A retrograde planet in a tight binary star system with a white dwarf

Area of Science:

• Astronomy and astrophysics
• Exoplanetary science
• Stellar evolution

Background:

• Close-in binary stars typically hinder the formation and stability of circumstellar (S-type) planets.
• The star ν Octantis was suspected to host a circum-primary planet in a wide, retrograde orbit, but its existence was doubted due to formation challenges.
• The observed scarcity of S-type planets in close binary systems supports this formation difficulty.

Purpose of the Study:

• To investigate the existence and orbital characteristics of a suspected planet around ν Octantis.
• To determine the nature of the companion star in the ν Octantis system.
• To explore the formation pathways of planets in binary star systems, particularly those with unusual orbital configurations.

Main Methods:

• New radial velocity measurements were acquired to analyze the orbital dynamics of the suspected planet.
• Adaptive optics imaging was employed to characterize the companion star.
• Simulations of primordial binary orbital settings were used to assess planet formation scenarios.

Main Results:

• Radial velocity data confirm the presence of a planet with a retrograde and nearly coplanar orbit.
• Adaptive optics imaging revealed the companion star to be a white dwarf.
• Initial binary separations indicate that coeval formation of the circum-primary planet is highly improbable.

Conclusions:

• The retrograde planet around ν Octantis likely formed from a circumbinary orbit or a second-generation protoplanetary disc.
• Binary stellar evolution plays a significant role in shaping planetary system architecture.
• This discovery challenges conventional models of planet formation in binary systems and highlights alternative pathways.