An exoplanet is any planet beyond our solar system. Most orbit other stars,
but free-floating exoplanets, called rogue planets, orbit the galactic
center and are untethered to any star.
What is an ExoPlanet?
Most of the exoplanets discovered so far are in a relatively small region of our galaxy, the Milky Way. We know
from NASA’s Kepler Space Telescope that there are more planets than stars in the galaxy.
By measuring exoplanets’ sizes (diameters) and masses (weights), we can see compositions ranging from very
rocky (like Earth and Venus) to very gas-rich (like Jupiter
and Saturn). Exoplanets are made up of
elements similar to those of the planets in our solar system, but
their mixes of those elements may differ. Some planets may be dominated by water or ice, while others
are dominated by iron or carbon. We’ve identified lava worlds covered in molten seas, puffy planets the
density of Styrofoam and dense cores of planets still orbiting their stars.
Planet Types
Exoplanets come in a wide variety of sizes, from gas giants larger than Jupiter to small, rocky planets about as
big around as Earth or Mars. They can be hot enough to boil metal or locked in deep freeze. They can orbit
their stars so tightly that a “year” lasts only a few days; they can orbit two suns at once. Some exoplanets
are sunless rogues, wandering through the galaxy in permanent darkness.
