Do Super-Earths Trap the Civilizations On Them?

The exoplanet HD 40307g is way bigger than Earth. The Habitable Exoplanets Catalog estimates its mass as 7.1 times that of Earth. Doesn’t that mean it would have a crushing gravity? Wouldn’t a petite 100-pound woman find herself lugging around a weight of 710 pounds? Gravity might make it impossible to get off the planet. A civilization would never become spacefaring.

That’s what I thought until Abel Méndez, who runs the Habitable Exoplanets Catalog, told me the formula for calculating a planet’s surface gravity: mass divided by the radius squared. That is, SG=M/R^2.

Let’s try it with HD 40307g, using data from the Habitable Exoplanet Catalog. Mass, 7.1 Earths. Radius, 2.56 times that of Earth. That gets you a surface gravity of 1.08 gees. (Earth is defined as having a gravity of 1 gee.)

It seems counterintuitive, doesn’t it? How can a planet be so much more massive than Earth yet have only 1.08 times the gravity? The answer lies in the radius. The further you are from the planet’s center, the further you are from most of its mass–so it pulls on you less.

Here’s another example of the importance of a large radius. Jupiter has 317.8 times Earth’s mass. Yet its surface gravity is “only” 2.53 gees. (Jupiter doesn’t actually have a surface you can stand on, since it’s a gas giant, so we’re just counting the cloud tops here.) But it’s very large, so most of its mass is far away from you. The mass is neutered, so to speak, by its large radius.

But mass does matter to a civilization that wants to get off a very large planet like HD 40307g. It would need bigger rockets because the planet’s escape velocity is still higher than Earth’s.

Put simply, escape velocity is how fast you have to go away from a planet to ensure that its gravity can never pull you back. For Earth, escape velocity is about 25,000 miles per hour.

Let’s consider a fictional planet that has eight Earth masses, but a surface gravity of 1 gee. You’d have to go 42,000 miles per hour to escape it. You’d need four Saturn Vs to launch an Apollo-style mission to a moon.

However, this planet has more resources than Earth, simply because it’s bigger. A single continent could have the entire surface area of Earth. An emerging civilization would have more land, more metals, more fossil fuels, and more room to sustain a large population.

Think of it this way: this single continent could have eight billion people, which means a lot of taxpayers. You could easily have the alien equivalent of NASA with four times the Apollo 11 budget. Relative to such a civilization, an Apollo 11-style mission could actually be easier than it was for 1960s America.

And it may be even easier than the equations make it sound. A spacecraft has go 25,000 mph to get to the Moon, but it has to go only 17,500 mph to get into low earth orbit.

Once you’re in orbit, things are easier. No air resistance. No need for rapid velocity changes. Once you’re in orbit you can accelerate to escape velocity slowly, using ion engines. In fact, the probe Deep Space 1 used a chemical rocket to get into orbit, and an ion engine to reach escape velocity so it could visit a comet and asteroid.

In short: once you can get off the planet, you have the technology you need to get away from the planet. It’s just a matter of assembling the resources. Far from being gravitational traps, super-Earths could be excellent incubators of spacefaring civilizations.

(Many thanks to @apollo18, who helped me with the math for this post.)

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