Double planet

In astronomy, a double planet (also called a binary planet) is a system where two planets or planetary-mass objects orbit each other. Both objects orbit a common center of gravity, called a barycenter, which lies outside of either body. This means neither object is truly orbiting the other; instead, they both revolve around a shared point in space.

While many stars have binary systems—meaning they have two stars orbiting each other—double planets are thought to be much rarer. This is because planets usually have a much larger parent star nearby, which can affect their stability. For a double planet to stay in a stable orbit, special conditions are needed.

Our Solar System does not have an official double planet, but the Earth–Moon system is sometimes described this way. In fact, during the SMART-1 mission, the European Space Agency called Earth and the Moon a double planet in some of their materials. Scientists have also considered Pluto and its large moon Charon as a double planet. Though this idea was discussed by the International Astronomical Union, it was not officially adopted. Other objects in space, such as Eris–Dysnomia, Orcus–Vanth, and Varda–Ilmarë, might also qualify as double planets because of their large moons.

Smaller objects in space, like some binary asteroids, can also be called double minor planets when they have components of similar size. Examples include 69230 Hermes, 90 Antiope, 79360 Sila–Nunam, and 1998 WW31. These systems help scientists learn more about how objects in space form and move.

Definition of "double planet"

The Pluto–Charon system is closer to being a double planet than the Earth–Moon system.

There is debate about how to define a "double planet." One idea is that both bodies should clear their orbits, meaning they are large enough to control their paths around the Sun. Another idea is to look at the mass ratio — if the two bodies have similar masses, they might be considered a double planet. The Pluto–Charon system has a mass ratio close enough that many scientists call them "double dwarf planets." The Earth–Moon system has a mass ratio that is also relatively close, leading some to suggest it could be a double planet, though this view is not widely shared.

A common way to define a double planet is by where the center of mass lies. If this point is outside both bodies, they may be a double planet. Pluto and Charon fit this description, but the Earth–Moon system's center of mass is still inside Earth. However, in the future, as the Moon moves farther from Earth, this might change.