During the Late Heavy Bombardment period roughly 3.8 to 4.1 billion years ago, inner Solar System planets encountered numerous impacts from outer Solar System comets and asteroids. This period, triggered by the giant planets’ migration, saw these celestial bodies hurtling towards Mercury, Venus, Earth, and Mars, potentially distributing water and life’s building blocks. Recent research from the University of Cambridge suggests that for comets to deliver organic material to other planets, they need to travel at speeds below 15 km/s (9.32 mi/s). Such comets are more likely in closely orbiting planetary systems, offering promising areas to seek extraterrestrial life indicators.
In planetary systems characterised by a closely knit group of orbiting planets, there exists the intriguing possibility of comets being transferred or redirected between these planets. This process can effectively decelerate the comet’s speed. Should cometary delivery play a pivotal role in life’s origins, these planetary systems emerge as highly promising areas for the quest to uncover signs of life elsewhere in the universe.
Comets in our Solar System primarily originate in the Kuiper Belt, a disk extending from 30 to 50 astronomical units (AUs), beyond Neptune’s orbit. When Kuiper Belt objects collide, Neptune’s gravity can redirect them sunward, and Jupiter’s gravity can capture them. Some of these comets, upon entering the inner Solar System, develop characteristic tails due to the sublimation of their frozen volatiles as they approach the Sun’s increasing heat.
If a comet changes orbits significantly, it can slow down sufficiently for certain prebiotic molecules to survive atmospheric entry. Research suggests that around Sun-like stars, the chances of these molecules surviving increase when planets are lower in mass. However, in systems revolving around low-mass stars like M-type red dwarfs, closely orbiting planets become crucial. Loosely packed rocky planets in such systems face more high-velocity impacts, posing a challenge for life.
The researchers stress that comets might not be essential for life’s origin on Earth or any other planet. Their focus was on understanding how comets effectively transport complex molecules like hydrogen cyanide (HCN).
Comets are reservoirs of various molecules known as life’s building blocks. Japan’s Hayabusa2 mission, in 2019, collected samples from the asteroid Ryugu, revealing evidence of Uracil and Niacin. Uracil is an RNA building block, while Niacin, also called Vitamin B3, plays a vital role in metabolism.
