George's Random Astronomical Object

Object 66: b Centauri

Podcast release date: 07 February 2022

Right ascension: 14:41:57.6

Declination: -37:47:37

Epoch: J2000

Constellation: Centaurus

Corresponding Earth location: Approximately 3650 km east of New Zealand in the Pacific Ocean

b Centauri (with a little b) is a binary star system at a distance of 325 light years (99.7 pc) in the constellation Centaurus [1,2]. Centaurus, by the way, is a giant constellation with a lot of bright stars, but because the constellation is primarily visible from areas in the tropics or the Southern Hemisphere, many people living in Europe, Asia, and North America have probably never seen it, which is unfortunate.

Anyway, the b Centauri star system contains two stars labelled A and B because astronomers have an uncreative and confusing system for naming stars within star systems. b Centauri A is a large, hot blue star with a mass of 5-6 times the mass of the Sun that astronomers have classified as a type B star (because this can be more confusing if we use letter-based classifications for stars) [3]. Much less is known about the smaller, fainter star, b Centauri B. It has not been possible to actually image b Centauri B because it's too close to b Centauri A and because the smaller star seems to be relatively faint compared to the brighter star. However, the smaller star is known to actually be present because of how it gravitationally pulls on the larger star as the two stars orbit each other.

The two stars are estimated to have a total mass of somewhere between 6 and 10 times the mass of the Sun [3]. The star system is also visible in the night sky. The system has a magnitude of 4 [4], which means that it is around 1-2 magnitudes brighter than the faintest stars that can be seen without a telescope. Having said this, please don't ask me how to find this star system in the sky. Centaurus is not only very large but also weird and complicated, and b Centauri is a relatively indistinct-looking star system near a bunch of other relatively indistinct-looking star systems in the constellation.

What makes b Centauri interesting is that is has an exoplanet named b Centauri b. This exoplanet should not be confused with the star named b Centauri B; just assume that, when I mention b Centauri b in the rest of this episode, I am referring to the exoplanet and not the star. If we want to be extra confusing, we could call the exoplanet b Centauri (AB) b, but I'm not going to do that.

Now, if you listened to my last episode, you would have heard me talk about another exoplanet orbiting a completely different star system, and you would have heard me explain how that exoplanet was so uninteresting that, if we were to compare exoplanets to Motown musicians, it would be comparable to performers like Gino Parks or the Velvetettes. Well, the exoplanet in the b Centauri system is more comparable to someone like Stevie Wonder or Lionel Richie. It's discovery really excited astronomers (and even had some of them dancing on the ceiling), and it also changed astronomers' view of how and where exoplanets can form.

The exoplanet b Centauri b was discovered in a survey called the B-star Exoplanet Abundance Study or BEAST [5]. (Now, none of the words in the survey name begins with T, but I suppose a survey named BEAST just sounds more awesome than BEAS, so the survey managed to find a way to force the letter T into the acronym.) Anyway, most exoplanet surveys detect planets orbiting other stars either by looking for how the stars' brightnesses change slightly when the planets pass in front of the stars or how the stars are moved slightly by the gravitational forces of the exoplanets in orbit around them. However, the BEAST (or BEAS) survey actually directly images the exoplanets in orbit around other stars, which is extremely difficult. To do this, the survey is using a specialized instrument named using the acronym SPHERE (although the instrument is not actually spherical) on one of the 8 m wide telescopes that are part of the Very Large Telescope in Chile [5].

The instrument uses two techniques to assist with imaging exoplanets in orbit around other stars. First, the SPHERE instrument uses a small disk called a coronagraph to block out the light from the stars themselves, which would otherwise saturate the detectors and make it impossible to see anything else. (This sounds really simple, but it is a notable technical innovation for imaging things near bright stars.) Second, the SPHERE instrument uses rapidly moving mirrors to correct for how the Earth's atmosphere tends to make everything look blurry. This mirror system is referred to as adaptive optics, and both the adaptive optics and the coronagraph make it possible to image faint things like exoplanets near very bright stars.

The first observations of the exoplanet b Centauri b by the BEAST (or BEAS) survey were made on March 20th, 2019 [3]. The instrument had actually detected three faint, point-like objects near the two stars at the center of b Centauri. Two of the fainter objects looked like stars behind the b Centauri system, but the infrared color of the exoplanet looked different, so the BEAS (or BEAST) survey took a second look on April 10, 2021, and they found that the exoplanet had moved between 2019 and 2021 in a way that was consistent with it being an exoplanet [3]. They also found some older observations from the year 2000 that had also imaged the exoplanet, but the object had been labelled as a background star and had been ignored by the people who made those images [3].

