What would you weigh on an exoplanet?

posted in: Science fact | 24
picture of an exoplanet

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 began to wonder 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.

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 https://phl.upr.edu/library/media/size-comparison-of-the-gliese-581-planetary-system

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?

24 Responses

  1. Paul Trembling

    If I ever get round to actually writing this variable gravity planet story, this discussion will have helped a lot! Just talking round the subject sparks ideas! Thanks, everyone.

  2. Richard Leonard

    The gravity of a planet is always strongest at its surface. If you burrow down towards the centre of the planet, gravity decreases to zero at the centre because you have some of the planet’s mass above you pulling you in the other direction. So there’s another solution if the planet is too massive. 🙂

    • Paul Trembling

      So do I, Greta!

      I have this vague idea for a story that requires, as its background, a planet with variable gravity. I thought that having it part of a orbital system involving several Jupiter sized planets (and perhaps a white dwarf star) might do the trick, but from this discussion, perhaps not. Back to the imagination with that one, then!

      • Steven J Pemberton

        Well… you could go for the classic Hal Clement approach (a big planet that rotates so fast that it bulges into a discus shape, so that gravity at the equator is a fraction of that at the poles).

        Or does gravity need to vary unpredictably? You might have lumps of extremely dense matter buried in the crust, so that gravity near to those would be stronger than elsewhere. Though you then have to explain why those lumps haven’t sunk into the planet’s core, or crushed the planet out of its spherical shape.

        If you’re not too concerned with scientific plausibility 🙂 make it a small planet where someone’s installed gravity generators in the crust so that they can run around without flying off into space. Some of the generators have stopped working, or (for added fun) are on the blink, so that you can never be sure whether your next step will have you breaking bones or breaking high jump records…

        • Paul Trembling

          All good ideas, Steve. I’m thinking round the idea of a technology that has some gravity control, but which finds that the varying gravity fields make their normal transportation methods unreliable. So they’re forced to find other solutions. Complicated by the fact that this particular world is an unpleasant place to live – unbreathable atmosphere, acid seas, violent storms, etc.

          Which give rise to the question, why would anyone want to live there anyway? Well, possibly because the entire planet is an anomaly, and therefore of scientific interest. Details still to be thought through!

          • Greta van der Rol

            Yes, I got as far as unbreathable atmosphere and thought huh? But then, the planet’s atmosphere in “Avatar” was unbreathable. The lure was the mineral Unobtanium – which seems to have been something to do with gravity?

      • Steven J Pemberton

        Other reasons for living on an inhospitable planet – nowhere else to live (it’s a ghetto, in other words), or because you choose to – because you know no one else will bother you, or because you want to test your strength or religious faith (“God created Arrakis to train the faithful.”) You could combine this with scientists trying to study the planet, which the people wanting to test themselves might not be too happy about.

        Avatar didn’t really explain what unobtainium does – they just showed it floating, and someone said it sells for 20 million a kilo back on Earth. The name is an engineering joke – “unobtainium” is any material that’s perfect in every way for whatever problem you’re trying to solve, except that it doesn’t exist. The ship in “The Core” (ashamed to admit I saw that…) had a hull made of unobtainnium.

        • Greta van der Rol

          Unobtainium was a stupid name and as you say, the substance wasn’t explained (except for the floating mountains – which may have been a clue). But it’s an idea to play with – a gravity-defying substance.

  3. Paul Trembling

    Thanks for clarifying that point, Steven. I had an feeling that there would be some good reason why it wouldn’t be likely to happen.

    Of course, that doesn’t mean that a story couldn’t be made out of it! As you say, the Puppeteers (or some other suitably advanced aliens) could be brought in to help make it work. And the discovery of such an anomaly would be a good starting point…

      • Paul Trembling

        Thanks Richard. I had heard of the Lagrangian point – anyone who reads SF probably has – but this explains it very clearly.

        I don’t think it would help with our original problem (which is to reduce the effective gravity on a very large planet to the point where it could be colonised (by humans!)), since the article specifies a small body of negligible mass. Might have to bend the rules a bit to fit a planet in!

  4. Paul Trembling

    Interesting possibilities there! But doesn’t a planet’s composition also influence its gravity? So that a very large planet which was seriously deficient in heavy elements might have a relatively light gravitational pull? I’ve read some SF that uses this idea to allow colonisation of some big planets (with interesting implications for technology given the very poor heavy element content!).

    Another way round it is to have a planet with a very fast rotation, so that centrifugal force counteracts the gravity at the equator. If memory serves (as it occasionally still does!) it was Hal Clement in ‘Mission of Gravity’ who used that.

    Of course, if you’ve got the technological ability to manipulate gravity, any sized planet could be terraformed, at least in part. The writers problem then is to show why it needs to be!

    • Steven J Pemberton

      Yes, a big planet with little in the way of heavy elements would have a low gravity at its surface. Saturn, for example, has an average density lower than that of water, and its surface gravity is only slightly greater than Earth’s.

      A dense planet that rotates quickly brings its own problems, such as ferocious storms if it has an appreciable atmosphere. If my calculations are right (which they probably aren’t) Gliese 581g would have to rotate about four times faster than Earth for centrifugal force to lower its gravity at the equator to the same strength as Earth’s.

      • Paul Trembling

        Good point about the atmosphere effects of fast rotation!

        How about opposing gravitational fields to lower the effective gravity on a planets surface? Just off the top of my head, I’m wondering about having two Jupiter sized planets orbiting each other, with another planet in a stable position somewhere between the two. If it didn’t get ripped apart, it might make for some interesting effects!

      • Steven J Pemberton

        (Paul, I can’t reply to your comment – maybe the blog has a limit on how deeply comments can be nested.)

        Systems where two bodies have similar masses aren’t gravitationally stable. Two Jupiter-sized planets wouldn’t orbit one another if there was a star nearby – they’d orbit the star instead. If they existed in interstellar space, a smaller planet between them would probably end up orbiting only one of them, or else being flung off into the darkness. The reason is that the region where the gravity of the two Jupiters cancels out is very narrow (the same is true of any body in orbit around another). A planet travelling through that region has a tricky job to stay balanced. Any deviation to one side or the other makes it fall more under the gravity of the Jupiter on that side, and once it’s gone down that path, the other Jupiter usually can’t get it back.

        So unless someone hired the Puppeteers at some point in the last few thousand years, I don’t think your colonists are likely to come across this sort of arrangement 🙂

  5. juliabarrett

    Oh my god, you understand math. Give me a planet where I weigh 90 earth pounds and I’ll be satisfied. My aunt always said, one can never be too rich or too thin. I’ll never be too rich, therefore another planet it is!

    • Greta van der Rol

      Hahahaha. No, I don’t understand math. (Or maths as we say in British English ;)) I can see the logic but like I said, I can’t add up to save my life. Just as well the OH can.

  6. Steven J Pemberton

    That close to the star, there’s a good chance the planet is tidally locked, which means it won’t be a very nice place to live. The same side of it would always face the star, making it very hot, while the other side would be very cold. See https://en.wikipedia.org/wiki/Gliese_581g#Tidal_locking_and_habitability

    The only way to measure its period of rotation, as far as I know, is to see a feature on its surface and measure how long it takes to come round again. As we can’t actually see the planet with our current instruments (it was discovered by Doppler spectroscopy – the “wobble method”), this is rather tricky 🙂

    • Greta van der Rol

      Thanks for the link. Puts a whole different complexion on things, doesn’t it? I think I’d be more inclined to believe the Wikipedia site. How often does that happen? 🙂

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