dwarf") with seven known exoplanets, all at highly circular orbital distances between 0.01 and 0.06 AUs (astronomical units), with orbital periods ranging from 1.5 to 19 days. Three or four of these exoplanets may be in the star's habitable zone, where liquid water could exist on the exoplanet's surface.

M dwarfs are the most common type of star in the Milky Way galaxy, comprising about 75% of all stars. Such close proximity to an M dwarf might also influence an exoplanet's atmospheric dynamics and chemistry, which would in turn affect the response of the atmosphere to impacts from comets.

These close-in exoplanets can undergo significant angular momentum exchange with their star via tidal torques. These are the types of exoplanets Sainsbury-Martinez and his two colleagues focused on to investigate.

To study the effect of a single cometary impact, the group coupled a 2024 cometary impact model with a common climate model that had previously been used to explore the atmospheric dynamics and chemistry of Earth-analog exoplanets and tidally locked exoplanets.

The cometary impact model included the physics of the dynamics of the comet's breakup and thermal ablation (melting) of the comet's surfaces. They assumed a 2.5 km radius comet made of pure water ice coming in perpendicular to the exoplanet's surface, delivering water and thermal energy onto an Earth-like atmosphere of the exoplanet TRAPPIST-1e, a significant object of interest in the search for a habitable, Earth-like exoplanet. (Such a comet would have a mass of about 65 gigatonnes, just over one-third that of Mt. Everest.)

As the comet enters the atmosphere, the density of the atmosphere increases, but so too does the atmospheric drag and stress on the comet, increasing thermal ablation (melting and evaporation).

Eventually, this ram pressure exceeds the tensile strength of the comet, and it begins to breakup. That process can be very complicated, but it's known that only considering ram-driven breakup is sufficient to reproduce the breakup locations of, for example, the Shoemaker-Levy 9 comet train that impacted Jupiter in 1994.

An exponentially decaying function was used to ensure that the breakup of all the comet's material and its kinetic energy occurs in the atmosphere, before the surface is reached.

Running the coupled models, the group found that it took about 20 years for the model's atmosphere to return to an approximate steady state. The comet changed the atmosphere's water content, with most of the water being delivered at pressures greater than the 10 Pascal (Pa) level (surface pressure on Earth is 101,000 Pa).

After one month of simulation time, there was a several order-of-magnitude increase for pressures below 100 Pa. Being in the outer atmosphere, on the surface there was almost no response to the influx of water, chiefly due to the atmosphere's exponentially increasing pressure near the surface. There was the longest-lasting enhancement in atmospheric water in the mid-atmosphere for over 15 years post-impact.

"Even a relatively small cometary impact can significantly disrupt the climate of a terrestrial (Earth-like) planet," said Sainsbury-Martinez, "with the changes being strong enough that we may even be able to observe them using space-based telescopes such as the current James Webb Space Telescope (JWST) or the future Habitable Worlds Observatory (HWO)."

For a follow-up paper, he is looking at a similar impact with an Earth-like planet—the mass of TRAPPIST-1e is only 70% that of Earth's—that is not tidally locked.

He expects that the differences from the present model—driven by the differences in circulation/winds—will be significant due to the horizontal transport around the planet in the atmosphere—while differences driven by the differences in circulation/winds are significant with horizontal transport playing a more important role in mixing.

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