Circumbinary Exoplanet Detected via Radial Velocity Measurements

Kepler-16b, a rare exoplanet that orbits around two stars, has been detected using the SOPHIE spectrograph at the 1.93-m telescope of the Observatoire de Haute-Provence.

First discovered in 2011, Kepler-16b is a giant circumbinary exoplanet similar to Saturn in both size and mass.

The alien world orbits its two parent stars, Kepler-16A and Kepler-16B, every 229 days at a distance of 65 million miles — similar to Venus’ 225-day orbit.

But it lies outside the system’s habitable zone, where liquid water could exist on the surface.

Both stars are smaller and cooler than our Sun: one is a K dwarf about 69% the mass of the Sun and the other is a red dwarf only 20% % the mass of the Sun.

As a result, Kepler-16b is quite cold, with a surface temperature of around minus 73 to minus 101 degrees Celsius (minus 100-150 degrees Fahrenheit).

“The typical process of planetary formation is thought to take place within a protoplanetary disk — a mass of dust and gas that surrounds a young star,” said University of Birmingham’s Professor Amaury Triaud and colleagues.

“However, this process may not be feasible within a circumbinary system.”

“Using this standard explanation it is difficult to understand how circumbinary planets can exist. That’s because the presence of two stars interferes with the protoplanetary disk, and this prevents dust from agglomerating into planets, a process called accretion.”

“The planet may have formed far from the two stars, where their influence is weaker, and then moved inwards in a process called disk-driven migration — or, alternatively, we may find we need to revise our understanding of the process of planetary accretion.”

“Circumbinary planets provide one of the clearest clues that disk-driven migration is a viable process, and that it happens regularly,” said Dr. David Martin, an astronomer at the Ohio State University.

In the new study, the astronomers were able to detect Kepler-16b via the radial velocity method.

“The radial velocity method is amongst the most robust and most established means of detecting exoplanets,” they said.

“However, until now, it failed to detect circumbinary planets despite their relatively high occurrence rates.”

The detection of Kepler-16b using a ground-based telescope at the Observatoire de Haute-Provence and the radial velocity method is an important demonstration that it is possible to detect circumbinary planets using more traditional methods, with greater efficiency and at a lower cost than by using space-based observatories.

“Kepler-16b was first discovered 10 years ago by NASA’s Kepler satellite using the transit method. This system was the most unexpected discovery made by Kepler,” said Dr. Alexandre Santerne, an astronomer at Aix-Marseille University.

“We chose to turn our telescope to Kepler-16 to demonstrate the validity of our radial-velocity methods.”

“Our discovery shows how ground-based telescopes remain entirely relevant to modern exoplanet research and can be used for exciting new projects,” said Dr. Isabelle Boisse, also from Aix-Marseille University.

“Having shown we can detect Kepler-16b, we will now analyze data taken on many other binary star systems, and search for new circumbinary planets.”

The discovery is reported in a paper in the Monthly Notices of the Royal Astronomical Society.

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Amaury H.M.J. Triaud et al. 2022. BEBOP III. Observations and an independent mass measurement of Kepler-16 (AB) b – the first circumbinary planet detected with radial velocities. MNRAS 511 (3): 3561-3570; doi: 10.1093/mnras/stab3712