Most Life-Hosting Planets in Milky Way Probably Formed Early

Most of the Milky Way's life-hosting planets probably formed early in the galaxy's history.
Brad Bergan

Rocky planets born early in the Milky Way's 13.5-billion years had a better chance of becoming a life-hosting world than planets forming later in the galaxy's life span, according to a recent Goldschmidt geochemistry conference.


Rocky planets born to young Milky Way best for life

Rocky worlds that formed during the early days of our Milky Way were more likely to harbor life than planets that were late to the galactic party.

This is because early-forming planets are more likely to develop magnetic fields, plate tectonics, and other features that enhance the development and persistence of life, reports

"Plate tectonics is important for habitability, and it looks like the optimum conditions [for] plate tectonics existed for planets forming early in the galaxy's lifespan, and may be unlikely to easily recur," said the study's lead author Craig O'Neill, director of the Macquarie Planetary Research Centre at Macquarie University in Syndey, Australia, in a statement made during a Goldschmidt geochemistry conference. "For life, maybe that was as good as it gets."

Computer simulations override limits of distance

O'Neill and his team studied exoplanets within the Milky Way — which are planets orbiting stars beyond our solar system.

"Because of the great distances involved, we have a limited amount of information on these exoplanets, but we can understand some factors, such as position, temperature and some idea of geochemistry of the exoplanets," said O'Neill. "This allows us to model how they develop."

O'Neill's team entered the guiding parameters into planet-development simulations that use processors at the National Computational Infrastructure in the Australian National University, in Canberra.

Plate tectonics act like planetary thermostats

Together, the researchers discovered that planets forming when their galaxy is relatively young are better candidates for plate tectonics — which work like a built-in thermostat, cooling the surface enough for familiar life forms to evolve.

Planets without plate tectonics harm a planet's life-supporting abilities in several ways, said O'Neill.

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"This doesn't just affect the surface temperature; this means that the core stays hot, which inhibits the development of a magnetic field," he said. "If there's no magnetic field, the planet is not shielded from solar radiation, and will tend to lose its atmosphere. So life becomes difficult to sustain. A planet needs to be lucky to have the right position and the right geochemistry at the right time if it's going to sustain life."

Galactic age lowers abundance of lighter chemicals

The geochemistry of planets also varies depending on when in their galactic era they form, reports, since the chemistry of galaxies evolve as they age. For example, later in a galaxy's life, heavier elements are eventually distributed via supernova explosions, leaving less lighter elements (like helium and hydrogen) floating around to condense into stars.

As the search for life beyond our solar system goes on, it's interesting to note that what in daily life is usually a drag (plate tectonics create earthquakes and contribute to volcanic eruptions) are actually a prerequisite for life as we know it in much of the Milky Way.