Cosmic Billiards: How “Bouncing” Comets Might Seed Life Across the Galaxy

Cambridge researchers propose that comets could transport life’s building blocks to other planets, especially in ‘peas in a pod’ systems. Their findings suggest that these molecules can survive on planets with closely orbiting neighbors, offering new insights into the search for extraterrestrial life.

How did the molecular building blocks for life end up on Earth? One long-standing theory is that they could have been delivered by comets. Now, researchers from the University of Cambridge have shown how comets could deposit similar building blocks to other planets in the galaxy.

The Role of Comet Speed and ‘Peas in a Pod’ Systems

In order to deliver organic material, comets need to be traveling relatively slowly – at speeds below 15 kilometers per second. At higher speeds, the essential molecules would not survive – the speed and temperature of impact would cause them to break apart.

The most likely place where comets can travel at the right speed are ‘peas in a pod’ systems, where a group of planets orbit closely together. In such a system, the comet could essentially be passed or ‘bounced’ from the orbit of one planet to another, slowing it down.

At slow enough speeds, the comet would crash on a planet’s surface, delivering the intact molecules that researchers believe are the precursors for life. The results, reported on November 15 in the Proceedings of the Royal Society A, suggest that such systems would be promising places to search for life outside our Solar System if cometary delivery is important for the origins of life.

Comets: Carriers of Prebiotic Molecules

Comets are known to contain a range of the building blocks for life, known as prebiotic molecules. For example, samples from the Ryugu asteroid, analyzed in 2022, showed that it carried intact amino acids<div class="cell text-container large-6 small-order-0 large-order-1">
<div class="text-wrapper"><br />Amino acids are a set of organic compounds used to build proteins. There are about 500 naturally occurring known amino acids, though only 20 appear in the genetic code. Proteins consist of one or more chains of amino acids called polypeptides. The sequence of the amino acid chain causes the polypeptide to fold into a shape that is biologically active. The amino acid sequences of proteins are encoded in the genes. Nine proteinogenic amino acids are called "essential" for humans because they cannot be produced from other compounds by the human body and so must be taken in as food.<br /></div>
</div>” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>amino acids and vitamin B3. Comets also contain large amounts of hydrogen cyanide (HCN), another important prebiotic molecule. The strong carbon-nitrogen bonds of HCN make it more durable to high temperatures, meaning it could potentially survive atmospheric entry and remain intact.

“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,” said first author Richard Anslow from Cambridge’s Institute of Astronomy. “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.”

The researchers do not claim that comets are necessary to the origin of life on Earth or any other planet, but instead, they wanted to place some limits on the types of planets where complex molecules, such as HCN, could be successfully delivered by comets.

Cometary Paths and the Influence of Solar Systems

Most of the comets in our Solar System sit beyond the orbit of NeptuneNeptune is the farthest planet from the sun. In our solar system, it is the fourth-largest planet by size, and third densest. It is named after the Roman god of the sea.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>Neptune, in what is known as the Kuiper BeltThe Kuiper Belt is a region in our outer solar system where a large number of "short-period" comets, with orbits less than 200 years, can be found. It is located from about 30 to 50 astronomical units (AU) away from the Sun, starting near Neptune's orbit. It is estimated that the Kuiper Belt could contain up to 100 million comets.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>Kuiper Belt. When comets or other Kuiper Belt objects (KBOs) collide, they can be pushed by Neptune’s gravity toward the Sun, eventually getting pulled in by JupiterJupiter is the largest planet in the solar system and the fifth planet from the sun. It is a gas giant with a mass greater then all of the other planets combined. Its name comes from the Roman god Jupiter.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>Jupiter’s gravity. Some of these comets make their way past the Asteroid Belt and into the inner Solar System.

“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 Anslow. “What kinds of comets, traveling at what kinds of speed, could deliver intact prebiotic molecules?”

Using a variety of mathematical modeling techniques, the researchers determined that it is possible for comets to deliver the precursor molecules for life, but only in certain scenarios. For planets orbiting a star similar to our own Sun, the planet needs to be low mass and it is helpful for the planet to be in close orbit to other planets in the system. The researchers found that nearby planets on close orbits are much more important for planets around lower-mass stars, where the typical speeds are much higher.

In such a system, a comet could be pulled in by the gravitational pull of one planet, and then passed to another planet before impact. If this ‘comet-passing’ happened enough times, 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 Anslow. “It’s possible that this mechanism could be how prebiotic molecules end up on planets.”

For planets in orbit around lower-mass stars, such as M-dwarfs, it would be more difficult for complex molecules to be delivered by comets, especially if the planets are loosely packed. Rocky planets in these systems also suffer significantly more high-velocity impacts, potentially posing unique challenges for life on these planets.

Implications for the Search for Extraterrestrial Life

The researchers say their results could be useful when determining where to look for life outside the Solar System.

“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.”

Reference: “Can comets deliver prebiotic molecules to rocky exoplanets?” by R. J. Anslow, A. Bonsor and P. B. Rimmer, 15 November 2023. Proceedings of the Royal Society A Mathematical Physical and Engineering Sciences.
DOI: 10.1098/rspa.2023.0434

The research 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.