Researchers at the University of Cambridge have proposed that comets could transport the building blocks of life to other planets, particularly in "peas in a pod" systems. Their results show that these molecules can survive on planets orbiting closely adjacent each other, providing new insights into the search for extraterrestrial life.
How did the molecular building blocks of life appear on Earth? One long-standing theory is that they may have been sent by comets. Now, researchers from the University of Cambridge have shown how comets deposit similar building blocks onto other planets in the Milky Way.
To transport organic material, comets need to travel relatively slowly - less than 15 kilometers per second. At higher velocities, elementary molecules wouldn't survive—the speed and temperature of the impact would cause them to fragment.
In the "Wandoujia" system, the comet is most likely to be traveling at the right speed. In such a system, a comet can pass or "bounce" from the orbit of one planet into the orbit of another, slowing it down.
At slow enough speeds, comets can impact planetary surfaces, bringing with them intact molecules that researchers believe are the precursors to life. The results, published Nov. 15 in Proceedings of the Royal Society A, suggest that if cometary delivery is important for the origin of life, then such systems would be a good place to search for life outside the solar system.
Comets: carriers of prebiomolecules
Comets are known to contain a range of life's building blocks, so-called prebiomolecules. For example, analysis of a sample of the Ryugu asteroid in 2022 showed that it contained complete 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 resistant to high temperatures, meaning it has the potential to remain intact after entering the atmosphere.
Lead author Richard Anslow, from the Institute of Astronomy at the University of Cambridge, said: "Our knowledge of exoplanet atmospheres is increasing all the time, so we wanted to see if complex molecules from planets could also be delivered by comets. It's possible that the molecules that gave rise to life on Earth came from comets, and therefore planets elsewhere in the galaxy."
The researchers are not claiming that comets are necessary for the origin of life on Earth or any other planet. Instead, they hope to impose some constraints on the types of planets to which comets can successfully deliver complex molecules like HCN.
Comet Paths and Solar System Impact
Most comets in the solar system lie 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 toward the Sun by Jupiter's gravity. Some of these comets pass through the asteroid belt and into the inner solar system.
"We wanted to test our theory on a planet similar to ours, since Earth is our only planet currently supporting life," Anslow said. "What kind of comet, traveling at what speed, could bring intact prebiotic molecules?"
Using a variety of mathematical modeling techniques, the researchers determined that comets have the potential to bring precursor molecules to life, but only under certain circumstances. For a planet to orbit a star similar to our sun, the planet's mass must be low, and the planet's orbit must be close to the other planets in the system. The researchers found that for planets around lower-mass stars, it is much more important for nearby planets to orbit closer because the typical velocities of these stars are much higher.
In such a system, a comet might be pulled in by one planet's gravity and pass by another before impacting. If such 'comet crossings' occur often enough, the comet's speed will slow down, allowing some prebiotic molecules to survive entry into the atmosphere.
"In these dense systems, each planet has a chance of interacting with a comet and trapping it," Anslow said. "It's possible that this mechanism is how prebiotic molecules end up on the planet."
For planets orbiting low-mass stars, such as M-dwarfs, it is more difficult for comets to transport complex molecules into their orbits, especially if the planet is loosely structured. The rocky planets in these systems would also be subject to more high-speed impacts, which could pose unique challenges for life on these planets.
Impact on the search for extraterrestrial life
The researchers say their findings could help determine where to look for life outside the solar system.
"What's exciting is that we can start to identify the types of systems that we can use to test different origin scenarios," Anslow said. "It's another way of looking at the great work that's been done here on Earth. What kind of molecular pathways lead to such a rich variety of life around us? Are the same pathways present on other planets? This is an exciting time when we are able to combine advances in astronomy and chemistry to study some of the most fundamental questions."