(SCIENCE TREK MUSIC) JOAN CARTAN-HANSEN, HOST: To understand more about exoplanets, you first should know what a planet is.
You would think this is an easy question to answer, but it isn't.
Scientists have been arguing about this for years.
The current definition of a planet is: A body or object that orbits a star and not a different planet.
It must be big enough to have enough gravity to force it into a spherical shape and it must be big enough that its gravity has cleared away any other objects of a similar size near its orbit.
Everything else in our solar system except satellites are called small solar system bodies.
Our solar system has 8 planets.
Dwarf planets like Pluto, dozens of moons and millions of asteroids, comets and meteoroids.
Now an exoplanet is any planet beyond our solar system.
Most are tied to other stars but there are free-floating or rogue exoplanets.
We've learned some exoplanets are a bit weird.
Some are entirely covered by water, some are covered by oceans of lava, and some may have atmospheres that rain rubies or glass.
For more information about exoplanets, check out the Science Trek website.
You'll find it at ScienceTrek.org.