From all of these observations, the mass of the exoplanet b Centauri b was measured to be 11 times the mass of Jupiter, which isn't that surprising for an exoplanet [3]. However, the planet orbits at a distance of about 560 Astronomical Units (AU) from the binary stars at the center of b Centauri [3]. For reference, the distance from the Sun to the Earth is 1 AU, and the distance from the Sun to Neptune is 30 AU, so the orbit of b Centuari b is very wide compared to the orbits of any of the planets in our Solar System. Also, because b Centauri b has such a wide orbit, it takes somewhere between 2600 and 7200 years to travel completely around the star system, which is a relatively long time [3].

Now, as I mentioned before, this was not just any boring old exoplanet discovery. It challenged some existing preconceptions about exoplanets. It's important to point out that b Centauri b formed near a very large, very hot blue star that is going to very strongly heat any gas or dust in the surrounding interstellar medium. You would not expect to find a large exoplanet in such an environment, and, in fact, before the discovery of b Centauri b, exoplanets have only been found orbiting smaller and cooler stars, many of which look like the Sun. The reason for this is, in part, because too many unimaginative astronomers have spent way too much time looking for planets around Sun-like stars because they really want to find things that look like Earth-like planets in places that look like our Solar System. These people simply never thought about looking for explanets orbiting bigger, hotter, bluer stars. However, it is also quite likely that exoplanets orbiting large blue stars are simply going to be rare because of how those stars heat up gas in their surrounding environments, and astronomers need to spend more time looking for exoplanets like b Centauri b to determine if that is actually the case.

One of the outstanding questions is how did b Centauri b form and how did it get to where it is. The standard process by which astronomers assume that a gas giant like Jupiter forms is called the core accretion hypothesis. In this scenario, a solid core of material slowly forms out of dust in the protoplanetary disk of material surrounding a newly-formed star. Once that core gets large enough, it can gravitationally attract gas from the surrounding protoplanetary disk and form a very large atmosphere of mostly hydrogen gas. This entire process takes about a few million years. The problem with this scenario as applied to b Centauri b is that the exoplanet is located very far away from the center of the star system where you would expect to find very little dust to form the core of such an exoplanet [3]. A solid core could potentially form in this location given enough time, but also keep in mind that the hot blue star at the center of the b Centauri star system would probably have formed more quickly than the exoplanet and would have heated up and blown away the gas in the surrounding gas disk before an exoplanet could form through the relatively slow core accretion process [3].

Another possible variation of the core accretion hypothesis is that b Centauri b originally formed through this process near the center of the star system where there was much more material to form a core relatively quickly. The exoplanet would have been later ejected into a much wider orbit by some sort of gravitational interaction. However, this would have placed the exoplanet on a very extended elliptical orbit. b Centauri b's orbit looks relatively circular, implying that it formed in the location where it currently orbits. So, it does not look like it got ejected from the center of the star system, either [3].

The most likely scenario in which b Centauri b could have formed is that some sort of gravitational instability in the outer part of the protoplanetary gas disk led to the gas itself collapsing very rapidly to form an exoplanet in this location, thus bypassing the relatively slow process of building a solid core to form the center of a gas giant [3]. This gravitational instability hyopthesis was proposed over a couple of decades ago as an alternate hypothesis for how planets could form, but it never really seemed applicable to either planets in our Solar System or most other exoplanets, and so most astronomers favored the core accretion hypothesis for planet formation. However, b Centauri b looks like a very clear case where the core accretion hypothesis doesn't work or at least a case where the core accretion process has a lot of problems. Hence, b Centauri b could be the first planet discovered anywhere that demonstrates that planets can indeed form through gravtiational instabilities [3].

The final question from this discussion is how many other exoplanets like b Centauri b are out there. The BEAST survey (which I will not refer to as BEAS one last time) is poised to answer this question. It will be interesting to see what other exoplanets they discover orbiting other hot blue stars in the upcoming years.

References

[1] Gaia Collaboration et al., The Gaia mission, 2016, Astronomy & Astrophysics, 595, A1

[2] Gaia Collaboration et al., Gaia Early Data Release 3: Summary of the contents and survey properties, 2020, arXiv e-prints, arXiv:2012.01533

[3] Janson, Markus et al., A wide-orbit giant planet in the high-mass b Centauri binary system, 2021, Nature, 600, 231

[4] Ducati, J. R., VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system., 2002, VizieR Online Data Catalog

[5] Janson, Markus et al., BEAST begins: sample characteristics and survey performance of the B-star Exoplanet Abundance Study, 2021, Astronomy & Astrophysics, 646, A164

Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

Sound Effects: Benjifiguried, Berwitz, CC0_Sound, Hockinfinger, Infernus2, ivolipa, J_Bond, jameswrowles, newagesoup, and noetzli at The Freesound Project

Image Viewer: Aladin Sky Atlas (developed at CDS, Strasbourg Observatory, France)

 

© George Bendo 2022. See the acknowledgments page for additional information.

Last update: 6 February 2022