Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: rellick1313 on 11/04/2015 00:50:33

Title: Would a planet larger than Earth be able to have the same gravitational force?
Post by: rellick1313 on 11/04/2015 00:50:33

So I started talking to someone at a party about whether this was possible or not, but I would like to know. Is it possible for a planet, say 4x the size of Earth, to have the same gravitational effect on people and animals on the surface of the planet? If so how? I do not know much about this but what if the planet was hollow? Or had a majority of the planet hollow? Or what if some of the planet was made primarily out of a different material than Earth? Is this completely impossible or just implausible?

Title: Re: Would a planet larger than Earth be able to have the same gravitational force?
Post by: evan_au on 11/04/2015 02:39:57

By "4x the size of Earth", I assume that the question means "4 times the mass of the Earth"?

Since gravitational attraction declines as the square of the distance from the center, this would imply a radius twice that of the Earth. This means a volume 8x more than the Earth.

The density of the Earth is 5.5 times heavier than water, due to its iron-rich core.

This alternative Earth would need to be made out of a material with 70% the density of water (on average).
This implies that it would be mostly gas (eg hydrogen, helium or methane), with a very small rocky core.

So there would be no solid surface on which humans or animals could live (unless they built hydrogen balloons). Just ensure that the hydrogen/methane atmosphere doesn't mix with the oxygen for the humans, or their lives would be short but spectacular!

A hollow plant could achieve the same surface gravity as Earth (and provide a solid surface), but we currently don't have enough construction material which could withstand such immense forces.

Title: Re: Would a planet larger than Earth be able to have the same gravitational force?
Post by: chiralSPO on 11/04/2015 13:16:20

It would probably be unlikely to form, but one could achieve a density similar to 70% that of water with a planet that is primarily made of solid hydrocarbons, which would have a real surface. The density would increase somewhat because the planet would compress itself under its own gravity.

Title: Re: Would a planet larger than Earth be able to have the same gravitational force?
Post by: Ciaran Murray Little on 29/04/2016 09:50:10

Elaborating on your question...

I was wondering how a rocky, earth-like planet, which was 4x the mass of the Earth could have only twice the amount of gravity instead of being stronger in normal situations?

Would the planet have to have a thick atmosphere; and would the planetary core be required to be made out of predominately aluminium with some iron, if not completely aluminium? Furthermore, would a planetary core that is made out of a metal other than iron even be plausible?

Title: Re: Would a planet larger than Earth be able to have the same gravitational force?
Post by: evan_au on 29/04/2016 13:00:59

The visible surface of Saturn has almost exactly the same gravity as Earth (1.07 Earth-normal), despite having 95 times the mass of the Earth.

But the visible surface of Saturn is not a solid surface.

See: https://en.wikipedia.org/wiki/Surface_gravity#Mass.2C_radius_and_surface_gravity