I made this up to help as a guideline for world design. The info was gathered online and may be incorrect. (On checking some on my less readable notes I learned the largest exoplanet I wrote down is no longer the largest). I originally planned to include all the data points Starfinder lists for planets and extend the data for stars but I grew tired of it. I was going to add some info on oddball planets (Iron planets, Cthonic Planets) but didn't despite notes as it didn't quite fit and their were oddball types I missed before (Carbon planets).
Yes I am aware Starfinder is fantasy and has no need to follow these rules.
Well here it is.
The Planetary Cheat Sheet
What are the size limits of a planet?
The smallest possible size for a rocky planet is 600 KM, for an ice planet it is 400 KM.
How big can a world get before it gets too thick of an atmosphere and becomes a ice or gas giant? 5-10 earth masses. (2 Earth radii).
A rocky planet Larger than Earth but smaller than Uranus or Neptune (who are 15-17 earth masses) is often termed a super-Earth.Those on the higher end of the scale may be more accurately termed gas dwarves (mini neptune, and transition planet have also been used). A gas dwarf may have hydrogen/helium atmosphere and deep layers of ice, rock, or liquid oceans of water, ammonia, heavier volatiles, or a mix of them.
I could not find a size cut off between a gas and ice giant sadly.
Hot big can a planet get?
A world 15 jupiter masses or larger will fuse deuterium becoming a brown dwarf star.
The largest known planet, Tres 4 in the constellation Hercules is 1.4 times the size of Jupiter (70 percent larger). 0.8474 Jupiters mass but low density. (Update: WASP 12b, WASP 17b, and CT Chamaeleontis are larger).
To put the Jupiter measures in perspective, Jupiter is 317 times more massive than Earth, 11 times larger, 1400 Earths can fit inside of it.
At what size is a planet habitable?
Larger than Mars but smaller than Earth. 0.3 Earth masses is the possible dividing line.
Smaller planets have a higher surface to volume ratio, lose energy quickly, and end of geologically dead.
To be habitable a world must not rotate too quickly. It needs a magnetic field, gravity, and atmosphere to hold water.
Can exotic materials effect things?
Yes, but this is very very unlikely. A planet of pure Platinum could hold Earth like gravity at a size slightly smaller than the moon. A planet of pure Osmium can hold Earth like gravity at half the size of Mercury.
