Comets deliver building blocks of life to planets around the galaxy

Researchers from the University of Cambridge have revealed how comets could transport life’s essential building blocks to exoplanets. The study reshapes our understanding of how life may emerge on other planets.

Slow speed delivery

The team’s key finding is that for comets to effectively deliver organic materials, they must be moving relatively slowly at speeds under 15 kilometers per second. At higher velocities, the vital molecules would not survive the impact due to the extreme speed and temperature, leading to their disintegration.

“In this work, we consider the potential of cometary impacts to deliver complex organic molecules and the prebiotic building blocks required for life to rocky exoplanets,” wrote the study authors. “Numerical experiments have demonstrated that for these molecules to survive, impacts at very low velocities are required.”

Peas in a pod systems

The research suggests that the most likely regions where comets can travel at the right speed are “peas in a pod” systems, where a group of planets orbit closely together. In such systems, a planet’s orbit could transfer or “bounce” a comet to another orbit. This would sufficiently reduce its speed for a safe impact. 

The slower approach would allow comets to deposit intact molecules upon collision, believed to be the precursors for life. The results of the study suggest that peas in a pod systems are promising places to search for life outside of our solar system, if cometary delivery is important for the origins of life.

Life’s building blocks

Comets are already known to harbor a variety of life’s building blocks, including prebiotic molecules. For instance, the Ryugu asteroid samples from 2022 confirmed the presence of amino acids and vitamin B3.

Furthermore, comets contain hydrogen cyanide (HCN), a robust prebiotic molecule, capable of withstanding high temperatures and possibly surviving atmospheric entry.

“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,” said study lead author Richard Anslow from Cambridge’s Institute of Astronomy. “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.”

Focus of the study

The researchers do not claim that comets are necessary to the origin of life on Earth or any other planet. Their goal was to define the planetary conditions conducive for successful delivery of complex molecules like HCN by comets.

Comets in our solar system are primarily found in the Kuiper Belt beyond Neptune. The team examined how gravitational pulls from Neptune and Jupiter influence the journey of these comets, leading some to the inner solar system.

“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,” said Anslow. “What kinds of comets, traveling at what kinds of speed, could deliver intact prebiotic molecules?”

Specific circumstances 

The researchers used various mathematical models to determine that comets can indeed transport life’s precursor molecules, but under specific circumstances. 

For planets around sun-like stars, certain criteria like low mass and proximity to other planets in the system enhance the chances of successful delivery. The significance of nearby planets is even greater around lower-mass stars due to typically higher speeds.

“In these tightly-packed systems, each planet has a chance to interact with and trap a comet,” said Anslow. “It’s possible that this mechanism could be how prebiotic molecules end up on planets.”

However, the scenario is more challenging for planets orbiting lower-mass stars like M-dwarfs, particularly in loosely packed systems. These planets often endure high-velocity impacts, creating additional hurdles for life.

Implications of finding life’s building blocks

“Our results highlight the importance of understanding a planet’s bulk properties, the stellar-mass and the surrounding planetary environment, as all of these factors individually can drastically affect a comet’s minimum impact velocity,” wrote the study authors. This could be useful when determining where to look for life outside the solar system.

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

“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.”

More about comets

Comets offer a fascinating insight into the early solar system and the origins of life. These celestial bodies primarily consist of frozen gases, rock, and dust.

As a comet approaches the Sun, its ice warms and vaporizes, releasing gas and dust that form a glowing head known as the coma. Radiation pressure and solar winds blow this material away, forming the comet’s characteristic tails. There are typically two tails. The first is a dust tail (curved and yellowish). The second is an ion tail (straight and bluish), pointing away from the Sun.

Comets and life’s building blocks

Astronomers believe that comets are remnants from the early solar system, formed over 4.5 billion years ago. Scientists refer to them as “icy time capsules.”

They mainly consist of water ice, along with frozen carbon dioxide, carbon monoxide, methane, and ammonia. Mixed within this icy conglomerate are dust and rocky particles, making them like dirty snowballs.

Types of comets

Short-period comets have orbits that last less than 200 years. They often originate from the Kuiper Belt, a region beyond Neptune filled with icy bodies.

Long-period comets have orbits extending over 200 years. Astronomers believe they come from the Oort Cloud, a distant spherical shell surrounding the solar system.

The Royal Society and the Science and Technology Facilities Council (STFC) supported this research. They are part of UK Research and Innovation (UKRI).

The journal Proceedings of the Royal Society published the full study.

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