What can binary star systems teach astronomers about the formation and evolution of planets orbiting them? This is what a recent study published in Nature hopes to address as a team of scientists investigated past studies that claimed a specific binary star system could host a planet demonstrating a retrograde orbit, meaning it orbits in the opposite direction of the star’s rotation. This study has the potential to help scientists better understand binary and multiple star systems, specifically the formation and evolution of their planets and what this could mean for finding life beyond Earth.
For the study, the researchers examined a planet in the binary star system, nu Octantis (nu Oct), which is located approximately 22.54 parsecs (73.5 light-years) from Earth, and is comprised of a K-type star (nu Oct A) that is approximately 1.5 times as massive as our Sun and a smaller star (nu Oct B) that is half as massive as our Sun. The planet—designated as nu Oct A b as it orbits nu Oct A—is estimated to be approximately 2.19 Jupiter masses while orbiting at approximately 1.24 astronomical units (AU) from its host star with an orbital period of approximately 402 days.
The researchers obtained radial velocity measurements from the European Southern Observatory (ESO)’s HARPS spectrograph to ascertain why nu Oct A b orbits at the distance it does, with past studies hypothesizing that it has a retrograde orbit due to its large orbital distance. In the end, not only did the researchers confirm that nu Oct A b has a retrograde orbit, but they also discovered that the smaller star in the system is a white dwarf, which are incredibly dense stars approximately the size of Earth that were once larger stars. For context, our Sun will eventually become a white dwarf star billions of years from now. Both the retrograde orbit of nu Oct A b and the white dwarf conformation of nu Oct B enabled the researchers to learn more about the system’s history.
“We found that the system is about 2.9 billion years old and that nu Oct B was initially about 2.4 times the mass of the Sun and evolved to a white dwarf about 2 billion years ago,” said Ho Wan Cheng, who is from The University of Hong Kong and lead author of the study. “Our analysis showed that the planet could not have formed around nu Oct A at the same time as the stars.”
In addition to the system’s age and stellar history, the researchers suggest two scenarios for the formation of nu Oct A b: a) It formed in a debris disk that was created from nu Oct B becoming a white dwarf, and b) It was initially in a normal orbit around the binary system (also called prograde) and was captured by nu Oct A’s gravity.
As noted, what makes this planet unique is its retrograde orbit. While Venus and Uranus in our solar system have retrograde rotations, they still exhibit prograde orbits. Therefore, the retrograde orbit of nu Oct A b could help scientists better understand the formation and evolution of exoplanets, and specifically within binary or multiple star systems. The closest multiple star system to Earth is the triple-star system, Alpha Centauri, which is located approximately 4.37 light-years from Earth and is also the closest star system, as well. While that system contains one (unconfirmed) rocky exoplanet, it exhibits a prograde orbit. There are currently only two other exoplanets that have retrograde orbits, HAT-P-7b and WASP-17b, which are part of a triple-star system and single star system, respectively.
What new discoveries about binary star systems and planetary formation and evolution will researchers make in the coming years and decades? Only time will tell, and this is why we science!
As always, keep doing science & keep looking up!