The building blocks of life have been found on comets, asteroids, and in interstellar space. But how they end up on the surface of planets is not exactly clear. One idea is that comets and asteroids can ferry these molecules to worlds where life can emerge, but they need to travel relatively slowly for molecules to survive the impact – no more than 15 kilometers (9 miles) per second.

Comets originate from the edges of star systems, so coming in towards the habitable zone they have a lot of speed. Researchers have worked out that it is possible for such comets to slow down if planets are arranged in a certain way. The architecture of a star system where Earth-like planets all orbit in close proximity (the “peas-in-a-pod” configuration) seems to be ideal to slow down comets.

These planets orbit stars smaller than our Sun and their close proximity often reveals resonant orbits, which means intriguing and complex gravitational interactions. Interactions that can make comets bounce from one orbit to another are enough to slow down a comet, so that it can get down onto a planet without destroying all its precious molecular cargo.

“In these tightly-packed systems, each planet has a chance to interact with and trap a comet,” first author Richard Anslow, from the University of Cambridge’s Institute of Astronomy, said in a statement. “It’s possible that this mechanism could be how prebiotic molecules end up on planets.”

It is not certain that comets did deliver such molecules, and this is not the only way to slow down comets (gas giants in a solar system can also shift them), but the team was interested in how particular molecules might have been spread in an intriguing multi-planet system in which several planets are in the habitable zone.  

“We’re learning more about the atmospheres of exoplanets all the time, so we wanted to see if there are planets where complex molecules could also be delivered by comets,” explained Anslow. “It’s possible that the molecules that led to life on Earth came from comets, so the same could be true for planets elsewhere in the galaxy.”

Observations by JWST and other telescopes have begun to deliver views of the atmospheres (or lack thereof) around these rocky worlds. And soon, astronomers might discover the presence of interesting chemicals in these atmospheres. This work suggests that some systems might be better bets than others, insofar as it is comets that delivered those molecules.

“It’s exciting that we can start identifying the type of systems we can use to test different origin scenarios,” said Anslow.

“It’s a different way to look at the great work that’s already been done on Earth. What molecular pathways led to the enormous variety of life we see around us? Are there other planets where the same pathways exist? It’s an exciting time, being able to combine advances in astronomy and chemistry to study some of the most fundamental questions of all.”

The study is published in Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences.