SOFIA, Bulgaria — A groundbreaking discovery led in part by Bulgarian astrophysicist Dr. Trifon Trifonov has revealed a never-before-seen planetary orbit in a nearby binary star system, reshaping scientific understanding of how planets can form and survive in extreme gravitational environments.
The international research, published this week in the prestigious journal Nature, confirms the presence of an exoplanet orbiting the Nu Octantis (ν Octantis) system, located 73 light-years from Earth. What sets this planet apart is its rare retrograde orbit — it moves in the opposite direction to its host stars, a first in the field of exoplanetary science.
Dr. Trifonov, who heads the Bulgarian exoplanet research group within the “EXO-RESTART” project at Sofia University’s Department of Astronomy, played a key role in the analysis and verification process. “This discovery is not just a technical achievement; it fundamentally challenges our models of planetary system formation,” Trifonov said in a statement.
The Nu Octantis system comprises two stars: ν Oct A, a subgiant roughly 1.6 times the mass of the Sun, and its companion, ν Oct B, which has now been identified as a white dwarf. These stars orbit each other every 1050 days. Conventional wisdom holds that such close binary systems, with their intense gravitational forces, should prevent planet formation. But this new discovery proves otherwise.
The planet’s existence was first hinted at in 2004, when Dr. David Ram, then a PhD student in New Zealand, noticed an unusual signal in radial velocity data — the subtle motion of a star caused by an orbiting body. However, the planet’s apparent proximity to ν Oct B raised doubts about its stability unless it had a retrograde orbit.
To confirm its presence, researchers combined 18 years of radial velocity data with cutting-edge imaging from the European Southern Observatory’s HARPS spectrograph and SPHERE instrument in Chile. The dual approach provided irrefutable evidence of the planet’s retrograde path.
Further analysis revealed a stunning twist: ν Oct B is not a main-sequence star as once thought, but a white dwarf — a dense stellar remnant that was once the heavier of the two stars. It likely began with 2.4 times the mass of the Sun before shedding its outer layers in a red giant phase roughly two billion years ago.
That revelation led scientists to reconsider how the planet formed. According to lead author Professor Man Hoi Lee, the planet may be a “second-generation” world — either forming from material ejected by ν Oct B during its death throes or captured later into orbit around ν Oct A.
Dr. Trifonov emphasized the importance of this finding: “We’re witnessing a completely new chapter in planetary evolution. This may be the clearest evidence yet of planets forming after — not before — their host stars evolved.”
Funded by Bulgaria’s National Science Program through the EXO-RESTART initiative, the research underscores the growing role of Bulgarian scientists in international astrophysics and opens new avenues for studying planets in complex star systems.