The possibility of binary terrestrial-type exoplanets

The possible existence of Earth-like binary planets is being described today at the American Astronomical Society’s Division for Planetary Sciences meeting in Tucson, AZ. Two bodies, each of mass similar to Earth, can form a closely orbiting pair under certain conditions present during the formation of planetary systems. (

There is a good reason to believe terrestrial binary planetary systems may be possible. In a grazing collision if the angular momentum is too high to be contained within a single rotating body it would fission and if the bodies barely touch then they could retain their identity. However, [the intermediate scenario in which a binary planet might form] requires an encounter where the bodies are initially approaching each other at low enough velocity.

To test for this possibility, a simulation technique called Smoothed Particle Hydrodynamics (SPH) was utilized. Smoothed Particle Hydrodynamics represents a body as a collection of tens of thousands of particles, and it has been used to study protoplanetary collisions as well as the giant impact hypothesis of the Moon’s formation.

Using SPH, collisions between two rocky Earth-sized bodies were simulated, with impact velocity and impact parameter (a measure of how head-on a collision is) being varied and the output observed. In the cases where the bodies underwent substantial collision, the scientists replicated previous results in which a did not arise but a moon might form. However, by including interactions where the bodies are close enough to undergo a large tidal distortion, initial conditions were found that led to a terrestrial binary planetary system.


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