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SYFY WIRE Bad Astronomy

Astronomers get a photo of a record-breaking exoplanet around massive binary stars!

Host stars for b Cen b are the most massive planet-bearing ones known.

By Phil Plait
Phil Plait Bad Astronomy Vlt Bcenb Exoplanet

Astronomers have found a giant exoplanet orbiting a pair of massive stars in a nearby binary system, and it's a record breaker: These are most massive stars ever found to have a planet. It turns out that's important, because it helps us understand how planets form… except for the small issue that it's not at all clear how this one did.

The star system is named HR 5471, also called b Centauri (note the lower case b, not to be confused with Beta Centauri, the second brightest star in the constellation Centaurus, or worse, (capital) B Cen, which gave me a lot of grief when I was looking up stellar data for this article). It's about 325 light years from Earth, and is part of a group of young stars called the Scorpius-Centaurus association. If that name sounds familiar, it may be because roughly 2.6 million years ago one of these stars exploded so violently it literally showered the Earth with material, including radioactive plutonium and tiny nodules of iron-60.

b Cen is a binary, with the brighter of the two stars, b Cen A, a decently big star of 5–6 times the Sun's mass. It has a temperature of ~18,000° C — replace the Sun with this star and the surface temperature of Earth would be hot enough to melt rock — and emits energy at a rate a couple of hundred times the Sun's. So it's a bruiser. Spectra reveal it has a binary companion, b Cen B, but not much is known about it; it likely has a mass 1–4 times the Sun, so the total system mass is 6–10 times that of the Sun.

Art depicting a gas giant planet in a wide orbit around a binary star

While planets are found around stars like the Sun and lower mass ones all the time, very few have been seen around higher mass stars, so astronomers started a dedicated campaign to observe them, called the B-star Exoplanet Abundance Study (BEAST). The "B stars" part refers to a classification system astronomers use for stars; B-type stars have masses very roughly from 3 to 18 times that of the Sun, and are hotter and more luminous. It's not known why we see them to have fewer planets. It might be a real effect, or it might be due to observational issues making them harder to find. BEAST aims to find out.

The Sco-Cen association is a good place to look because it's loaded with these kinds of stars, it's nearby, and it's young. Planets are very hot after they form and therefore glow in infrared light, making them easier to spot. BEAST uses the SPHERE camera on the Very Large Telescope in Chile to observe these stars in infrared, using their incredible sensitivity and resolution to look for planets by taking direct images of them.

The new exoplanet orbiting b Cen was seen in data taken in 2019, but wasn't confirmed until observations in 2021 showed it was moving along with the star through space, proving it wasn't a background star or galaxy. Turns out it was even seen in observations taken in 2000 but overlooked! That's important, because that long time baseline really helps see the movement of the planet and star.

The planet is about 80 billion kilometers out from the star, which is really far, about 100 times farther out than Jupiter is from the Sun. Models of how planets cool after formation indicate it has 10.9 ± 1.6 times the mass of Jupiter. Big, but still definitely a planet.

Image of the planet b Cen b (arrowed)

The planet is circumbinary, meaning it orbits both stars, making its name, technically, b Cen(AB) b, but "b Cen b" is OK too. The host stars are by far the most massive binary ever seen to host a planet — second place falls to the binary HD 106906 AB at 2.7 solar masses. The most massive single stars seen to host them are around 3 times the Sun's mass, so again b Cen is the clear winner.

The real question is: What's it doing there at all? Planets form from a disk of material swirling around a forming star (or in this case, binary stars). There are two main ways this can happen. They can grow from the bottom up, with tiny grains of matter sticking together to get bigger and bigger, a process called core accretion. But 80 billion km is a long way out, and the disk shouldn't have enough material there to make a big planet. Worse, once the stars switched on their intense radiation would blow away gas in the disk, so this planet would've been further robbed of growth material. It's possible this planet formed closer in and was tossed way out by gravitational interactions with another planet (or the binary itself). However, the observations of b Cen b over time show some orbital motion of the planet, and the astronomers deduce the orbit is likely not to be very elliptical. A planet ejected this far out should have a highly elongated orbit, so this formation scenario seems unlikely.

The other way to make a planet is for the disk to gravitationally fragment in one spot, and that material collapses to form a planet. That makes more sense here, but it's also possible that the planet formed directly out of the gas and dust in which the Sco-Cen association is buried, and then was captured by the stars. That's tougher to do, but would be extremely cool — a rogue orphan planet adopted by a pair of stars! — but it's not possible at this time to know.

And that particular mystery does nothing to lessen the amazing nature of this discovery. First, massive stars can have planets. That's a big deal! Second, we can find them, which means it's important to keep looking. And third, this shows that we have quite a bit to learn about how these planets form. We have lots of ideas but precious little observational evidence to use to cull them.

This is a big first step. Now we just need to keep taking more.