Comets Probably Did Not Deliver Life's Ingredients to Earth

Comets are icy travellers from the outer Solar System, and they are known to carry many of the molecules that life is built from. That made them an attractive answer to a big question: where did the chemical building blocks of life on Earth come from? If comets rained those molecules down on the young planet, the ingredients for life could have arrived ready-made from space.

A new study led by the University of Cambridge challenges that idea. Using computer modelling that combined recent advances in comet impact simulations, comet-atmosphere interactions and models of how comets form and evolve, the researchers found that delivering those building blocks by comet would have been, at best, a "one-off fluke" — not the steady rain of prebiotic molecules that had been theorised. The results were published in the journal Scientific Reports on 28 October 2026.

Too fast, too steep

The problem is speed and angle. The modelling found that comets travel too fast and arrive at too sharp an angle for fragile organic molecules to survive the impact with Earth's surface: they would almost certainly vaporise.

"Historically, scientists thought that comets were important for the origin of life on Earth, because they carried carbon- and nitrogen-bearing molecules, so these giant space snowballs were almost like a 'get out of jail free card' that brought all the molecules a planet would need to seed life," said lead author Dr Richard Anslow, from Cambridge's Institute of Astronomy. "It's kind of a romantic idea, but our results suggest it's highly unlikely."

The idea the team tested has been around for close to a century. In 2023, Anslow and several co-authors on the new paper had theorised that in planetary systems where planets orbit closely together, comets might slow down by "bouncing" from one planet to the next, delivering organic molecules as they went. The new work asked whether such a scenario could have worked in our own Solar System — and found it extremely unlikely.

A narrow window, even when delivery worked

For the study, the researchers focused on hydrogen cyanide (HCN), an important prebiotic molecule that is regularly found on comets in our Solar System. Their simulations showed that even in the rare event that HCN molecules survived impact with Earth, impacts delivering a useful concentration were extremely uncommon.

Time was also against the idea. The number of comets hitting Earth declined rapidly in our planet's first 500 million years, leaving at most a handful of opportunities in Earth's early history for HCN delivery on a comet. And even if HCN carried on a comet did reach Earth's surface, it would not have stayed concentrated for long, so any prebiotic chemistry would have needed to progress very quickly from impact — which the researchers say is highly unlikely.

"It would really have to be a one-in-a-million occurrence, because not only would you need an impact at just the right speed and just the right angle, you would also need everything to happen after that point, in terms of the concentration and the speed at which the chemical origins of life got going," Anslow said. "We can't rule out for certain that it happened that way. But it seems to be a really rare occurrence — at best, a one-off fluke."

Mars, asteroids — and Earth itself

The researchers also found that Mars could be a slightly more promising candidate for cometary delivery, because it is smaller than Earth and has a much thinner atmosphere, so comets would be more likely to reach its surface during the period when comets were far more frequent visitors. Even so, the researchers stress that this is still highly unlikely. "This is not to say it definitely happened on Mars, of course, but suggests it would be unlikely if it happened on Earth and didn't happen on Mars," Anslow said. "Or the other, perhaps more likely, alternative is that it didn't happen on either planet."

So where did the molecular seeds of life on Earth come from? The researchers say several possibilities remain, including molecules made after asteroid impacts — asteroids hit Earth far more frequently than comets — or chemical precursors that formed through processes on Earth itself, with no outside help. The results could also help future searches for life elsewhere: identifying planetary systems where "bouncing" comets could exist may help astronomers narrow their search when successors to the James Webb Space Telescope, such as the proposed Habitable Worlds Observatory, come online.

A negative result like this is still progress. It does not answer how life began, but it tells scientists which delivery route probably did not work — and where to look instead. Book your first lesson and compare verified science tutors for help with the Solar System, the origins of life and GCSE or A-level science, online, in person or in a group class.

Sources:

Phys.org, "Comets probably didn't seed life on Earth after all, new study suggests", 28 October 2026, reporting Anslow et al., "Cometary impacts an unlikely pathway towards the origins of life on Earth", Scientific Reports (2026), DOI: 10.1038/s41598-026-74214-y, led by the University of Cambridge: https://phys.org/news/2026-10-comets-didnt-seed-life-earth.html