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View Full Version : What's the surface gravity of Gliese 581 d?



Noclevername
2010-Mar-16, 03:43 AM
And Gliese 581 e, for that matter? Just curious if super-Earths are places we can stand upright, or if we'll need to crawl the whole time.

WayneFrancis
2010-Mar-16, 05:08 AM
not sure if there we know much more about it aside from its possible mass so hard to say. If its 14x the mass but we don't know the radius of the planet...then it is hard to talk about the gravity on it

EDG
2010-Mar-16, 06:53 AM
And Gliese 581 e, for that matter? Just curious if super-Earths are places we can stand upright, or if we'll need to crawl the whole time.

For 581d - assuming a 7 earth mass planet and a bulk density of 5000 kg/m3 (roughly earthlike - given the high mass of the planet, this means a lot of its mass is actually water/volatiles, which fits the idea that it's an icy body that migrated inwards. If it was all rocky then the density would be significantly higher), I get a radius of about 12,650 km. Given that mass, density and radius, that would mean the surface gravity is about 1.8g.

For 581e, assuming a 2 earth mass planet, and a bulk density of 6000 kg/m3 (a rocky/metal planet like Earth, but a bit denser due to closeness to the star) I get a radius of 7,900 km, which yields a surface gravity of 1.35g.

That's all assumption, of course. ;)

eburacum45
2010-Mar-16, 09:22 AM
1.8 gee isn't all that bad, when all is said and done.

Since these planets are might well have fluid surfaces you might not have anything solid to stand on. But floating in an ocean on a relatively high gravity planet might be preferable to walking on it.

Noclevername
2010-Mar-17, 08:48 PM
Thanks for the answers. Being a product of the illustrious U.S. public school system, I don't have the slightest clue about the formulas for calculating mass-density-gravity relationships.

Hungry4info
2010-Mar-17, 11:56 PM
Thanks for the answers. Being a product of the illustrious U.S. public school system, I don't have the slightest clue about the formulas for calculating mass-density-gravity relationships.

I thought they had Google in America?
http://en.wikipedia.org/wiki/Surface_gravity

The density can be guessed at by guessing a radius of the planet. d=m/V

Of course, the mass has only a lower bound (with the true mass being the minimum mass divided by the sine of the inclination)

Noclevername
2010-Mar-18, 12:47 AM
I thought they had Google in America?


Yes, but they didn't teach any methods of research beyond "ask the librarian" when I was in school, and the only "google" back then was what Cookie Monster's eyes did.

What I lacked was not a way to look up the formula, but the knowledge of mathematics needed to solve said formula, and as I said, I went to a public school.

Hungry4info
2010-Mar-18, 07:39 AM
Ahh. I apologise.

Van Rijn
2010-Mar-19, 05:24 AM
And Gliese 581 e, for that matter? Just curious if super-Earths are places we can stand upright, or if we'll need to crawl the whole time.

You might like this article, which discusses what the surface gravity might be for Gliese 581 d for different assumptions about density (it apparently assumes a mass 7.7 times that of Earth):

http://scienceblogs.com/authority/2009/04/its_life_jim_but_not_as_we_kno.php

At the low density end, it might have a lower surface gravity than Earth, but there's no way to know yet. Keep in mind that it might not have any true solid surface.