From reading various sources online, I see that some people talk about binary planets in very tight orbits (around 1 or 2 planet radii). I am wondering if it is possible for a binary planet (Double planet) system to exist at a much greater distance from eachother.

Setup: Binary planets orbit their star at 1.19AU. Planet 1 is 1.254 Earth masses and planet 2 is 0.8087 Earth masses. They orbit eachother with a apoapsis of 750,600km and a periapsis of 742,500km.

According to calculations, they are within the sweet spot of each other's hill sphere and of course far from the Roche limit.

Bonus info to further complicate the situation, the star system is a binary star system, Alpha Centauri. The planets orbit the A star and are well within the stable orbital region of that star and should theoretically not be majorly affected by the B star.

Is this a stable planetary system? Any reasons why it wouldn't work would be great.

  • 1
    $\begingroup$ The earth - moon system is a binary (in some sense) with much greater separation $\endgroup$ – planetmaker Mar 4 '20 at 7:29
  • $\begingroup$ Pluto and Charon might be another example, their barycenter is outside of either one. $\endgroup$ – uhoh Mar 4 '20 at 11:17
  • 1
    $\begingroup$ Yes, Essentially this system is the Earth-moon system except the moon is substantially larger and double the distance away. $\endgroup$ – Markitect Mar 4 '20 at 12:58
  • $\begingroup$ I recently read a comment (that I can't find again) where the person said that a star would need to be at least 1.3 solar masses in order to have a binary planet system where the planets rotate on their own axis. Has anyone else heard of this before? $\endgroup$ – Markitect Mar 6 '20 at 22:12

Your Answer


Browse other questions tagged or ask your own question.