Gravity: Does Size Matter?

by Robert Scanlon

When you think of epic science-fiction, do your thoughts turn to slow-motion zero-gravity scenes, or fancy space stations with complex mechanisms for simulating gravity?

But what do you think of when you imagine a planet several times the size of Earth? A colossus, right? So the gravity would be unbearable, wouldn’t it? I mean a massive planet must need high gravity just to stay together.

This is not the case. It’s possible to have massive planets with lower gravity than Earth, and planets smaller than Earth with high gravity.

It’s all a question of density and mass. And of course, size matters.

For example, Jupiter, a massive planet and the biggest in our solar system, is 31 times the mass of Earth, and 12 times the diameter. But Jupiter’s gravity is only just two and a half times that of Earth.

Mars, is half the diameter of Earth, and one-tenth of the mass, but has one-third of Earth’s gravity.

Mercury, the smallest and least massive planet in our solar system, is quite dense (only slightly less dense than Earth). Its gravity is about 40% of the Earth’s, and higher than Mars.

So what makes a planet dense? (And therefore more likely to have a higher gravitational attraction.)

For the most part, if the planet is solid, it will be down to the composition and distribution of the heavier elements. Metals such as iron — one of the galaxy’s most common metals — will contribute strongly to the planet’s density, and therefore its gravity.

But some planets are not entirely solid!

For example, Jupiter is a “gas giant,” meaning, although it is huge, it’s not that dense (compared to Earth). But because the outer portion of Jupiter is thought to be gas or liquefied gas, it is hypothesized that below all that gas is a solid core. So if you try to stand on Jupiter’s gaseous surface, you’d fall through the gas (and at two and a half times earth gravity, you’d fall fast!), and probably end up on a solid surface somewhere (note: don’t try this at home. It’s probably fatal).

Back to the sci-fi. Just because the intrepid explorers land on a big planet, it doesn’t mean it will be hard to move due to the high gravity. In actual fact, it’s more likely the g-forces will be less.

Which brings to mind an interesting observation. Given the number of extraterrestrial planets visited in sci-fi history, don’t you think it’s incredibly lucky that most of them seem to have a gravitational pull the same as Earth’s?

Makes it easier (and cheaper) to film, I suppose!

Maybe this is one reason I liked the movie, “Gravity” so much – they took the time to make sure the gravitational science was true-to-life, even though it is science-fiction!

Robert Scanlon is the author of Constellation, the first book in the Blood Empire series and a Space Opera Science-Fiction Epic. In which there are two planets and one moon with completely different gravity to that of the Earth’s. And lots of zero-gee floating!



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