One of the keys to human life is the atmosphere of the earth. For a planet to keep an atmosphere of gas, it needs to have enough mass that the gas at the surface of the planet doesn’t escape into space. The amount of mass necessary depends on the temperature of the planet (because hot gas is more likely to escape because the gas molecules are moving faster) and the density of the planet. The density of the planet is important, because gravitational attraction becomes weaker with distance.

To calculate the gravitational field of a planet, the distance from the center of the planet is used. The further from the center of the planet you are, the weaker the gravitational field. Therefore, two planets with the same mass but different sizes—and therefore different densities—will have different strengths of gravitational fields at the surface of the planet: the planet with the larger volume and lower density will have a weaker gravitational field at the surface and will be able to trap less gas at the surface. The amount of planetary mass needed to trap an atmosphere like the earth’s guarantees that anything living on the surface won’t fly off. In fact, even the moon has a strong enough gravitational field to prevent things on the surface from flying off, and the moon has much less mass than the earth and has almost no atmosphere.

Once the conditions of the earth, including the atmosphere, temperature, and the presence of liquid water, allowed life to form, life evolved to tolerate the gravitational force at the earth’s surface. If the earth had much more mass and the gravitational field at the surface were much stronger, it is likely that life would still exist, but would have adapted to the increased gravity. It is interesting to think about what life on Earth would look like if the planet were much smaller or much larger.

Regarding the probability of a planet able to support life, thousands of planets have been discovered so far, with none conclusively determined to be appropriate for human inhabitation, so the chances are low. Just how low, though, won’t be determined until astronomers find a planet that can support human life.