Under certain conditions, comets could transport organic molecules to planets

This artwork depicts a rocky planet being bombarded by comets. Image credit: NASA/JPL-Caltech.

About 4.1 to 3.8 billion years ago, the planets of the inner solar system experienced numerous impacts from comets and asteroids that came from the outer solar system.

This is known as the Late Heavy Bombardment (LHB) phase, where (according to the theory) the migration of the giant planets knocked asteroids and comets out of their regular orbits and hurled them towards Mercury, Venus, Earth and Mars.

This bombardment is believed to have distributed water into the inner solar system and possibly the building blocks of life itself.

Comets must travel slowly – less than 15 km/s (9.32 mi/s) – to carry organic material to other planets, according to a new study from the University of Cambridge.

Otherwise, the essential molecules would not survive the high speeds and temperatures created by entry into the atmosphere and impact.

As the researchers found, it is likely that such comets only appear in closely connected systems where planets orbit close together. Their results show that these systems would be a good place to look for evidence of life (biosignatures) outside the solar system.

The research was conducted by Richard Anslow and Amy Bonsor, a Ph.D. Student and Royal Society University Research Fellow at the Institute of Astronomy, University of Cambridge.

They were joined by Paul Rimmer, a SCOL Senior Fellow in the Astrophysics Group at the Cavendish Laboratory at the University of Cambridge.

Their paper, “Can comets deliver prebiotic molecules to rocky exoplanets?”, appeared Nov. 15 in the Proceedings of the Royal Society A.

In our solar system, most comets come from the Kuiper Belt, the circumstellar disk that extends from 30 astronomical units (AU) – beyond the orbit of Neptune – to about 50 AU. When Kuiper Belt Objects (KBOs) collide, they can be “hurled” toward the Sun by Neptune’s gravity and eventually be captured by Jupiter’s gravity. Some of these comets are then thrown past the asteroid belt and enter the inner solar system.

These comments become “tails” as they approach the Sun as rising temperatures cause their frozen volatiles to sublimate.

Scientists have also found that comets may contain prebiotic molecules, which are the building blocks of life. These include hydrogen cyanide, methanol, formaldehyde, ethanol, ethane and more complex molecules such as 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 also known as vitamin B3.

However, not all of these elements can remain intact when they enter a planet’s atmosphere and hit the surface. As Anslow said in a University of Manchester press release:

“We are constantly learning more about the atmospheres of exoplanets, so we wanted to see if there are planets where complex molecules could also be transported by comets.

It is possible that the molecules that gave rise to life on Earth came from comets. So the same could apply to planets elsewhere in the galaxy.

“We wanted to test our theories on planets similar to our own, as Earth is currently our only example planet where life is possible. What types of comets could deliver intact prebiotic molecules and at what rate?

In these densely packed systems, every planet has a chance of interacting with and capturing a comet. It is possible that this mechanism is responsible for prebiotic molecules ending up on planets.”

For their research, the team tried to set limits on the types of planets on which comets could successfully transport complex molecules. Using various mathematical models, the researchers found that comets can provide the precursor molecules for life, but only in certain scenarios.

Their results showed that comets traveling at the right speed are most likely to be found in “peas in a pod” systems, which consist of planets orbiting close together. In these systems, comets can be attracted by the gravitational pull of one planet and then “bounce” off another before impacting.

When the comet is shifted from one orbit to another, it slows down enough that some prebiotic molecules could survive entering the atmosphere.

Their results also showed that for sun-like stars, the chances of survival of prebiotic molecules were even better when the planets had a low mass. But for planets orbiting low-mass stars (e.g. M-type red dwarfs), close-orbiting planets were particularly important.

If rocky planets in these systems were loosely packed, they would be subject to much stronger, high-velocity impacts, posing a significant challenge to life on these planets.

These results could help astronomers figure out where to look for signs of life (biosignatures) outside our solar system. Answer said:

“It’s exciting that we can start to identify the types of systems that we can use to test different origin scenarios.

It is a different way of looking at the great work that has already been done on Earth. What molecular pathways led to the enormous diversity of life we ​​see around us?

Are there other planets where the same paths exist? It is an exciting time to combine advances in astronomy and chemistry to investigate some of the most fundamental questions of all.”

Written by Matt Williams/Universe today.

Luke Plunkett

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