Size matters: A planet needs to be at least as big as Mars for alien life to survive on it

Planets like Earth orbiting a star in the habitable zone are considered the gold standard for habitability. But a rocky world, apart from being at the perfect distance from its star, must also be able to hold onto its atmosphere to sustain life. And it turns out, size is a major factor in determining if a rocky world can hold onto its atmosphere for long enough for life to bloom. Scientists at the University of California Riverside recently conducted a study to determine how small a planet could be and still retain its atmosphere. Using simulations for different Earth-like exoplanets of varying sizes, they found that a planet must be at least as big as Mars for life to thrive. 

In their research published in The Planetary Science Journal, the authors noted that a planet should be 80 percent as wide as Earth but could technically be as small as 60 percent as well. For comparison, Mars is slightly over half the size of Earth, according to NASA. This finding could help scientists narrow down their search for potentially habitable exoplanets.

Why bigger planets are better candidates 

The size of a planet seems to have a lot to do with the preservation of atmosphere. In their model, scientists created different versions of exoplanets—some with chemical makeup the same as Earth and others with different amounts of carbon, larger or smaller cores, or different starting temperatures. The model traced what these planets went through in a billion years based on how quickly stellar wind stripped them of their atmospheres and how quickly volcanoes pumped out gas (mostly carbon dioxide) to replace them.

Small planets have weaker magnetic fields and gravity, which render them unable to hold onto their atmosphere. If there is no magnetosphere, a region dominated by a magnetic field, the stellar winds and radiation will start chipping away at a planet's atmosphere. It is believed that a loss of magnetosphere caused Mars to lose its once-thick atmosphere. 

In the latest research, the authors also noted that simulated planets not as big as Mars lost their atmosphere relatively sooner, as volcanoes, which pump carbon dioxide through eruption, couldn’t release it to replace the stripped gases fast enough. The magma churning beneath the crust releases less volcanic gas over time while the upper layers cool and harden much faster, thereby cutting off volcanic eruptions quite early. On the flip side, even planets just 60 percent of Earth’s size but with higher carbon concentrations would have better chances of survival. That's because mantles with higher carbon content tend to release more carbon dioxide in eruptions. And carbon dioxide, being a heavy molecule, is difficult to lose.

A cooler mantle could also help in atmospheric retention. If the mantle is cool and there are no serious eruptions for a long time, the planet will hold onto its reservoir of gases until its star is past the phase of violent bursts of radiation and plasma that could wipe out the released gases early on. Interestingly, the study underscores that just because a planet lost its initial atmosphere doesn’t mean it would stay this way forever. Impacts of comets and asteroids on the planet might deliver volatile elements like hydrogen, oxygen, and carbon to build an atmosphere again.

The model, appropriately named the 'Smaller Than Earth Habitability Model,' could be used in future simulations to study worlds around smaller, cooler red dwarf stars, which make up about 75% of our galaxy. Their dimmer light also makes it easier for observatories like the James Webb Space Telescope to study their planets' atmospheres.

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