Astronomers identify an actual planet with two suns – like Star Wars’ ‘Tatooine’

Artist’s impression of Kepler-16b, the first known planet to permanently orbit two stars – what is called a circumbinary planet. The planet, visible in the foreground, was discovered by NASA’s Kepler mission. Credit: NASA/JPL-Caltech/T. Pyle

A study proves that ground-based telescopes can search for planets with two suns.

Astronomers have used a new technique to confirm a real Tatooine, the fictional planet with two suns that was home to Luke Skywalker in ‘Star Wars’.

The planet, Kepler-16b, is about 245 light-years from Earth, is a gas giant, and is roughly the size of Saturn. Scientists already knew the planet existed, but in a recent study, an international team of astronomers explained how they successfully applied a technique that had never been used before to observe a planet orbiting two stars.

“This is confirmation that our method is working,” said David Martin, co-author of the study and " data-gt-translate-attributes="[{" attribute="">Nasa Sagan Fellow in the Department of Astronomy at Ohio State University. “And that creates an opportunity for us to apply this method now to identify other systems like this.”

The technique, called the radial velocity method, has long been used in astronomy. (The first planet ever found around a sun-like star was found using radial velocity – and was found using the same telescope astronomers used to find this one.)

The method of radial velocities consists of analyzing the spectra of light produced by stars. Astronomers collect spectral data through ground-based telescopes – in this case, from a France-based telescope, the Observatoire de Haute Provence. This spectral data comes in the form of a line, but the line “wobbles” as the planet orbits the two stars, producing a shaky line in the light spectra. The wobble indicates a planet is there, and astronomers can use it to derive a number of other pieces of information about a planet, including its mass.

Measuring radial velocity is, according to Martin, one of the best tools astronomers have for identifying exoplanets, or planets outside our solar system. But until this study, astronomers hadn’t been able to use it to find planets outside our solar system that orbit two stars.

The study was published this week in the Royal Astronomical Society Monthly Notices.

In the past, these planets – called circumbinary planets – were identified by watching the passage of one star in front of another. This method, known as the “transit method,” identified 14 such planets, including Kepler-16b. The first confirmed circumbinary planet was described in a 2011 paper; others followed. But until this article, none had been found using radial velocity.

“What people had to deal with is that having two sets of two star spectra makes it really tricky, and people were having trouble getting enough precision to see the wobble caused by the planet,” Martin said. “And we got around that by doing a survey of systems with two stars orbiting each other where one star is large and the other is quite small.”

The survey, called Binaries Escorted by Orbiting Planets, or BEBOP, was created specifically to search for planets like this.

One of Kepler-16b’s stars is about two-thirds the mass of Earth’s sun, and the other about 20% the mass.

Astronomers had been monitoring this system since July 2016.

Proving that measuring radial velocities can identify planets orbiting two stars, Martin said, opens the door to the technique’s wider application. This is important to astronomers for a number of reasons, but the main one is that planets that orbit two stars tend to exist at a distance that would make them good candidates for life.

“These planets are frequently in the habitable zone, at a distance from stars where one would expect to find liquid water,” Martin said.

Kepler-16b, which is composed mostly of gas, is unlikely to be a candidate where life could be found, Martin said. But using the radial velocity method could help astronomers find other similar planets.

Reference: “BEBOP III. Observations and independent mass measurement of Kepler-16(AB)b – the first circumbinary planet detected with radial velocities” by Amaury HMJ Triaud, Matthew R Standing, Neda Heidari, David V Martin, Isabelle Boisse, Alexandre Santerne, Alexandre CM Correia , Lorena Acuña, Matthew Battley, Xavier Bonfils, Andrés Carmona, Andrew Collier Cameron, Pía Cortés-Zuleta, Georgina Dransfield, Shweta Dalal, Magali Deleuil, Xavier Delfosse, João Faria, Thierry Forveille, Nathan C Hara, Guillaume Hébrard, Sergio Hoyer, Flavien Kiefer, Vedad Kunovac, Pierre FL Maxted, Eder Martioli, Nicola J Miller, Richard P Nelson, Mathilde Poveda, Hanno Rein, Lalitha Sairam, Stéphane Udry and Emma Willett, February 25, 2022, Royal Astronomical Society Monthly Notices.
DOI: 10.1093/mnras/stab3712

Martin’s portion of this work was funded in part by NASA.

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