25Oct2012

Could Humans Colonize A Planet With Stronger Gravity?

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With the recent explosion of extrasolar planet discoveries, some nearby, astronomers and space buffs 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. Not as often referenced 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. A planet becomes a gas giant when it has 10x Earth’s mass, or more. However, don’t let the name fool you. A super-Earth planet could very easily be a gas planet too, and are generally thought to be “gas dwarf planets” the closer their mass gets 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 interesting to astronomers because if they lie within their stars Goldilocks zone, there is a greater chance they could be habitable.

What About Gravity?

We're used to the conditions on our humble planet... but what about others?

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 Ames Research Center’s expert on humans in space and gravitational biosignatures, a person has, in fact, survived 2x Earth’s gravity for 24 straight hours. They also claim it is theoretically possible for a human to adapt to a gravity environment that is between 2x and 3x that of the Earth. However, at 4G or above, humans can not sustain an adequate 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.

Steven Ludmon for ‘Astro Space News

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Image: Planetary protection workers inspect the Mars Phoenix Lander before its interplanetary flight. (NASA/JPL/UA/Lockheed Martin)
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