Sometimes a new beginning only comes from parting ways.
Relatedly, scientists have come far in their ability to simulate the formulation of planets, peering into the ancient history of our solar system, and thereby learning how distant exoplanets might come into being and eventually support alien life. But one planet's origin has continued to leave scientists at a loss: Mercury. And an advancing model might reveal that Mercury's hidden beginnings are joined-at-the-hip to the very early life of Earth and Venus, according to a recently shared preprint study.
Mercury might look like a dumb dead rock broiling mindlessly in the sun's furious glare, but unlocking its secrets could bring us one step closer to knowing which distant star systems may host Earth-like planets, and which won't.
Earth and Venus 'sculpted' Mercury from protoplanetary material
Mercury's underwhelming mass, comparably isolated orbit, and the clear lack of other planets orbiting between it and the sun have baffled numerical models of planetary accretion. Accretion is the process by which gaseous particles and rocky bodies of vastly varying size leftover from a star's formation slowly gather into entire planets, and the fact of Mercury's existence isn't up to snuff with the most common models. Previous work in the field has suggested that "if massive embryos (or even giant planet cores) formed early in the innermost parts of the sun's gaseous disk, they would have migrated outward," according to the study. In other words, the larger planetary bodies we've witnessed beyond our solar system orbiting far closer to their sun-like stars than Mercury does to ours might have escaped their original sun-adjacent positions, and drifted outward. "This migration may have reshaped the surface density profile of terrestrial planet-forming material and generated conditions favorable to the formation of Mercury-like planets."
If larger Earth-, super-Earth- or sub-Neptune-sized protoplanets had formed close to our star and then drifted outward, they may have dragged or swept up enough planetary material that only a Mercury-like planet may have formed. The scientists, from the Carnegie Institution for Science's Earth and Planet's Laboratory, in addition to France's Laboratoire d'Astrophysique de Bordeaux, of the University of Bordeaux, think that this model best describes how Mercury came into being. And it turns out Mercury could have Earth and Venus to blame for its scaldingly-hot existence. "We favor a scenario where Earth and Venus' progenitor nuclei form closer to the sun and subsequently sculpt the Mercury-forming region by migrating towards their modern orbits."
Sun-like stars with mercury-like planets could be a sign of nearby life
In successful simulations of the beginning of the solar system, Earth and Venus sweep up mostly dry, "Enstatite Chondrite-like material" on their way out from the extreme inner region of the solar system to their present-day positions. And there's a bonus to the researcher's extended model of planetary migration: it predicts that Venus' composition should be a lot like Earth's, potentially formed from a far greater fraction of dry material.
But notably, while the exodus of Earth and Venus from their sun-adjacent positions left less material for similar planets to form within modern Mercury's orbital trajectory, the scientists' simulation of the little messenger planet's formation also makes statistical room for the planet to achieve alternative compositions. In other words, if there are stars a lot like ours out in the galaxy, and they have a tiny, low-mass planet orbiting within a similar radial distance to its host star, it may have been created when an Earth-like planet wandered away to a more habitable region. And, if that rocky planet was lucky, it could host alien life.