With the recent explosion of extrasolar planet discoveries, some relatively nearby, astronomers and space enthusiasts wonder how habitable some of these planets would be for humans.

The Goldilocks Zone is often referenced (an area around a planet’s host star which could be ‘just right’ for liquid water to exist) when it comes to habitability. What’s not mentioned as often, however, is the planet’s gravity.

With many descriptions of these planets including words like “super-Earth”, it’s hard not to wonder if the gravity is also ‘super’ and how habitable these planets could be for us.

What Is A Super-Earth?

Super-Earth planets are planets with more mass than our own, but less than that of a gas giant. Astronomers classify a gas giant as a planet with at least 10x Earth’s mass. Don’t let the name fool you, however. A super-Earth planet could very easily be a gas planet too. These smaller gas planets are referred to as “gas dwarf planets” and are thought to be more common the closer a planet is to the 10x gas giant threshold.

A rocky super-Earth planet is one that has a solid surface, like Earth, but has a larger mass and/or radius. These types of planets are incredibly interesting to astronomers because if they lie within their stars Goldilocks zone, the greater the chance is that they could be habitable.

What About Gravity?

It’s a tricky question because planets that are larger than Earth in size may not be as dense. If a planet is less dense, it will result in a weaker gravitational pull on the surface.

The question becomes even more complicated because the gravitational force felt on the surface of a planet is proportional to the planet’s mass divided by its radius squared.

This means that a planet with twice the radius (size) of Earth, yet similar in density (mass), would only have twice as strong gravity at its surface compared to Earth, despite having 8x Earth’s total mass.

To put it another way, a planet twice as massive as Earth would have the same surface gravity as Earth, as long as it’s radius is also proportionally larger — 4x the mass with 2x the radius, or 16x the mass and 4x the radius. g = G*M/R²

How Much Gravity Can Humans Handle?

Humans are a very adaptable species. We do have our limits though. According to NASA’s Ames Research Center’s expert on humans in space, a person has survived 2x Earth’s gravity for 24 straight hours without ill effects. They go on to claim that it is theoretically possible for a human to adapt to a gravity environment that is between 2x and 3x that of the Earth. However, they say that at 4 times Earth’s gravity (4G) or above, human physiology cannot maintain sufficient blood-flow to the brain.

In the future, it may be possible to break the 4G limit with enhancements in genetic manipulation and extremely strong mechanical replacement organs to keep our body’s systems running, but that technology is a long way off.

References:

The Astrophysical Journal 656 (1): 545–551. arXiv:astro-ph/0610122

National Science Foundation. September 29, 2010.

“The quest for very low-mass planets”, M Mayor, S Udry – Physica Scripta, 2008 Pg. 20 (PDF)