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‘Bouncing’ comets could deliver building blocks of life to exoplanets, say researchers at Institute of Astronomy, Cambridge

The molecular building blocks of life could be delivered to planets by ‘bouncing’ comets, University of Cambridge astronomers have shown.

There is a long-standing theory that organic material that kicked off life on Earth arrived in this way.

An illustration of Comet Neowise passing the sun and releasing gases creating a tail and coma.
An illustration of Comet Neowise passing the sun and releasing gases creating a tail and coma.

And by showing how this could happen on exoplanets, researchers have identified systems that could be promising places to search for life outside our own solar system.

Richard Anslow, first author of a new study from Cambridge’s Institute of Astronomy, said: “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

“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.”

Comets need to be travelling at speeds below 15 kilometres per second to deliver organic material as at higher speeds, the essential molecules would not survive. Instead, the speed and temperature of impact would cause them to break apart.

Astronomers say ‘peas in a pod’ systems, where a group of planets orbit closely together, are the most likely places where a comet could travel at these lower speeds, as it is effectively passed or ‘bounced’ from the orbit of one planet to another, slowing it down.

In this scenario, a comet could crash on a planet’s surface, delivering intact molecules that researchers believe to be the precursors for life.

“We wanted to test our theories on planets that are similar to our own, as Earth is currently our only example of a planet that supports life,” said Richard. “What kinds of comets, travelling at what kinds of speed, could deliver intact prebiotic molecules?”

Comets contain a range of ‘prebiotic molecules’. Samples from the Ryugu asteroid, analysed in 2022, showed it carried intact amino acids and vitamin B3.

Comets also contain large amounts of hydrogen cyanide (HCN), another important prebiotic molecule, which has strong carbon-nitrogen bonds that make it more durable to high temperatures. This means it is more likely to survive atmospheric entry.

While the researchers are not claiming comets are necessary to the origin of life on Earth or elsewhere, they were looking to understand the types of planets where complex molecules, such as HCN, could be delivered.

Comet Neowise and its gas and dust tails
Comet Neowise and its gas and dust tails

Most of the comets in our Solar System are beyond the orbit of Neptune in the Kuiper Belt.

When comets and other Kuiper Belt objects (KBOs) collide, they can be pushed by Neptune’s gravity toward the Sun, eventually getting pulled in by Jupiter’s gravity. But some of these comets make their way past the Asteroid Belt and into the inner Solar System.

The researchers used mathematical modelling techniques to work out that it is possible for comets to deliver the precursor molecules for life, but only in certain scenarios.

A planet orbiting a star similar to our own Sun would need to have a low mass and, ideally, be in close orbit to other planets. Having nearby planets would be particularly key in systems with lower-mass stars, where the typical speeds are much higher, as a comet could be pulled in by the gravitational pull of one planet, then passed to another before impact.

If this ‘comet-passing’ happened enough times, the researchers say the comet would slow down enough so that some prebiotic molecules could survive atmospheric entry.

“In these tightly-packed systems, each planet has a chance to interact with and trap a comet,” said Richard. “It’s possible that this mechanism could be how prebiotic molecules end up on planets.”

Where a planet is in orbit around a lower-mass star, such as M-dwarfs, the researchers say it would be more difficult for complex molecules to be delivered by comets, especially if the planets are loosely packed.

In such systems, rocky planets feature significantly more high-velocity impacts.

“It’s exciting that we can start identifying the type of systems we can use to test different origin scenarios,” said Richard. “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 research, published in the Proceedings of the Royal Society A, was supported in part by the Royal Society and the Science and Technology Facilities Council (STFC), part of UK Research and Innovation (UKRI). Richard Anslow is a Member of Wolfson College, Cambridge.

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