[LINK] "A tale of two worlds: habitable, or colonizable?"
In a post at his blog, Canadian science fiction writer Karl Schroeder makes the distinction between "habitable" worlds and "colonizable" worlds. To illutrate, he uses two recently discovered exoplanets: Alpha Centauri Bb, a planet somewhat more massive than the Earth orbiting Alpha Centauri B in a scorching three-day orbit; and, Gliese 667Cc, a super-Earth that orbits stable red dwarf Gliese 667Cc squarely in its habitable zone. Gliese 667Cc could support liquid water on its surface, and thus conceivably an Earth-like environment. Alpha Centauri's world, though, might be a better prospect, for all that the half of its surface permanently exposed to its sun is a magma sea. Why?
Because 581g is a super-earth, the gravity on its surface is going to be greater than Earth's. Estimates vary, but the upper end of the range puts it at 1.7g. If you weigh 150 lbs on Earth, you'd weigh 255 lbs on 581g. This is with your current musculature; convert all your body fat to muscle and you might just be able to get around without having to use leg braces or a wheelchair. However, your cardiovascular system is going to be under a permanent strain on this world--and there's no way to engineer your habitat to comfortably compensate.
On the other hand, Centauri Bb is about the same size as Earth. Its surface gravity is likely to be around the same. Since it's tidally locked, half of its surface is indeed a lava hell--but the other hemisphere will be cooler, and potentially much cooler. I wouldn't bet there's any breathable atmosphere or open water there, but as a place to build sealed domes to live in, it's not off the table.
Also consider that it's easier to get stuff onto and off of the surface of Bb than the surface of a high-gravity super-earth. Add to that the very thick atmosphere that 581g is likely to have, and human subsistence on 581g--even if it's a paradise for local life--is looking more and more awkward.
Colonizable worlds, Schroeder goes on to suggest, have accessible surfaces, elements needed for life and industry in sufficient quantity, and a "manageable flow of energy at the surface" (Venus' surface fails as its uniformly superhot). Mars comes off badly, actually, on account of its low nitrogen content.
Because 581g is a super-earth, the gravity on its surface is going to be greater than Earth's. Estimates vary, but the upper end of the range puts it at 1.7g. If you weigh 150 lbs on Earth, you'd weigh 255 lbs on 581g. This is with your current musculature; convert all your body fat to muscle and you might just be able to get around without having to use leg braces or a wheelchair. However, your cardiovascular system is going to be under a permanent strain on this world--and there's no way to engineer your habitat to comfortably compensate.
On the other hand, Centauri Bb is about the same size as Earth. Its surface gravity is likely to be around the same. Since it's tidally locked, half of its surface is indeed a lava hell--but the other hemisphere will be cooler, and potentially much cooler. I wouldn't bet there's any breathable atmosphere or open water there, but as a place to build sealed domes to live in, it's not off the table.
Also consider that it's easier to get stuff onto and off of the surface of Bb than the surface of a high-gravity super-earth. Add to that the very thick atmosphere that 581g is likely to have, and human subsistence on 581g--even if it's a paradise for local life--is looking more and more awkward.
Colonizable worlds, Schroeder goes on to suggest, have accessible surfaces, elements needed for life and industry in sufficient quantity, and a "manageable flow of energy at the surface" (Venus' surface fails as its uniformly superhot). Mars comes off badly, actually, on account of its low nitrogen content.