Spotted: a Tatooine-like exoplanet orbiting two stars

Happy Star Wars Day! And May the 4th be with you.

Like Tatooine – the home world of Anakin and Luke Skywalker – a rare exoplanet with its own binary sunset (but in our galaxy, the Milky Way) was spotted by astronomers earlier this year using a ground-based telescope.

We all know that the best films come in trilogies, like Star Wars and A Spacetime Trilogy with Jared Kaplan – a 2021 SCINEMA International Film Festival entry. In the series, Jared Kaplan, theoretical physicist at Johns Hopkins University in the US, looks to black holes, gravity, and even a dazzle of zebras, to examine profound and whimsical questions about spacetime and our universe.

Watch the full trilogy here.

A rare exoplanet orbiting two stars at once has been spotted by researchers using a ground-based telescope, in a first-of-its kind demonstration that less expensive, on-the-ground methods could be used to find such rare planets. 

The exoplanet, Kepler-16b, is 245 lightyears from Earth and, just like the home worlds of both Luke Skywalker (Tatooine) and Doctor Who (Gallifrey), has two sunsets a day as it drifts around its twin stars. It was first discovered 10 years ago by NASA’s Kepler satellite.

Kepler-16b orbits two stars, and those two stars orbit one another. Pairs of stars that orbit one another, known as binary systems, are not rare in themselves – estimates range, but CSIRO predicts possibly up to 85% of all stars are in binary systems. But circumbinary planets (planets that orbit these twin stars) are rare, and before Kepler-16b we only knew of ten.

In fact, scientists aren’t sure how circumbinary planets can come to exist. Typically, planets are formed from the mass of dust and gas that surrounds a young star, known as the protoplanetary disc. But this process may not be possible in a binary system.

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

The team observed the exoplanet using a telescope based at the Observatoire de Haute-Provence, in France. They were able to detect the exoplanet using the radial velocity method, observing the change in the velocity of a star as a planet orbits around it. 

The researchers say this demonstration is important, because it shows we can detect exoplanets using cheaper and more efficient methods. They say the radial velocity method is more sensitive to additional planets in a system, and can also measure the mass of a planet.

Binary sunset on the exoplanet kepler-16b
Most star systems in the universe are probably binary systems – two stars orbiting one another. But planets that orbit two stars (like Tatooine in the Star Wars series) are rare, and would experience two sunsets. Image credit: NASA/JPL-Caltech

They hope to continue the search for more circumbinary planets, and hopefully answer the pressing question of how these planets form. 

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

Isabelle Boisse, from the University of Marseille, is the scientist in charge of the SOPHIE instrument used to collect the data.

“Our discovery shows how ground-based telescopes remain entirely relevant to modern exoplanet research and can be used for exciting new projects,” Boisse says. “Having shown we can detect Kepler-16b, we will now analyse data taken on many other binary star systems, and search for new circumbinary planets.”

The study is published in Monthly Notices of the Royal Astronomical Society. 

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