Jay Wesley Richards, Ph.D.

A planet’s habitability depends on its mass in manyh ways: terrestrial planets significantly smaller or larger than Earth are probably less habitable. Because its surface gravity is weaker, a less massive Earth twin would lose its atmosphere more quickly, and because of its larger surface-area-to-volume ratio, its interior might cool too much to generate a strong magnetic field. And…smaller planets tend to have more dangerously erratic orbits.

In contrast, without getting more habitable, a more massive Earth-twin would have a larger initial inventory of water and other volatiles, such as methane and carbon dioxide, and would lose less of them over time. Such a planet might resemble the gas giant Jupiter rather than our terrestrial Earth. In fact, Earth may be almost as big as a terrestrial planet can get.

While life needs an atmosphere, too much atmosphere can be bad…High surface pressure would slow the evaporation of water and dry the interiors of continents. It would also increase the viscosity of the air at the surface, making it more difficult for big-brained, mobile creatures like us to breath.

In addition, more surface gravity would create less surface relief, with smaller mountains and shallower seas. Even with more vigorous tectonic churning, rocks could not support mountains as high as those we enjoy. The planet probably would be covered by oceans and too mineral-starved at the surface (and too salty throughout) to support life…

To add insult to injury, the surface gravity of a terrestrial planet increases with mass more rapidly than you might guess. Intense pressures compress the material deep inside, so that a planet just twice the size of Earth would have about fourteen times its mass and 3.5 times its surface gravity. This higher compression would probably result in a more differentiated planet, gases like water vapor, methane, and carbon dioxide would tend to end up in the atmosphere. Earth has kept dry land through its long history, in part, because some of its water has been sequestered in the mantle; in contrast, a more massive planet would probably have degassed more than Earth.