I love exoplanets: alien worlds orbiting alien stars. For centuries we’ve wondered if other stars hosted planets, and then in the 1990s they were confirmed. Now we’ve just passed 6,000 such discoveries, with more coming in all the time.

One of the most interesting flavors of exoplanets are ones orbiting binary stars, which are systems with two stars orbiting each other. Binary systems are extremely common, so finding planets around them opens the possibility of many such planets existing.

A lot of these planets are circumbinary, meaning they orbit both stars farther out (think Tatooine). If the planet is far enough away the orbit can be stable, but too close and the constantly varying distance between the two stars can make an orbit unstable.

The other type of planet orbits just one star in the binary system. A lot are known like that as well.

But now we have a new category to add: a binary system where there are planets orbiting both stars.

Well, maybe. One of the planets isn’t confirmed yet, but looks likely.

The star system is called TOI-2267 — that’s for TESS Object of Interest, meaning a star or stars that may have planets, in a catalog of stars observed by the Transiting Exoplanet Survey Satellite, or TESS, which looks for planets around other stars. TESS stares at stars and looks for tiny dips in their light as planets orbit around them and cause mini-eclipses. This is called the transit method and it’s the most successful way we have of finding these exoplanets.

The binary is about 72 light-years from us (note the press release is wrong; it says 190) and is composed of two very faint red dwarfs. They’re very similar, with one being a tad bit smaller and fainter than the other (their spectral types are M5 and M6). The bigger one, called the primary and named TOI-2267A, is about 0.2 times the mass of the Sun, 0.17 times the radius, and glows at a feeble 0.0033 times the Sun’s luminosity. The secondary, TOI-2267B, is 0.1 times the Sun’s mass, 0.13 the radius, and 0.0011 times the luminosity. These stars are faint.

They orbit each other at a distance of about 1.2 billion kilometers, or roughly the distance of Saturn from the Sun. That’s close enough that any planets would have to stick tight to their respective host stars, or else the gravity of the other star would destabilize the orbit.

And that’s the fun part: a team of astronomers found planets orbiting each star, very close in! [link to journal paper] With some caveats, though. 

They definitely detect two planets, called TOI-2267b and c (note the lower case letters, which are used for planets, while upper case is used for stars). Both planets are about the same size as Earth, and take 2.28 and 3.49 Earth days to orbit their host star. In addition, they found a strong detection of a third planet from TESS, but haven’t been able to confirm it with ground-based telescopes. So for now it’s a candidate planet. If it exists, it has a period (a year) of 2.03 Earth days and is also about the same size as Earth.

The thing is, they don’t know which planet orbits which star! The two stars are close enough together that, from Earth, they blend into one object. They’re so similar to each other that it’s not possible to know which planet orbits which star.

But they simulated the orbits of the planets and found that, if the third one exists, then there is no stable configuration where all three orbit the same star. Two must orbit one star, and the third the other star. Since planet b and the candidate planet (let’s call it d for simplicity) have such similar periods, they can’t orbit the same star. They’d be too close together, and their mutual gravity would quickly destabilize them. So either b and c orbit one star and d the other, or c and d orbit one star and b the other. And, because why not, they don’t know which set of planets orbits which star.

Become a paying subscriber of Premium to get access to this post and other subscriber-only content.

Already a paying subscriber? Sign In.

A subscription gets you:

• • Three (3!) issues per week, not just one
• • Full access to the BAN archives
• • Leave comment on articles (ask questions, talk to other subscribers, etc.)