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To deliver organic material, comets must travel relatively slowly – at speeds of less than 15 kilometers per second. At higher speeds the essential molecules would not survive; the speed and temperature of the collision would cause them to fall apart.
The most likely places where comets could travel at the right speed are ‘peas in a pod’ systems, where a group of planets orbit close 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 sufficiently low speeds, the comet would crash onto a planet’s surface, delivering the intact molecules that researchers believe are the precursors of life. The results, reported in the Proceedings of the Royal Society A, suggest that such systems would be promising places to search for life beyond our solar system if the arrival of comets is important for the origins of life.
Comets are known to contain a series of building blocks for life, known as prebiotic molecules. For example, samples from the Ryugu asteroid analyzed in 2022 showed that it contained intact amino acids and vitamin B3. Comets also contain large amounts of hydrogen cyanide (HCN), another important prebiotic molecule. HCN’s strong carbon-nitrogen bonds make it more durable against high temperatures, meaning it could potentially survive entry into the atmosphere and remain intact.
“We’re learning more and more about the atmospheres of exoplanets, so we wanted to see if there are any planets where complex molecules could also be delivered by comets,” said first author Richard Anslow of the Cambridge Institute of Astronomy. “It’s possible that the molecules that gave rise to life on Earth came from comets, so the same could be true for planets elsewhere in the Milky Way.”
The researchers are not claiming that comets are necessary for the origins of life on Earth or any other planet, but instead they wanted to put some limits on the types of planets where complex molecules, such as HCN, can be successfully passed by comets could be delivered.
Most of the comets in our solar system are located outside the orbit of Neptune, in what is known as the Kuiper Belt. When comets or other Kuiper Belt Objects (KBOs) collide, they can be pushed toward the Sun by Neptune’s gravity and ultimately pulled inward by Jupiter’s gravity. Some of these comets make their way along the asteroid belt into the inner solar system.
“We wanted to test our theories on planets similar to ours, as Earth is currently our only example of a planet that supports life,” says Anslow. “Which types of comets, traveling at what speed, can 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 must have a low mass and it is helpful if the planet is in a close orbit around other planets in the system. The researchers found that nearby planets in close orbits are much more important to planets around lower-mass stars, where typical velocities are much higher.
In such a system, a comet could be pulled in by the gravity of one planet and then transferred to another planet before impact. If this ‘comet passing’ happened often enough, the comet would slow down enough that some prebiotic molecules could survive entering the atmosphere.
“In these tightly packed systems, every planet has a chance to interact with and trap a comet,” Anslow says. “It is possible that this mechanism is how prebiotic molecules end up on planets.”
For planets orbiting 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 are also significantly more likely to experience high-speed impacts, potentially posing unique challenges for life on these planets.
The researchers say their results could be useful in 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,” Anslow said. “It’s another way to look at the great work that’s already been done on Earth. What molecular pathways have led to the enormous diversity of life we see around us? Are there other planets where the same routes exist? It is an exciting time to combine advances in astronomy and chemistry to study some of the most fundamental questions of all.”
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 fellow of Wolfson College, Cambridge.
RJ Anslow, A. Bonsor and PB Rimmer. ‘Can comets deliver prebiotic molecules to rocky exoplanets?’ Proceedings of the Royal Society A (2023). DOI: 10.1098/rspa.2023.0434
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