Astronomers have found a giant planet in a nearby system, and it's the most massive star ever found to have a planet. It's important because it helps us understand how planets form, but it's not clear how this one did.
The star system is called HR 5471, which is the second brightest star in the constellation Centaurus, and gave me a lot of grief when I was younger. It is part of a group of young stars called the Scorpius-Centaurus association. If that name sounds familiar, it may be because one of these stars exploded 2.6 million years ago, showering the Earth with plutonium and iron-60.
The brighter of the two stars, b Cen A, is 5 times the Sun's mass. The surface temperature of Earth would be hot enough to melt rock if it were replaced with this star, and it emits energy at a rate a couple of hundred times the Sun's. It's a bruiser. The total system mass is 6–10 times that of the Sun, but not much is known about it.
Calculating the number of planets around higher mass stars is called the B-star Abundance Study (BEAST). The "B stars" part refers to a classification system that is used to classify stars, and they are hotter and more Luminous than the Sun. We don't know why they have fewer planets. It could be due to observational issues or it could be a real effect. BEAST wants to find out.
The Sco-Cen association is a good place to look because it's young and has lots of stars. Planets are very hot after they form and therefore glow in the light, making it easier to spot. The Very Large Telescope in Chile is where BEAST uses the SPHERE camera to look for planets by taking direct images of the stars.
In data taken in 2019, the new exoplanet was seen, but wasn't confirmed until observations in 2021, which showed it was moving along with the star through space. It was seen in 2000 but not taken into account. The long time baseline helps see the movement of the planet and star.
Jupiter is from the Sun, but the planet is about 100 times farther out. The models show that planets have more mass than Jupiter. It is definitely a planet.
The planet is circumbinary, meaning it is in the same general area as both stars. The host stars are the most massive ever seen to host a planet, and second to the HD 106906AB at 2.7 solar mass. b Cen is the clear winner because he is the most massive single star seen to host them.
What is it doing there? Planets form from a disk of material around a star. There are two ways this can happen. They can grow from the bottom up, with tiny grains of matter sticking together to get bigger and bigger. The disk shouldn't have enough material to make a big planet, because 80 billion km is a long way out. The stars would blow away gas in the disk if they switched on their intense radiation. It's possible that this planet formed closer in and was thrown out by the other planet. The observations of b Cen b over time show some motion in the planet's atmosphere, and the astronomer thinks the orbit is not very elliptical. This formation scenario is unlikely because a planet ejected this far out should have a very long, very long, very long, very long, very long, very long, very long, very long, very long, very long, very long, very long, very long, very long, very
One way to make a planet is for the disk to fragment in one spot, and the material to collapse into a planet. It's possible that the planet formed from the gas and dust in the Sco-Cen association, and then was captured by the stars. It's not possible at this time to know, but a rogue orphan planet adopted by a pair of stars would be extremely cool.
The amazing nature of this discovery is unaffected by that particular mystery. Massive stars can have planets. That's a big deal. It's important to keep looking because we can find them. This shows that we have a lot to learn about how planets form. We have a lot of ideas, but little observational evidence to use.
This is the first step. We need to keep taking more.
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