I had this nice cute idea about a small inhabitable planet that is, say, 1km around equator (that's about 320m in diameter). Is it possible that such planet has gravity the strength of the Earth's gravity? (or, to rephrase the question, from what matter should that 320m diameter planet be made of (it's nucleus (so there is fertile land on top)) in order to have Earth's gravity) ------------------------- (I guess something must "hold down" the atmosphere thick as the one on the Earth (hmm... and what atmosphere would it be (what about rain, thunderstroms etc?))) (also: what rotation speed must it be in order for it to have the Earth temperature range (same distance from our Sun as the Earth)) OR!: To rephrase the first queston even more: what diameter a planet must have in order to have Earth's gravity strength if its nucleus is made of material (known to us) with greatest gravity potential (non-radioactive material mind you). So: " diameter = ? "