Title: Can comets deliver prebiotic molecules to rocky exoplanets?
Authors: R. J. Anslow, A. Bonsor and P.B. Rimmer
First Author’s Institution: Institute of Astronomy, University of Cambridge,Cambridge, UK
Status: Published in Proceedings of The Royal Society A [open access]
Don’t leave your umbrellas at home for this one!
Picture this: life-building molecules delivered to Earth by comets, crashing onto the planet’s surface and sparking the beginnings of life. Or maybe these molecules were already here, slowly forming over time from the Earth. This is the mystery of abiogenesis – how did life begin? Was it sparked by prebiotic molecules from outer space (exogenous) or did it emerge from Earth’s own resources (endogenous)?
The building blocks of life – prebiotic molecules – include amino acids, nucleotides, and lipids. The sources of these molecules have been hotly debated for more than a century and identifying them is outlined as one of the key objectives in NASA’s astrobiology strategy.
Comets are believed to be the prime delivery driver when it comes to exogenous prebiotic molecules. Not only have the ingredients for DNA been found by the Rosetta mission to 67p, but comets have also been suggested as the source of the amino acids found on the Asteroid Ryugu.
With an ample supply of molecule-laden comets in the Kuiper belt (a source that is also thought to be present around other stars), the issue comes down to figuring out how they could be safely delivered to the surface of the Earth in the inner system.
As opposed to meteorites, which are able to carry an, albeit far smaller, amount of prebiotic material through a fiery descent to the surface, comets traveling too fast risk burning up their precious cargo of organic molecules during impact. To avoid this, the so-called ‘warm comet pond’ has been proposed as a specific origin scenario, where a ‘soft-landing’ of a cometary nucleus creates a small crater and forms a messy mixture of ice, dust and organic molecules from the comet’s contents. Compared to other delivery mechanisms, this scenario requires low-velocity impacts, bringing us to today’s authors.
Slowing down the comets
If abiogenesis truly depends on a supply of slow impacting comets, then we would expect to find possible future biosignatures to be correlated with conditions that are conducive to such impacts. This leads the authors of the paper to try and answer the question “Which planets, and planetary systems are the most susceptible to successful cometary delivery?”
Through a series of analytical considerations and numerical n-body simulations, the authors explore the impact velocities of comets that are flung from beyond the snow-line and into the habitable zone of an idealised exoplanet system, shown in Figure 1.
Figure 1: The architecture of the idealised planetary system with comets either scattered directly from the snow line to the habitable zone or as a series of interactions between a number of Earth-mass planets that are equally-spaced apart.
There are three key factors to consider when determining the minimum velocity with which a comet will arrive at a planet: the planet’s escape velocity (which is determined by its mass), the planet’s orbital velocity (dependent on the mass of the host star) and the comet’s orbit relative to the planet. The authors predict that the minimum impact velocity increases for stars with lower mass as the habitable zone moves much closer to the central star where orbital velocities are increased. This leads them to conclude that comets traveling directly from the snow-line to an Earth-like planet in the habitable zone will have impact velocities that exceed the threshold for safe delivery of a sample prebiotic molecule if the host star is significantly less massive than our Sun. However, the velocity can be reduced by scattering off other planets in the system before impact (as is demonstrated with the chain of lower arrows in Figure 1), making the overall system architecture an important consideration. The velocity can be predicted analytically as shown in Figure 2.
Figure 2: Analytical predictions for the minimum impact velocity (y-axis) as a function of spacing between the scattering planets (x-axis). As the mutual spacing between planets grows, comets that are scattered directly from the snow-line or only rarely by planets have more elliptical orbits and thus a higher minimum impact velocity. This effect is increased for low-mass stars. The dashed horizontal lines indicate rough survivability rates for an example molecule (HCN) with inefficient delivery indicating a less than 1 percent survivability rate.
Analytical models can only go so far though. To test their predictions, the authors perform a series of N-body simulations to model the inward flow of comets from beyond the snow line. This approach enables them to assess the effects of varying planetary spacings and stellar mass on the resulting velocity distribution of the comets coming in from beyond the snow line.
The result of these N-body simulations are shown in Figure 3. The results show that there is a much larger fraction of low-velocity impacts around Solar-mass stars regardless of how the planets in the system are distributed. For lower mass stars like M-dwarfs, the velocity distribution and minimum impact velocity become highly dependent on the spacing between planets in the system. For an M-dwarf with 10 percent of the mass of the Sun, most comets hitting a planet will be travelling too fast for any organic molecules to survive – unless the planets are very close together. Even then, there will still be a lot of fast-moving comets. This suggests that the most promising planets for cometary delivery at low impact velocities will be found around F- and G-type stars. Meanwhile a planet around an M-dwarf will need several nearby planets to increase the chances of slower impacts and even then must contend with a higher amount of high-velocity impacts that could be detrimental to the development of life.
Figure 3: The impact velocity distribution from the N-body simulations of a comet impacting an Earth-like planet in the habitable zone around different stellar bodies. Lower mass stars have a higher minimum impact velocity (dashed line), as well as an increased frequency of high velocity impacts. Decreasing the planetary spacing, Δa, in the system reduces this effect for low-mass stars while the distribution for a Solar-mass star (shaded) is independent of the architecture.
The authors suggest that if cometary delivery of prebiotic molecules at low velocities is important for the origin of life, there will be a correlation between the presence of biosignatures and exoplanets around higher-mass stars (~ 1 M☉) and in tightly packed systems around lower mass stars. Additionally, a lack of any such correlation might provide an argument against cometary delivery as a prerequisite for abiogenesis. With a number of new programs dedicated to the characterisation of exoplanet atmospheres and the search for potential biosignatures, we may soon have the data we need to confirm or challenge these ideas. The places where we do potentially find biosignatures could either back up or question this theory, helping guide future research into the kinds of environments where life might emerge and might even tell us a thing or two about our possible origin.
Astrobite edited by Kaz Gary and Sahil Hedge
Featured image credit: Edmund Weiß, Public domain, via Wikimedia Commons
