A common theme in SF is “heavy-gravity” planets, where the indigenous lifeforms – or adapted human colonists – are portrayed as extraordinarily strong by our standards. But in our Solar System, you won’t find any. Earth is the most massive body, except for the gas giants which may have no “surface” as we understand it. (See this thread: http://boards.straightdope.com/sdmb/showthread.php?t=367097) Is that just how it worked out in this particular system, or is there some upper limit to the size of a non-gas-giant planet? Does anybody have a theory?
I do not have a definitive answer, but some extra-solar planets been found that were not gas giants and have far greater masses than Earth.
So, I strongly suspect that far larger non-gas-giant planets than Earth are quite common in the grand scheme of things.
I don’t know; but consider… As planets get larger, their gravity increases a lot. In other words, a planet twice the diameter of earth of similar density would have a gravity several times that of earth, wouldn’t it? As a result, it would have a much denser and thicker atmosphere. Do the SF stories of high-gravity planets ever consider that?
Once a rocky planet gets big enough it’ll retain hydrogen from the protoplanetary disk and become a gas giant. This site claims that process starts when the mass reaches 10-20 times that of earth, but IIRC, I’ve seen other claims that 2-3 earth masses are all that’s needed.
Surface gravity is proportional to density times radius, so a planet with twice the radius of the Earth and the same density would have twice the surface gravity. Of course, SF writers don’t usually bother listing the mass, density, or radius, just the gravity. So you say that, say, Sphinx has a gravity of 1.7 times Earth’s, but don’t worry about its size or density.
And atmospheric thickness would not necessarily increase with increasing mass. Venus is a bit lighter than the Earth, and much hotter, both of which should contribute to a thinner atmosphere, all else being equal. Yet Venus’ atmosphere is actually about 100 times thicker than the Earth’s. Both Earth and Mars have atmospheres much thinner than they could theoretically support, based on their masses and temperatures, and there’s no reason, in principle, why any planet absolutely must have an atmosphere at all.
For the specific question of the Earth in the solar system, I don’t think that there’s a large enough sample to really say anything at all. Depending on how you count the Moon and Ceres, there’s maybe six Earthlike planets in the Solar System, and Earth happens to be the largest of them. Well, there’s a 1 in 6 chance that would happen anyway. No statistician is going to notice a 1 in 6 chance.
Time for some field research on Krypton…
Poul Anderson’s “Flandry” SF story A Stone in Heaven was largely set on a planet that was a former gas-giant core orbiting a star which had gone supernova or reasonable facsimile. Outgassing from the core remnant, after virtually all the H and He had been driven off, had given it essentially earthlike conditions. However, its size, mass, and radius combined to give it a surface gravity of 7 G’s.
There was at one time some speculation that the Earth’s Moon was responsible for our particular conditions, as opposed to those of Venus: that a world as large as Earth or Venus would retain a dense atmosphere largely of CO[sub]2[/sub] and nitrogen unless it had a large companion “stirring things up” and lifting off some of the volatiles.
One of them has to be the largest, so why not the earth?
Ah yes. Niven used it and I asked about it here in some other thread. I think the consensus was that the idea is wrong.
Just to slightly clarify Chronos’s post, in case it’s not obvious.
- Gravitational force from a sphere is just the same as if all the mass was at the center of the sphere.
- G.F. is proportional to mass
- G.F. drops off with distance, as the square of the distance.
- A sphere with twice the radius has 2^3 or 8 times the mass.
With a sphere twice the radius, the mass goes up (by a factor of 8), but the surface is also twice as far from the center (so G.F. drops by 4). The result is that a sphere twice the radius has twice the surface gravity.
:smack: I remembered the mass but forgot the distance. Nobody tell my 11th grade physics teacher!
A statistician would probably also consider Venus and Earth to be equivalent in size, with only about a 5% difference in diameters. This is actually a pretty big coincidence, unless some physical process drove them to those dimensions. If rocky planetary bodies can form in sizes from a pebble up to maybe two or three earth diameters, the fact that two neighbors wound up being almost identical in size seems rather unlikely. Much more so than the 1/6 chance that Earth would be the biggest of the bunch.
Well, unlikely things happen all the time. It’s probably something to make a note just of in case we find another star with a family of planets.
Well, assuming the same density. Our planet’s density is 5,515 kg/m^3. What if there is a planet who’s density is higher? Is it really possible to have a higher density and still be able to support life?
Depends on how you look at it… A 5% difference in diameter corresponds to more than a 15% difference in mass, which doesn’t look so coincidental, any more.
That’s a good point, but as David Simmons says, it’s only interesting at all as a data point when our sample size gets smaller. As we discover rocky planets around other star systems, if the ones at the same distances from equivalant stars all fall in the same size range, it tells us something about the way that they developed.
Who knows? We have no examples. Robert Forward wrote two novels (Dragon’s Egg and Starquake) about neutronium-based lifeforms on the surface of a neutron star. (From our POV, they live very fast.)