Bulgarian Scientists Confirm First Retrograde Exoplanet in Close Binary Star System

Dr. Trifon Trifonov, head of the Bulgarian exoplanet research group within the “EXO-RESTART” project at Sofia University’s Department of Astronomy, is a key contributor to a groundbreaking study published in the renowned journal Nature. The study confirms the presence of an exoplanet orbiting a close binary star system located 73 light-years from Earth. Remarkably, this planet follows a retrograde orbit - moving opposite to the direction of its companion star. This unique orbital pattern is a first in exoplanetary science and offers crucial insights into how planets form and evolve in tightly bound binary systems.

The binary system in question, Nu Octantis (ν Octantis), consists of two stars orbiting each other every 1050 days. The primary star, ν Oct A, is a subgiant with about 1.6 times the mass of the Sun, while its companion, ν Oct B, is roughly half the Sun’s mass. While most stars in the universe exist as part of multiple star systems, the gravitational influence of a close stellar companion typically disrupts planet formation. This discovery challenges previous assumptions, showing that planets can indeed exist in such complex environments.

The discovery process involved careful analysis of radial velocity data spanning 18 years. Radial velocity measures the motion of a star along the line of sight, indicating gravitational effects of orbiting planets. The first signal suggesting a planet around ν Oct A was detected in 2004 by Dr. David Ram during his PhD research in New Zealand, but skepticism remained due to the planet’s orbit being unusually close to ν Oct B. Stability in such a system seemed possible only if the planet moved retrograde, contrary to standard orbital dynamics. However, no conclusive evidence had yet confirmed this theory.

To settle the question, the research team used cutting-edge instruments at the European Southern Observatory (ESO) in Chile, combining data from the HARPS spectrograph and the SPHERE imaging instrument. This comprehensive approach validated the planet’s existence and its retrograde orbit beyond doubt, marking a first in the field of exoplanetary studies.

Another major finding from the study involves the nature of ν Oct B. Analysis with the SPHERE instrument revealed that this companion star is most likely a white dwarf - an extremely dense stellar remnant with a mass around 60% that of the Sun but squeezed into a volume comparable to Earth. This indicates ν Oct B has passed through its red giant phase, shedding much of its mass. Initially, ν Oct B was the heavier star in the system, with an estimated mass 2.4 times that of the Sun, before evolving into a white dwarf about two billion years ago. The entire system is approximately 2.9 billion years old.

This evolutionary history has important implications for the planet’s origin. The research suggests the planet likely did not form alongside the stars initially. Instead, it may have arisen in a retrograde accretion disk of material surrounding ν Oct A, composed of mass lost by ν Oct B during its transformation. Alternatively, the planet could have been captured into its unusual orbit from the broader binary environment, rather than forming in place. Professor Man Hoi Lee, the study’s lead author, highlights these possibilities as key to understanding this rare planetary configuration.

Dr. Trifonov stresses the significance of this discovery as a probable example of a second-generation planet - either formed from stellar ejecta or captured after the companion star’s mass loss. Such a planet reflects a dynamic and previously unseen chapter in planetary evolution, reshaping our understanding of how planets can survive and form in challenging conditions.

This research is supported by the EXO-RESTART project, funded by the Bulgarian Science Fund under the National Science Program for top-tier research and European scientific development. The ongoing search for exoplanets continues to uncover complex systems, with this study emphasizing the valuable insights gained from planets orbiting evolved stars in tight binaries. These discoveries deepen our knowledge of planetary formation processes and the diversity of planetary systems in the universe.

Source: https://www.phys.uni-sofia.bg/?p=30733