in a paper published in the Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, researchers at the University of Cambridge analyzed the potential of comet impacts to release the initial prebiotic molecules necessary for the origin of life on rocky exoplanets. They discovered that in order to release prebiotic molecules, comets need to travel relatively slowly, at speeds less than 15 km per second; at higher speeds, the molecules would not survive: the speed and temperature of the impact would cause them to break apart. The most likely place where comets can travel at the right speed is compact planetary systems, where a group of planets orbit close together; In such a system, the comet could essentially pass or “bounce” from the orbit of one planet to another, slowing it down.
Cometary impacts on the surface of early Earth are a potential way to supply the complex organic molecules necessary for the origins of life. For this to be successful, the prebiotic molecules must survive the high temperatures experienced during impacts, so low-velocity impacts are required. Anslow et al. Determine which types of rocky planets withstand low-speed impacts by calculating how the central star and other nearby planets affect the comets’ orbits. Image credit: Murayama, Osaka University.
Comets are known to contain a variety of building blocks for life, known as prebiotic molecules.
For example, samples from the asteroid Ryugu, analyzed in 2022, presented that carried intact amino acids and vitamin B3.
Comets also contain large amounts of hydrogen cyanide, another important prebiotic molecule.
Hydrogen cyanide’s strong carbon-nitrogen bonds make it more durable at high temperatures, meaning it could survive entry into the atmosphere and remain intact.
“We’re learning more about exoplanet atmospheres all the time, so we wanted to see if there are planets where comets can also carry complex molecules,” he said. Dr Richard Anslowresearcher at the Institute of Astronomy of the University of Cambridge.
“It is possible that the molecules that gave rise to life on Earth came from comets, so the same could be true for other planets in the Milky Way.”
Most comets in our Solar System are found beyond the orbit of Neptune, in what is known as the Kuiper Belt.
When comets or other Kuiper Belt objects collide, Neptune’s gravity can pull them toward the Sun and eventually be pulled by Jupiter’s gravity.
Some of these comets pass through the asteroid belt and enter the inner Solar System.
“We wanted to test our theories on planets similar to our own, since Earth is currently our only example of a planet that supports life,” Dr. Anslow said.
“What types of comets, traveling at what speed, could release intact prebiotic molecules?”
Using a variety of mathematical modeling techniques, the researchers determined that it is possible for comets to release life’s precursor molecules, but only in certain scenarios.
For planets orbiting a star similar to our own Sun, the planet must be low mass and it is useful for it to be in close orbit with other planets in the system.
Scientists found that nearby planets in close orbits are much more important for planets around lower-mass stars, where typical velocities are much higher.
In such a system, a comet could be attracted by the gravitational pull of one planet and then pass to another planet before impact.
If this “comet pass” occurred enough times, the comet would slow down enough that some prebiotic molecules could survive entry into the atmosphere.
“In these very compact systems, each planet has the possibility of interacting with and trapping a comet,” Dr. Anslow said.
“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 comets to deliver complex molecules, especially if the planets are loosely packed.
Rocky planets in these systems also experience significantly more high-speed impacts, potentially posing unique challenges for life on these planets.
The results could be useful in determining where to look for life outside the Solar System.
“It’s exciting that we can begin to identify the type of systems we can use to test different origin scenarios,” Dr. Anslow said.
“It’s a different way of looking at the great work that has 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 paths 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.”
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RJ responds et al. 2023. Can comets transport prebiotic molecules to rocky exoplanets? Proc. R. Soc. A 479 (2279): 20230434; two: 10.1098/rspa.2023.0434
