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> Gliese 581g, has a mass three to four times that of Earth > The surface gravity would be about the same or slightly higher than Earth's I'm confused. Maybe

> Gliese 581g, has a mass three to four times that of Earth > The surface gravity would be about the same or slightly higher than Earth's I'm confused. Maybe it's way less dense than earth, so on the surface you're far from the center of gravity?

hugh3 16y ago
The density is unknown, but we assume it to be rocky because... well, at 2 Earth masses there's not much else it could be made of. It can't be ice because it's too warm, and it can't be gas because gas planets that size won't hold together. But by my calculations a four Earth-mass planet with the same density as Earth should have gravity of about 1.73g. (using http://www.ericjamesstone.com/weird_stuff/gravitator.htm http://www.ericjamesstone.com/weird_stuff/gravitator.htm) Which is not that similar to Earth gravity, but not too far off either, in the scheme of things.
- astrofinch 16y ago
  Potentially habitable planets that have less mass than Earth does are inherently more difficult to detect, am I right?
  - ugh 16y ago
    Yep. But we are getting closer.
- ww520 16y ago
  That's a very cool calculator. I tried to work out the math based on sphere volume: V=4/3 x PI x R^3. Earth Mass Radius Surface Gravity 1X 6378km 1.00g 2X 8035km 1.26g 3X 9198km 1.45g 4X 10124km 1.59g 5X 10906km 1.71g If it's 2X or 3X, it's just adding 30% to 40% body weight. For 4X or 5X, it's like adding 100lb to an 150lb body. We have plenty of people in the 250lb to 350lb range and they have no problem in mobility. Human body can handle the load. It might still be helpful to have exoskeleton help.
  - hugh3 16y ago
    You'd probably still have issues if you tried to live there. Your legs can support twice your body weight if they need to, but can your neck support a double-weight head? Human spines are already pretty under-spec for their task, so people living in 2g gravity would probably suffer from a lot more back problems. It's a pretty small-beer problem on the scale of colonise-another-star-system problems though.
    - roc 16y ago
      Not to mention that large humans do take time to become large. A 200 pound man doesn't wake up one day weighing 340 pounds. Yes, a human body can adapt to it, massive health problems notwithstanding, but that doesn't mean you can step off a space craft into that situation. To say nothing of doing so after a multi-generation trip through bone- and muscle-wasting space.

hugh3 16y ago

The density is unknown, but we assume it to be rocky because... well, at 2 Earth masses there's not much else it could be made of. It can't be ice because it's too warm, and it can't be gas because gas planets that size won't hold together. But by my calculations a four Earth-mass planet with the same density as Earth should have gravity of about 1.73g. (using http://www.ericjamesstone.com/weird_stuff/gravitator.htm http://www.ericjamesstone.com/weird_stuff/gravitator.htm) Which is not that similar to Earth gravity, but not too far off either, in the scheme of things.

astrofinch 16y ago
Potentially habitable planets that have less mass than Earth does are inherently more difficult to detect, am I right?
- ugh 16y ago
  Yep. But we are getting closer.
ww520 16y ago
That's a very cool calculator. I tried to work out the math based on sphere volume: V=4/3 x PI x R^3. Earth Mass Radius Surface Gravity 1X 6378km 1.00g 2X 8035km 1.26g 3X 9198km 1.45g 4X 10124km 1.59g 5X 10906km 1.71g If it's 2X or 3X, it's just adding 30% to 40% body weight. For 4X or 5X, it's like adding 100lb to an 150lb body. We have plenty of people in the 250lb to 350lb range and they have no problem in mobility. Human body can handle the load. It might still be helpful to have exoskeleton help.
- hugh3 16y ago
  You'd probably still have issues if you tried to live there. Your legs can support twice your body weight if they need to, but can your neck support a double-weight head? Human spines are already pretty under-spec for their task, so people living in 2g gravity would probably suffer from a lot more back problems. It's a pretty small-beer problem on the scale of colonise-another-star-system problems though.
  - roc 16y ago
    Not to mention that large humans do take time to become large. A 200 pound man doesn't wake up one day weighing 340 pounds. Yes, a human body can adapt to it, massive health problems notwithstanding, but that doesn't mean you can step off a space craft into that situation. To say nothing of doing so after a multi-generation trip through bone- and muscle-wasting space.