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Second Planet Found In a “Tatooine System” Orbiting Two Suns

Astronomers went looking for information about one planet and ended up discovering a different one instead.


Stephen Luntz

Stephen has a science degree with a major in physics, an arts degree with majors in English Literature and History and Philosophy of Science and a Graduate Diploma in Science Communication.

Freelance Writer

A drawing of the elements of the BEBOP-1 system behind the telescopes  (and laptop) used to discover them.

A drawing of the elements of the BEBOP-1 system behind the telescopes  (and laptop) used to discover them.

Image Credit: Amanda Smith/ University of Birmingham

One of the few planets known to orbit two stars, TOI-1338b, has a giant companion in the same system – only the second time that what is known as a Tatooine System has been found to be multi-planetary. Most known stars are part of binary systems, but we’re still just starting to learn how well they form and maintain planets, so the discovery fills a big hole in our knowledge. 

Binary systems can contain planets when stars are so wide apart that a planet orbits one and is barely affected by the other. Circumbinary systems, named Tatooines after Luke Skywalker’s home planet, are much more interesting, however. There, planets orbit around both stars.


Tatooines were once considered unlikely, the suspicion being the system’s evolution would at some point kick the planet out. We’ve now discovered a few, but not enough to look for patterns. Moreover, we don’t know if Tatooines are mostly lonely worlds, a system’s only surviving planet, or if they come in groups. A paper helps answer that. 

Since 2018, the TESS space telescope has been seeking planets crossing the face of their stars in systems dubbed TOIs (Tess Objects of Interest). A subset of TOIs are binary systems, for which the catchy name Binaries Escorted By Orbiting Planets (BEBOPs) was reserved should we find any

In 2020 a planet TOI-1338b was discovered in the TESS data by teenager Wolf Cukier on the third day of his NASA internship, giving the system the name BEBOP-1. Unfortunately, while TESS can identify a planet’s presence and the length of its orbit, it leaves big questions unanswered. “The transit method permitted us to measure the size of TOI-1338b, but not its mass which is the planet’s most fundamental parameter,” said Dr Matthew Standing, formerly of the University of Birmingham, in a statement

Standing and co-authors went looking for BEBOP-1b’s mass using the radial velocity method, also known as Doppler wobble, by which the first planets were found outside the solar system. They didn’t find it – the complications of having two stars affecting each other make such detections particularly difficult. Instead, however, they found a more massive planet almost twice as far out: BEBOP-1c. “BEBOP-1c has an orbital period of 215 days, and a mass 65 times larger than Earth, which is about five times less than Jupiter’s mass,” said Standing. With an orbital distance similar to Venus, that leaves plenty of room further out in the BEBOP-1 system for even more planets, but we may struggle to detect them unless their masses are also very large.


“Only 12 circumbinary systems are known so far, and this is only the second that hosts more than one planet,” said Dr David Martin of Ohio State University. It’s also the first time a circumbinary planet has been first discovered using this method.

Astronomers are keen to find more Tatooines, despite the greater challenges compared to other systems, because of what they can teach us about planetary formation more generally.

“Planets are born in a disc of matter surrounding a young star, where mass progressively gathers into planets,” said Dr Lalitha Sairam of the University of Birmingham. "In the case of circumbinary geometries, the disc surrounds both stars. As both stars orbit one another, they act like a giant paddle that disturbs the disc close to them and prevents planet formation except for in regions that are quiet and far away from the binary.”

This, Sairam added, makes it; “Easier to pinpoint the location and conditions of planet formation in circumbinary systems compared to single stars like the Sun”. From there we can retrace the planet’s movements to its current position.


Even the failure to find TOI-1338b/BEBOP-1b taught us something dramatic. Had its mass been greater than 22 times that of the Earth, the authors are confident they would have found it. Given its size, to be below that mass, BEBOP-1b’s density must be at most 36 percent that of water, making it a potential target for the JWST’s exploration of the chemistry of freakishly light planets. “If we are to unveil the mysteries of circumbinary Tatooine-like exo-atmospheres,” the authors write, “The TOI-1338/BEBOP-1 system provides a new hope.”

BEBOP-1b and BEBOP-1c are both gas giants and also too close to the stars to support life. However, if there are any living beings further out in the system, they would not see the stars as anything like equals. One of the pair is more than three times more massive and around four thousand times brighter than the other.

The study is published in Nature Astronomy.


spaceSpace and PhysicsspaceAstronomy
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