An exoplanet is definitely orbiting two stars

Do you remember that iconic scene from Star Wars, where a young Skywalker comes out to the surface of Tatooine and watches two suns set? It turns out that may be what is happening to life forms on the exoplanet known as Kepler-16, a rocky planet that orbits in a binary star system. Originally discovered by NASA’s Kepler mission, an international team of astronomers recently confirmed that this planet orbits two stars at once – what’s known as a circumbinary planet.

The international team, led by Professor Amaury Triaud from the University of Birmingham, includes members of the BEBOP collaboration. This observing campaign began in 2013 and relied on telescopes around the world to perform radial velocity readings for circumbinary planets. The team’s research is published in Royal Astronomical Society Monthly Notices.

The planet, known as Kepler-16b, is located about 245 light-years from Earth and orbits its binary stars with a period of 228.8 days. Like Tatooine, lifeforms on this planet would look up at the sky and see two suns rise and set. However, the planet orbits outside the “habitable zone” of its two stars, which means conditions on the surface are likely very cold. It was discovered in 2011 by Kepler using the transit method (aka. transit photometry).

An illustration of the Kepler-47 circumbinary planetary system. Credit: NASA/JPL Caltech/T. Pyle

For this method, astronomers observe stars for periodic dips in brightness that indicate the presence of orbiting planets. Astronomers also rely on this method because it effectively sets constraints on the size of an exoplanet. For the purposes of their study, the team relied on the SOPHIE scale spectrograph on the 193 cm telescope at the Observatoire de Haute-Provence to perform radial velocity measurements (or Doppler spectroscopy) on the system.

This method involves observing the stars for signs of “wobble”, which indicates that gravitational forces are acting on them (caused by one or more planets). As co-author Dr. Alexandre Santerne (researcher at the University of Aix-Marseille) explained in a press release from the Royal Astronomical Society:

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

Their measurements confirmed that Kepler-16b orbits both stars (which orbit each other), a finding that could help answer an open question about binary star systems. According to the most widely accepted model of planet formation, planets are thought to form in a disk of dust and gas surrounding young stars – aka. a protoplanetary disc. This presents some difficulties with respect to binary systems, as the model predicts that gravitational forces could interfere with planet formation.

TOI 1338 b is a circumbinary planet orbiting around its two stars. Credit: NASA GSFC/Chris Smith

In recent years, the discovery and statistical significance of “hot Jupiters” has also raised questions for astronomers. According to the protoplanetary disk model, gas giants cannot form so close to their stars due to insufficient mass and excessive heat. The only possible explanation, according to astronomers, is that planets (while still forming) migrate inside the disk as a result of gravitational interactions with other bodies.

These results indicate that disk-driven migration is a viable process and a relatively common occurrence. Professor Triaud said:

“Using this standard explanation, it is difficult to understand how circumbinary planets can exist. This is because the presence of two stars interferes with the protoplanetary disk, which prevents dust from clumping into the planets, a process called accretion.

“The planet may have formed away 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 that we need to revise our understanding of the planetary accretion process.”

Artist’s impression of a hypothetical planet orbiting the star Alpha Centauri B, a member of the Solar System’s neighboring triple star system. Credit: ESO

The detection of Kepler-16b using a ground-based telescope and the radial velocity method was also significant. Essentially, he demonstrated that it is possible to detect circumbinary planets using more traditional methods with greater efficiency and lower costs than space observatories. With this success under their belt, the team plans to continue to search for previously unknown circumbinary planets and help answer questions about planetary formation.

As co-author Dr. Isabelle Boisse, scientist in charge of the SOPHIE instrument at Aix-Marseille University, summarized:

“Our discovery shows how ground-based telescopes remain highly relevant to modern exoplanet research and can be used for exciting new projects. Having shown that we can detect Kepler-16b. We will now analyze data taken from many other binary star systems and search for new circumbinary planets.

Further reading: Royal Astronomical Society

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