A thoroughly sporadic column from astronomer Mike Brown on space and science, planets and dwarf planets, the sun, the moon, the stars, and the joys and frustrations of search, discovery, and life. With a family in tow. Or towing. Or perhaps in mutual orbit.

What is a planet?

Not surprisingly, I frequenltly write about the topic of planetary classification. It is a topic that can be complex, subtle, contentious, and silly. I try to dissect the debate in some of the essays below.

  •  Why do so many people believe that Pluto should be a planet? I think it is because very few people have the right mental picture of the solar system. These wrong impression start early.
 Planetary placemats (understand the solar system in one picture)
  •  If it weren't for that magical word "planet" we wouldn't have any of these problems. So why does it matter so much?
 What's in a name? (who should get to be called a planet?)
  • Things that aren't quite planets are now called "dwarf planets." Yes, it is a generally poor choice of words, but what does this generally poor choice of words mean?  
 What is a dwarf planet? (it's like a real planet, only dwarfier)
  • The debates about Pluto and planets are full of seriously flawed arguments from people who should know better. There are good arguments to be made on both sides. Find them here. 
 Ground rules for debating the definition of "planet" (if you learn these logical arguments you are ahead of 99% of the crowd, regardless of which side you are on)
  • A reminder of why Pluto was demoted in the first place and a tribute to the beauty and order of the solar system.
 So is Pluto a planet after all? (if Pluto and Eris are the same size does that make Pluto a planet again?)