In this episode of Our Island Universe, we take a look at how we define a planet... and how that definition is up for debate as we learn more about our universe.
Transcript:
What is a planet? We live on one, so you’d think someone would have come up a definition that works by now. Earth is a big ball of mostly rock but so is the Moon and it’s not a planet. Jupiter, Saturn, Uranus and Neptune are planets but they’re not big balls of mostly rock? Any why isn’t little old Pluto a planet any longer?!
This question is not new. In 1801, Ceres was discovered. Ceres is a small rock, about 600 miles across, that is found between the orbits of Mars and Jupiter. When it was discovered, it was labelled a planet. Soon after, Vesta was found nearby and it too was called a planet. In fact, we now know of more than 50,000 small pieces of rock between the orbits of Mars and Jupiter... we call it the Asteroid Belt, with Ceres and Vesta being its largest members.
In 2006, astronomers found themselves in the same situation. Objects like Sedna, Eris, and Makemake were all found just beyond Pluto. Are they planets too? In a vote, astronomers decided to call Pluto, Ceres, and these newly discovered objects "dwarf planets". And that Pluto, Sedna, Eris are just the largest members of something called the Kuiper Belt.
Science is always discovering new phenomena. Today we know of more than 2,000 planets around just the stars nearest to us! We may not know what a planet is exactly but we know our Galaxy is littered with them.