In our solar system, Jupiter is the largest planet we have, but what is the upper limit of planetary size?
If you gather too much mass into a single object, its core will fuse lighter elements into heavier ones.
At about eighty times the mass of Jupiter, you’ll achieve “true star” status, burning hydrogen into helium.
But lower than that, at about 14 times the mass of Jupiter, you’ll initiate deuterium fusion, where the remaining fuel from the Big Bang slowly self-generates its own energy.
This line – between a gas giant and a brown dwarf – defines the most massive planet.
In terms of physical size, however, brown dwarfs are actually smaller than the larger gas giants.
Above a certain mass, the atoms inside large planets will begin to compress so tightly that adding more mass will actually shrink your planet.
This happens in our solar system, explaining why Jupiter is three times the mass of Saturn, but only slightly larger physically.
But many solar systems have puffy planets made of much lighter elements, without big rocky cores inside.
Larger planets, like WASP-17b, can be up to twice the size of Jupiter before becoming stars.
This article has been reprinted with permission from Big Think, where it was originally published.