Rotating Question Curious About Astronomy? Ask an Astronomer

Are Earth and the Moon "twin planets"?

Is the Moon a satellite of Earth or are Earth and the Moon a twin planet system?

I would say both. Whenever a group of bodies sharing an orbit the Sun also orbit each other, the largest one is called the planet and the rest are considered satellites. So, theoretically, if you have a double planet where one component is only 1% larger than the other, we would still call them "planet" and "satellite".

Definition of a double planet is less clear. There is no obvious lower limit to how big (or massive) a satellite has to be relative to the planet in order for the system to be considered a "double planet". Pluto and Charon (1/10 of Pluto's mass) are definitely a double planet and most people would consider Earth and the Moon (1/81 of Earth's mass) a double planet.

It is possible that once some Earth-sized extrasolar planets with moons are discovered, the term "double planet" will be defined more strictly, but so far it is completely unofficial.

January 2003, Matija Cuk (more by Matija Cuk) (Like this Answer)

Still Curious?

Get More 'Curious?' with Our New PODCAST:

Related questions:

More questions about The Earth: Previous | Next

More questions about The Moon: Previous | Next

How to ask a question:

If you have a follow-up question concerning the above subject, submit it here. If you have a question about another area of astronomy, find the topic you're interested in from the archive on our site menu, or go here for help.

Table 'curious.Referrers' doesn't existTable 'curious.Referrers' doesn't exist

This page has been accessed 33926 times since January 12, 2003.
Last modified: January 13, 2003 11:53:12 PM

Legal questions? See our copyright, disclaimer and privacy policy.
Ask an Astronomer is hosted by the Astronomy Department at Cornell University and is produced with PHP and MySQL.

Warning: Your browser is misbehaving! This page might look ugly. (Details)