How did the molecular building blocks for life end up on Earth? A long-standing theory is that they may have been delivered by comets. Now, researchers at the University of Cambridge have shown how comets can lay similar building blocks to other planets in the galaxy.
To deliver organic matter, comets must travel relatively slowly—less than 15 kilometers per second. At higher velocities, essential molecules do not survive – the speed and temperature of the impact will cause them to break apart.
Where comets can travel at the right speed are 'pieces in a pot’ systems, where a group of planets orbits closely together. In such a system, the comet can be sent or 'bounced’ from one planet’s orbit to another, slowing it down.
At slow enough speeds, the comet could crash into a planet’s surface, delivering intact molecules that researchers believe are precursors to life. The results are reported in Proceedings of the Royal Society aIf cometary birth is important for the origin of life, suggest that such systems could be promising places to search for life outside our solar system.
Comets are known to contain a range of building blocks for life, known as prebiotic molecules. For example, Ryugu asteroid samples analyzed in 2022 showed that it carried intact amino acids and vitamin B3. Comets also contain high levels of another important prebiotic molecule, hydrogen cyanide (HCN). HCN’s strong carbon-nitrogen bonds are more stable at higher temperatures, meaning it survives atmospheric entry and remains intact.
„We are always learning more about the atmospheres of exoplanets, so we want to see if there are planets that could be supplied with complex molecules by comets,” said first author Richard Anslow, from the Institute of Astronomy in Cambridge. „The molecules that led to life on Earth probably came from comets, so the same applies to other planets in the galaxy.”
The researchers did not claim that comets were necessary for the origin of life on Earth or any other planet, but rather wanted to place some limits on the types of planets that comets could successfully deliver complex molecules like HCN.
Most of the comets in our solar system sit beyond the orbit of Neptune, in what is known as the Kuiper Belt. When comets or other Kuiper Belt Objects (KBOs) collide, they are pushed toward the Sun by Neptune’s gravity and eventually pulled by Jupiter’s gravity. Some of these comets cross the asteroid belt into the inner solar system.
„We wanted to test our theories on planets like ours because Earth is currently the only example of a planet that can support life,” Onslow said. „What kind of comets, traveling at what kind of speed, could deliver intact prebiotic molecules?”
Using various mathematical modeling techniques, the researchers determined that it is possible that comets could provide precursor molecules for life, but only under certain circumstances. For planets orbiting a star similar to our own Sun, the planet must be less massive, and it helps that the planet orbits closer to the other planets in the system. The researchers found that nearby planets are more important in orbits closer to planets around low-mass stars, where typical velocities are much higher.
In such a system, a comet could be pulled by the gravitational pull of a planet and then sent to another planet before impact. If this 'comet-entrainment’ happens enough times, the comet will slow down enough so that some prebiotic molecules can survive atmospheric entry.
„In these tightly packed systems, each planet has the potential to interact with and become entangled with a comet,” Anslow said. „This mechanism is likely how prebiotic molecules end up on planets.”
For planets orbiting low-mass stars such as M-dwarfs, it would be more difficult for complex molecules to be supplied by comets, especially if the planets are loosely packed. Rocky planets in these systems also experience high-velocity impacts, posing unique challenges to life on these planets.
The researchers say their results could be useful when deciding where to look for life outside the solar system.
„It’s exciting to start identifying the type of systems we can use to test different look 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 variety of life we see around us? Are there other planets with similar pathways? It’s an exciting time to combine advances in astronomy and chemistry to study some of the most fundamental questions of all.”