Discovering planets in orbit around binary star systems

A consortium of scientists has discovered a novel technique for discovering exoplanets that orbit around binary star systems known as, Cataclysmic Variables.

Over the last few decades, a huge number of exoplanets have been discovered around individual ‘normal’ stars. However, a novel investigation has indicated that there could be exceptions to this general pattern. Now, a multi-institutional study, including scientists from the National Autonomous University of Nuevo León (UANL), the National Autonomous University of Mexico (UNAM), and New York University Abu Dhabi (NYU Abu Dhabi), has uncovered a new method of detecting dim bodies, including planets that are orbiting exotic binary star systems called Cataclysmic Variables.

What are Cataclysmic Variables?

Cataclysmic Variables are binary star systems whereby the two stars are situated incredibly close to each other, so close that the smaller object transfers mass to the more massive star. Cataclysmic Variables are generally composed of a small, close type of star, known as a red dwarf star, and a hot, dense star called a white dwarf.

Red dwarf stars have a mass between 0.07 and 0.30 solar masses and a radius that is approximately 20% of the Sun’s, while white dwarf stars have a typical mass of around 0.75 Solar masses and a very small radius, close to Earth’s.

In the Cataclysmic Variables system, the transfer of matter from the small star forms an accretion disk around the compact, more massive star. The brightness of a Cataclysmic Variables system originates primarily from this disk and acts to overpower the light coming from the two stars. A third dim body orbiting a Cataclysmic Variable can have a significant impact on the mass transfer rate between the two stars, and consequently, the brightness of the whole system. The technique outlined in the team’s new research is based on the change of brightness in the accretion disk as a result of perturbations of the third body that orbits around the inner two stars.

What advancements have been made by the team’s research?

In their study, team leader, Dr Carlos Chavez, and his associates have approximated the mass and distance of a third body orbiting four different Cataclysmic Variables, utilising the changes in the brightness of each system. In accordance with the researchers’ calculations, these brightness variations have incredibly long periods in contrast to the orbital periods in the triple system. Out of the four Cataclysmic Variables, two appear to have bodies reminiscent of planets in orbit around them.

Commenting on these findings, Dr Chavez concluded: “Our work has proven that a third body can perturb a cataclysmic variable in such a way that can induce changes in brightness in the system. These perturbations can explain both the very long periods that have been observed – between 42 and 265 days – and the amplitude of those changes in brightness.” He added that “of the four systems we studied, our observations suggest that two of the four have objects of the planetary mass in orbit around them.”

The team are hopeful that this new technique can play a key role in discovering planets in orbit around binary star systems, adding to the thousands already discovered in the last three decades.

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