Gas Giant Planet Midway Between Two Stars

ν Octantis is a spectroscopic binary consisting of two stars identified as ν Octantis A (1.61 times the Sun's mass) and ν Octantis B (0.55 times the Sun's mass). Nearly 13 years of radial velocity measurements of ν Octantis indicate the presence of a Jupiter-like planet with ~2.6 times the mass of Jupiter. The remarkable fact about this planet is that it orbits ν Octantis A at half the separation between ν Octantis A and ν Octantis B. The planet takes ~415 days to orbit ν Octantis A, while ν Octantis A and ν Octantis B take ~1050 days to go around one another. The mean separation between v Octantis A and ν Octantis B is ~2.6 AU.

If the orbit of the planet around ν Octantis A is prograde, then its orbit can only be stable if its distance from ν Octantis A is less than ~0.25 times the separation between ν Octantis A and ν Octantis B. Nevertheless, if the orbit of the planet around ν Octantis A is retrograde, its orbit can remain stable even if its separation from ν Octantis A is as large as ~0.5 times the separation between ν Octantis A and ν Octantis B. Since the planet orbits ν Octantis A midway between v Octantis A and ν Octantis B, it clearly indicates a retrograde orbit. Dynamical modelling suggests the orbit of the planet can remain stable for ~100 million years or more.

Reference:
Ramm et al. (2016), "The conjectured S-type retrograde planet in nu Octantis: more evidence including four years of iodine-cell radial velocities", arXiv:1605.06720 [astro-ph.EP]