Join PopSci+ to read science’s greatest stories.
It’s just $1 per month »
JOIN
LOG IN
Channels
PopSci+
Science
Archaeology
Ask Us Anything
Biology
Dinosaurs
Physics
Space
Technology
AI
Aviation
Best of What’s New
Engineering
Internet
Military
Robots
Security
Vehicles
Environment
Agriculture
Animals
Climate Change
Conservation
Energy
Sustainability
Weather
DIY
Life Skills
Projects
Tech Hacks
Health
Diseases
Fitness & Exercise
Medicine
Nutrition
Psychology
Gear
Audio
Cameras
Computers
Fitness Gear
Gaming
Gift Guides
Home
Home Theater
Outdoor Gear
Phones
Tablets
Wearables
More
Merch
Newsletter Sign-Up
PopSci Shop
Podcasts
Video
FIND US ON
Social
Facebook
Twitter
LinkedIn
Instagram
Pinterest
Youtube
Flipboard
Apple News+
RSS
SOCIAL
PopSci +
Popsci+
Subscribe
Log in
Science
Archaeology
Ask Us Anything
Biology
Dinosaurs
Physics
Space
Technology
AI
Aviation
Best of What's New
Engineering
Internet
Military
Robots
Security
Vehicles
Environment
Agriculture
Animals
Climate Change
Conservation
Energy
Sustainability
Weather
DIY
Life Skills
Projects
Tech Hacks
Health
Diseases
Fitness & Exercise
Medicine
Nutrition
Psychology
Gear
Audio
Cameras
Computers
Fitness Gear
Gaming
Gift Guides
Home
Home Theater
Outdoor Gear
Phones
Tablets
Wearables
Newsletter Sign-Up
MERCH
PopSci +
Christopher Palma/The Conversation
Get latest from Christopher Palma/The Conversation
Why the definition of a planet is always changing
By Christopher Palma/The Conversation / Dec 18, 2019
