‘Bouncing comets’ could spread the seeds of life
Scientists have debated for decades about whether comets – icy messengers from the outer regions of planetary systems – contain life’s building blocks, for example, amino acids. Could it be comets that brought the first “seeds of life” to the early Earth, billions of years ago? And do comets in distant solar systems also carry the life’s seeds? On November 15, 2023, researchers at the University of Cambridge in the U.K. said it’s possible. They described what they called “bouncing comets” going from one planet to the next in a young planetary systems. They said it might happen most easily in systems where several planets orbit fairly close together, like “peas in a pod.”
Bouncing comets and peas in a pod
Speed in orbit is an important factor for the. A comet needs to be traveling slower than usual when it gets near a planet. The researchers said this should be about nine miles per second (15 km/s), or slower. Otherwise, the speed and temperature of the impact could destroy the prebiotic molecules.
Is this scenario possible? The study said it is, ideally if the comets are in a “peas in a pod” planetary system. This means that the system has multiple planets with orbits that are fairly close together. An incoming comet could “bounce” between the orbits of the planets. In essence, the comet would be passed along from one planet to the next and the effect would be to slow the comet down.
Then, if the comet did end up crashing on one of the planets, it could deliver its cargo of life-enabling goodies. And if they survived the impact on a planet that was habitable, life could potentially emerge.
Bouncing comets could deliver building blocks of life
Scientists already know that comets and asteroids carry many of the prebiotic molecules needed for life. There are amino acids, of course, but others as well. In 2022, scientists analyzed samples from the asteroid Ryugu. They found 10 types of amino acids, carbon, hydrogen and nitrogen. In addition, comets contain hydrogen cyanide. That’s another prebiotic molecule essential for life (as we know it, at least). Notably, it can withstand high temperatures, meaning that it could likely survive the impact on a planet.
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?
Which planets are the best?
The researchers also note that they are not claiming that comets – or even bouncing comets – must be the answer to how life arises on planets. Rather, they wanted to narrow down what kinds of planets would be the most likely to receive the cometary deliveries. That would be planets in the “peas in a pod” scenario.
The researchers used mathematical modeling to test various scenarios. If the planets are orbiting a sun-like star, then they should be low mass and have orbits relatively close to each other. This is even more necessary for planets that orbit lower mass stars, such as red dwarfs. In this scenario, the gravity of one planet would bring the comet in closer. But before the comet could hit the planet, it would be “slingshotted” away from the planet again.
Subsequently, the comet would then approach the next planet the same way, and so on. Ultimately, the comet might impact one of the planets. Notably, however, by that time the comet would be moving slowly enough (relatively) that any prebiotic molecules could survive the impact. If conditions on that planet were suitable, this might then lead to the emergence of life.
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.
In fact, in our own solar system, the gravity of Neptune can sometimes push comets or other objects in the Kuiper Belt closer to the sun. Then, Jupiter does the same thing, sending the objects into the inner solar system. There, they can potentially collide with planets like Earth, Mars, Venus or Mercury.
Which stars are the best?
The type of star also plays a role. The researchers said that this planet-to-planet scenario is most likely around stars similar to our sun. It would be more difficult around smaller stars like red dwarfs (M-dwarfs). Impacts on rocky planets around red dwarf stars also tend to be higher velocity and thus more violent, which increases the chances that any prebiotic molecules would be doomed. The paper stated:
The minimum impact velocity is always lower for planets orbiting solar-type stars than M-dwarfs. Using both an analytical model and numerical N-body simulations, we show that the lowest velocity impacts occur onto planets in tightly packed planetary systems around high-mass (i.e. solar-mass) stars, enabling the intact delivery of complex organic molecules. Rocky planets around M-dwarfs also suffer significantly more high velocity impacts, potentially posing unique challenges for life on these planets.
But as Anslow noted, there is still a lot to learn:
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.
Bottom line: Researchers in the U.K. said in a new study that ‘bouncing comets’ in some planetary systems could deliver prebiotic molecules needed for life to young planets.