View Full Version : Gliese 581c a "Big Earth"

2007-Oct-04, 05:52 PM
Gliese 581c is estimated to be about 5 earth masses, so in terms of our solar system it falls between Earth and Uranus (14 earth masses IIRC). Why it is described as a 'big Earth' rather than a 'small Uranus'? Is there any evidence that its terrestrial?

Do theories of planet formation predict a cut-off point between terrestrial and gas giant planets that is greater than 5 earth masses?

2007-Oct-04, 06:08 PM
Its the accretion disk material that dictates what is the composition of a planet. I can think of no reason why a solid surface planet ( terrestrial ) could not be five Earth masses. Gas giant formation seems to be the most common observed. Is this simply a result of the larger masses.? Gliese 581c is a long way from here. We may not know much of this for we can not see this object directly. I will await more information eagerly.

2007-Oct-04, 06:58 PM
Gas planet, or rocky planet may depend partly on the mass of the star. I think Gliese 581 has less mass, so gas giant may be more likely.

The distance from the star may be mostly what determine, rock vs gas. Since 581c is closer than Earth is to our sun, perhaps rocky is most likely. Neil

John Mendenhall
2007-Oct-04, 07:19 PM
Important difference between gas giants and rocky planets: gas giants have no solid surface, as far as we know. At 5 Earth masses, there's a reasonable chance that Gliese is a rocky planet, with a solid surface. That's a lot more fun to think about than another mush ball.

Tim Thompson
2007-Oct-04, 07:53 PM
Why it is described as a 'big Earth' rather than a 'small Uranus'?
Well, if you were writing a press release, and you really wanted lots of people to read it, so more people would hire you to write press releases, would you say "small Uranus" or "big Earth"?

Is there any evidence that its terrestrial?
Not that I know of, but with a mass that small it seems unlikely to be a gas giant planet.

John Mendenhall
2007-Oct-05, 07:18 PM
There's a good older book by Jack Vance with the above title, about a world with 1 g gravity and not much iron, mostly lighter minerals. The result is a planet with a diameter larger than Earth, the same gravity, and much more surface area. Neat idea.

2007-Oct-06, 01:17 PM
This planet, and any other similar sized world, can have varying proportions of four types of material; atmosphere, water or ice, crust/mantle, and iron-rich core.
The relative proportion of each of these materials dictates the density of the world, and therefore its radius and surface gravity; in the case of Gliese 581c we don't know the radius so we don't know what type of world it is.

If it is mostly atmosphere, it would have a very low density and therefore a low gravity and escape velocity- that would mean it wouldn't be able to hold onto much hydrogen, so the planet would gradually evaporate. But perhaps some planets are mostly made of some other gas, like nitrogen or carbon monoxide? This isn't impossible, although probably unlikely. In any case such a world would have some sort of solid core, be it ice or crust or iron-rich core, or perhaps all three. A small gas giant type might be possible then- a gas dwarf?

Some planets might be mostly water- many moons in the outer solar system are mostly water ice, and perhaps some large icy bodies migrate inwards to become worlds with huge deep oceans. If a planet is mostly water and/or ice it will be a bit denser than a gas world, but not as dense as a rocky world like Earth.

Some planets could be mostly crust, and have a minimal core component and a minimal water component- the Moon is like that, A Moon-like world with a mass of five Earth's is possible, but it would have quite a high density and gravity, and so would almost certainly have a considerable atmosphere unlike our Moon. The composition of this crust could be aluminium compounds and silicates, or perhaps carbon minerals, or perhaps silicon carbide; there are plenty of other materials that could make up a crust and mantle.

And a planet could have a very large iron-rich core component, making it very dense and therefore high in gravity. Such a world would retain a dense atmosphere, but not necessarily a very tall one.

Many worlds will probably be a mixture of all these options, a bit like the Earth, but there are so many combinations that you can't really make any definite predictions just by knowing a planet's mass (which is all we know in the case of Gliese 581c).

2007-Oct-12, 11:52 AM
Whether it's a gas giant or a terrestrial planet:

I think the key here is to look at the escape velocity of hydrogen. If the planet can't hold hydrogen or helium for long periods of time, then it's more likely (but not guaranteed) to be a terrestrial planet. Gliese 581c's ability to hold hydrogen is directly influenced by at least two factors: its gravity and its temperature

Temperature - Correct me if I'm wrong, but I think the estimated temperature of 0 to 40 C was for its blackbody temperature (in effect, only taking into account its distance from the sun [i.e. NO albedo, NO atmosphere, etc.].

Gravity - I think, but not sure, that it's about 2G or so; certainly considerably above 1G. Assuming this is close to the right figure, the planet almost certainly will hold its hydrogen better than Earth can, though probably still short of what's required to enable gas giant formation.

Given a 2G planet with a blackbody(?) temperature of 0 to 40 C, I'd be very surprised if it did not have an atmosphere of some sort (at least Nitrogen and CO2, possibly other gases as major components as well). This definitely creates a greenhouse effect for the planet.

(By comparison, Earth's blackbody temp is around -20 C while it's greenhouse effect makes it about 33 - 35 C warmer than if it had no atmosphere)

Therefore, I think Gliese 581c is quite a bit hotter than Earth (at coolest, with a polar climate that's hot but berable for humans) - unless either/or

*Its atmosphere is unusually deficient in CO2, H20, or other strong greenhouse gases

*It has an extremely high albedo (which would cool the planet considerably)

Furthermore, there's a good chance the planet's tidally locked (one side always facing the sun, in which case the part of the planet with eternally overhead sun is likely very near or above 100C).

Me? I'll wager that Gliese 581c is a large, tectonically active terrestrial planet whose temperature is right at the edge of safe human habitability without climate control equipment - and even then only at the poles. Furthermore, its atmosphere is probably at least as thick as Earth's, if not thicker (possibly too dense for safe human habitation?).

Anyway, that's my two cents.