The Space & Beyond Blog
What is a planet?
The three things that make a planet a planet
The International Astronomical Union adopted the latest definition of a planet in 2006, with three criteria that a celestial body must meet in order to be classified as a planet.
First, a planet must orbit a star (for our solar system, that would be the Sun). A planetary system’s multiple worlds tend to orbit in the same direction and in nearly the same plane.
A planet also must have sufficient mass to create a nearly round shape. Lastly, it must be big enough that its gravity clears away any other objects of similar size near its orbit around the Sun.
Debate and discovery among astronomers and planetary scientists continues to evolve, and while most of us are familiar with the defined planets of our solar system, there are many other objects in the sky that fall in between, such as stars and brown dwarfs.
Stars form when huge clouds of dust and gas collapse in on themselves and condense into hot cores that absorb the material around it. If the core becomes dense enough, nuclear fusion is triggered and hydrogen atoms start converting to helium, causing it to shine bright in the sky for billions of years.
Brown dwarfs are thought to form the same way, but their cores don’t become dense enough for nuclear fusion to occur, which excludes them from official star classification. And since they form differently and exceed the maximum weight for a planet (which is about 13 times the mass of Jupiter), they’re technically excluded from planet status, too. Our partners at Astronomy provide a detailed explanation of the difference between a brown dwarf and a planet!