A Death in the Solar System

Say good-bye to the old nine planets. Say hello to a whole new celestial family.

By Neil deGrasse Tyson
Nov 27, 2006 6:00 AMNov 12, 2019 5:38 AM

Newsletter

Sign up for our email newsletter for the latest science news
 

It's official. Pluto is not a red-blooded planet. As decreed in August by a vote of the General Assembly of the International Astronomical Union, Pluto is now a dwarf.

At first, the IAU seemed ready to defend Pluto. On August 16, the union's seven-member Planet Definition Committee released a draft Planet Definition Resolution, which stated that round objects in orbit around the sun are planets. Pluto is a round object in orbit around the sun. Therefore, Pluto is a planet. This definition would have given everyone the right to utter "Pluto" and "Jupiter" in the same breath, even though Jupiter is 250,000 times larger. The draft resolution would also have opened the door to granting planet status to at least three objects that had, until recently, been considered unworthy.

Plutophiles had about a week to rejoice before the astronomers returned from their deliberation with a change of heart. According to the final IAU definition, a planet must still be round but must also dominate the mass of its orbital zone. In other words, a full-fledged planet must have no competitors in its zone.. Poor Pluto is crowded by thousands of other icy bodies in the outer solar system, some bigger than Pluto itself, so it fails the test. To soothe Pluto's boosters, the IAU elected to call it a dwarf planet, without entirely quantifying what that is.

0 free articles left
Want More? Get unlimited access for as low as $1.99/month

Already a subscriber?

Register or Log In

0 free articlesSubscribe
Discover Magazine Logo
Want more?

Keep reading for as low as $1.99!

Subscribe

Already a subscriber?

Register or Log In

Stay Curious

Sign up for our weekly newsletter and unlock one more article for free.

 

View our Privacy Policy


Want more?
Keep reading for as low as $1.99!


Log In or Register

Already a subscriber?
Find my Subscription

More From Discover
Stay Curious
Join
Our List

Sign up for our weekly science updates.

 
Subscribe
To The Magazine

Save up to 40% off the cover price when you subscribe to Discover magazine.

Copyright © 2025 LabX Media Group