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Largest possible rocky planet
Anders Sandberg has written an interesting article about large-scale worlds; the link is here
Much of the article is about large scale megastructures, most of which is interesting but not really news to OA readers and contributors. But one section contains some very useful data and equations about the largest possible rocky or icy worlds.

As most worldbuilders know, if you increase the size of a gas giant it will eventually reach a maximum size, a little larger than Jupiter, and after that limit the planet will just get denser due to compression. (There is also an adjsutment for temperature, but adding extra mass will not affect the diameter so long as the temperature is comparable).

It turns out that rocky, metallic and icy worlds follow a similar pattern. Here is a diagram that Anders has made, using an equation from Seager et al. 2007
[Image: planetradius.png]
As you can see, the largest rocky worlds would be around 3 times Earth's radius, with 300 x Earth's mass. Any heavier than this, and the planetary material becomes increasingly denser, and indeed appears to contract.

In reality such a planet would be surrounded by a large hydrogen/helium atmosphere, as neither of those gases could escape - it would probably have a lot of water too, although Jupiter seems to have less water than expected, so this isn't a given.

Waterworlds would have a maximum radius of 4.5 Earths, with a similar mass, while the unlikely case of an iron-only planet would have a maximum radius of 2.3 Earths, at a slightly lower mass of about 250 Earths.

The figures in the paper apparently are sightly larger, with iron's maximum diameter at a mass of 274 Earths, perovskite (rock) at 378 Earths, and for ice at 359 Earths.

It would be interesting to imagine a maximal Earth with a mass 30 times our own world's, and certain clades could probably tolerate the 3+ gee conditions on such a world; but to get to this state the hydrogen/helium atmosphere would need to be removed somehow. Perhaps this could be a project for a slightly eccentric transap obsessed with terraforming, or it might be a relic of some ancient vanished civilisation. But I doubt it could form naturally.

Messages In This Thread
Largest possible rocky planet - by stevebowers - 06-25-2015, 02:54 AM
RE: Largest possible rocky planet - by Rynn - 06-25-2015, 03:00 AM
RE: Largest possible rocky planet - by Cray - 06-25-2015, 06:13 AM
RE: Largest possible rocky planet - by Cray - 06-25-2015, 09:53 PM
RE: Largest possible rocky planet - by radtech497 - 07-04-2016, 10:26 AM

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