How much would you weigh on an exoplanet?

Kepler 69c

I posted this article on my own site some time ago (2012) and it garnered so much interest I thought it worth repeating for the SFR audience. Yes, there's maths, but it's easy maths because that's the only kind I can do.  Don't believe anybody who tells you a computer programmer has to be able to add up.

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I was reading an article from somebody, all enthusiastic about the exo-planets the Kepler probe keeps finding. They’re all many times larger than planet Earth even if they’re in the ‘Goldilocks’ zone. You know the one – not too close, not too far, just right. That is, a planet neither too close to its primary nor too far away, where liquid water could exist. My immediate reaction was ‘sure, but we’d weigh too much’.
Then I wondered how much more. I’m not a mathematician – never have been. In truth, I can’t add up to save my life. So I’m counting on you (ha ha) to correct me if I get this wrong. Please note that this all assumes that the planet we're talking about is rocky, like Earth and Mars. I can't see us wanting to settle on a place like Jupiter or Neptune any time soon.

I discovered this site http://www.exploratorium.edu/ronh/weight/ and learned that gravitational pull weakens by the radius squared. So let’s say you weighed 60kg on planet Earth. Planet Gliese 581g is estimated at 2.6 Earth masses and 1.4 Earth radii. So yes, you’re going to weigh more on Gliese 581g, but not 2.6 times as much. If I’ve got this right, the increased diameter of the planet means you’ll weigh about 1.3 times as much – so about 78kg. That’s certainly not a huge imposition. And all of a sudden, I’m bouncing in my chair, going oooh oooh.
Here’s some estimated figures about Gliese 581g, taken from this fascinating website http://phl.upr.edu/projects/habitable-exoplanets-catalog

Mass = 2.6 Earth Radius = 1.4 Earth  Temp = average surface temperature, so this place, at 10, is rather cooler than our 15 degrees (NASA’s figure from 2008), but the estimate of average temperature assumes an Earth-like atmosphere, which is a pretty big assumption. On the face of it the planet zips around its sun in a fraction of the time it takes ours, taking only 32 days as compared to 365. But that might not be the case, since the Gliesean day may be much longer than Earth’s. The figures don’t mention period of rotation, which I find a tad surprising. As a comparison, Venus’s ‘day” (the time it takes to rotate on its axis) is actually longer than its year (the time it takes to orbit the Sun.) (http://www.universetoday.com/14282/how-long-is-a-day-on-venus/)

So there you have it. I found out today that a candidate for Torreno (capital of the Coalition of Worlds in Morgan’s Choice) may be only 20.2 light years away. And with the shift drive of the future, that’ll be a place to add to your holiday plans.
Ain’t science grand?

Greta van der Rol loves writing action-packed adventures with a side salad of romance. Most of her work is space opera, but she has written paranormal and historical fiction.

She lives not far from the coast in Queensland, Australia and enjoys photography and cooking when she isn't bent over the computer. She has a degree in history and a background in building information systems, both of which go a long way toward helping her in her writing endeavours.

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