Under Some Conditions, Comets Could Deliver Organic Molecules to Planets

Approximately 4.1 to 3.8 billion years ago, the planets of the inner Solar System experienced many impacts from comets and asteroids that originated in the outer Solar System. This is known as the Late Heavy Bombardment (LHB) period when (according to theory) the migration of the giant planets kicked asteroids and comets out of their regular orbits, sending them hurtling towards Mercury, Venus, Earth, and Mars. This bombardment is believed to have distributed water to the inner Solar System and maybe the building blocks of life itself.

According to new research from the University of Cambridge, comets must travel slowly – below 15 km/s (9.32 mi/s) – to deliver organic material onto other planets. Otherwise, the essential molecules would not survive the high speed and temperatures generated by atmospheric entry and impact. As the researchers found, such comets are only likely to occur in tightly bound systems where planets orbit closely to each other. Their results show that these systems would be a good place to look for evidence of life (biosignatures) beyond the Solar System.

The research was conducted by Richard Anslow and Amy Bonsor, a Ph.D. Student and a Royal Society University Research Fellow from the Institute of Astronomy at the University of Cambridge (respectively). They were joined by Paul Rimmer, an SCOL Senior Fellow with the Cavendish Laboratory’s Astrophysics Group at the University of Cambridge. Their paper, titled “Can comets deliver prebiotic molecules to rocky exoplanets?” appeared on November 15th in the Proceedings of the Royal Society A.

In our Solar System, most comets originate in the Kuiper Belt, the circumstellar disk extending 30 astronomical units (AUs) – beyond Neptune’s orbit – to approximately 50 AU. When Kuiper Belt objects (KBOs) collide, they can get “kicked” by Neptune’s gravity towards the Sun, eventually getting captured by Jupiter’s gravity. Some of these comets will then be hurled past the Asteroid Belt and make their way into the inner Solar System. These comments will grow “tails” as they approach the Sun as rising temperatures cause their frozen volatiles to sublimate.

Scientists have also learned that comets can contain prebiotic molecules, which are the building blocks of life. This includes hydrogen cyanide, methanol, formaldehyde, ethanol, ethane, and more complex molecules like long-chain hydrocarbons and amino acids. For example, samples returned from the Ryugu asteroid in 2022 showed evidence of intact amino acids and nicotinic acid, an organic molecule otherwise known as vitamin B3. However, not all these elements can remain intact when entering a planet’s atmosphere and hitting the surface. As Anslow said in a University of Manchester press release:

“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. 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.

“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. What kinds of comets, traveling at what kinds of speed, could deliver intact prebiotic molecules? In these tightly packed systems, each planet has a chance to interact with and trap a comet. It’s possible that this mechanism could be how prebiotic molecules end up on planets.”

For their research, the team sought to place some limits on the types of planets where comets could successfully deliver complex molecules. Using various mathematical models, the researchers determined that comets can deliver the precursor molecules for life, but only in certain scenarios. Their results showed that the most likely place to find comets that travel at the right speeds are “peas in a pod” systems, which are made up of planets that orbit closely together. In these systems, comets can be attracted by the gravitational pull of one planet and then “bounced” to another before impact.

If the comet is transferred from one orbit to another enough, it will slow down enough that some prebiotic molecules could survive atmospheric entry. Their results also showed that for Sun-like stars, the odds of prebiotic molecules surviving were even better if the planets were low-mass. But for planets orbiting low-mass stars (such as M-type red dwarfs), closely-orbiting planets were especially important. If rocky planets in these systems were loosely packed, they would suffer far more high-velocity impacts, creating a significant challenge for life on these planets.

These results could help astronomers determine where to look for signs of life (biosignatures) beyond our Solar System. Said Anslow:

“It’s exciting that we can start identifying the type of systems we can use to test different origin scenarios. 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.”

Further Reading: University of Cambridge, Proceedings of the Royal Society A