Big Planet, Wide Orbit, Unsolved Mystery: b Centauri(AB)b

J.L. Dauvergne/G. Hüdepohl/ESO's photo of Paranal Observatory's four VLTs and ATs, and the VST (foreground); VISTA (background) (2009) from ESO, via Wikimedia Commons, used w/o permission.Scientists in Sweden, taking part in 2019’s BEAST program that used SPHERE on the VLT discovered an ‘it might be an exoplanet.’

In 2021, they got confirmation. They’d spotted an exoplanet.

And SPHERE on the VLT1 has an picture of the newly-discovered world.

I’d planned on talking about BEAST, SPHERE, VLT; comparing ground-based and space observatories; and whatever else came to mind. Then, on Tuesday, I started running a fever. It was of the ‘nothing serious’ variety, but quite enough to slow me down.

After that, the household got a brief visit from number-two daughter, son-in law and granddaughter: a happy occasion. Which also didn’t help me do what I’d planned.

So I trimmed my plans back to what was possible, and this is the result.

b Centauri(AB)b, AKA HD 129116 b: Big Bright Stars and a Big Mystery Planet

ESO/Janson et al.'s image of b Centauri (AB) and the binary star's exoplanet.
(From ESO, used w/o permission.)
(Binary star HD 129116 and HD 129116 b, its huge exoplanet.)

Giant Planet Imaged Around Massive Stars
Monica Young, Sky and Telescope (December 8, 2021)

“Astronomers have imaged a giant planet around a massive pair of stars known as b Centauri. While Jupiter’s orbit around the Sun is five times wider than Earth’s, this planet is 100 times farther out, circling its star at 560 times the average Earth-Sun distance….

“…Markus Janson (Stockholm University, Sweden) and colleagues discovered the planet in observations taken in 2019 as part of the B-star Exoplanet Abundance Study (BEAST), using the SPHERE instrument on the European Southern Observatory’s Very Large Telescope in Chile. Follow up in 2021 confirmed the planet’s existence. The real kicker, though, was an observation taken two decades earlier as part of a different project. The planet was visible back then but discarded for being too faint. Including the earlier observation enabled the researchers to trace the object’s orbit and confirm that not only is it moving with the b Centauri system, it’s actually orbiting the central stars….”

Monica Young’s Sky and Telescope article doesn’t mention the planet’s designation. She calls the planet’s star b Centauri. That’s the designation European Southern Observatory’s announcement used.

I called b Centauri’s planet HD 129116 b partly because Wikipedia’s page on the star uses the “HD” or Henry Draper Catalogue designation.

And partly because the “b Centauri” designation invites confusion in my language.

b Centauri is a legitimate Bayer Designation. But it also ‘reads’ as Beta Centauri. Maybe Beta Centauri has planets, but we if so we haven’t spotted them yet.2

Awkward Designations, Pondering “Fred”

Jim Cornmell's sky chart of Caldwell Objects (September 3, 2006)) via Wikipedia, used w/o permission.The Sky and Telescope article calls b Centauri’s planet b Centauri(AB)b: an accurate of not euphonious designation. I’ve called it HD 129116 b, but I haven’t seen that designation used.

But whether I call it HD 129116 b or b Centauri(AB)b, the designation’s awkward. For me, at any rate.

If I need to refer to it often, then I may start calling it “Fred.” Or maybe not.

I talked about names, stars, catalog designations and alphanumeric gibberish last month.3

HD 129116 b, Beta Pictoris b and More: We See You!

Sky and Telescope's IAU Centaurus chart. (2011) position of b Centauri/HD 129116/HIP 71865 circled in red
(From IAU, Sky & Telescope magazine/Roger Sinnott & Rick Fienberg; used w/o permission.)
(Location of b Centauri (AB)b/HD 129116 circled.)

HD 129116 b isn’t the first exoplanet that astronomers have seen directly. That’d be Beta Pictoris b, imaged in 2008. Or ROXs 42Bb in 2013. Or maybe some other planet.

We’ve been finding a great many exoplanets, and who’s found what and when — to say nothing about what’s being learned and how — hasn’t been tidied up into well-organized lists yet.

On the other hand, today’s information technology helps scientists share, analyze and organize information — and lets someone like me stay more-or-less up to date with what humanity’s learning about this wonder-packed universe.

Which reminds me — b Centauri(AB)b is being presented as “a planet that shouldn’t exist:”

“Shouldn’t exist” headlines get attention, which is what headlines are supposed to do. I wouldn’t call them clickbait, since b Centauri(AB)b is hard to explain.4

Incomplete and Growing Knowledge

B. Saxton (NRAO/AUI/NSF)/ALMA (ESO/NAOJ/NRAO)'s infrared image of Elias 2-27's protoplanetary diak.
(From ESO/H. Avenhaus et al./E. Sissa et al./DARTT-S and SHINE collaborations, via Wikimedia Commons, used w/o permission.)
(SPHERE/VLT’s images of dusty discs surrounding nearby young stars.)

B. Saxton (NRAO/AUI/NSF)/ALMA (ESO/NAOJ/NRAO)'s infrared image of Elias 2-27's protoplanetary diak.The nebular hypothesis is still a hypothesis.

Immanuel Kant, possibly developing an idea of Emanuel Swedenborg’s, said that the Solar System could have formed when a nebula collapsed into a rotating disk which then became our star and planets.

That was in 1755.

Pierre-Simon Laplace came up with a similar model in 1796.

We’ve learned a great deal since then. And over the last few decades we’ve found collapsing nebulae and protoplanetary disks.5

Assuming that they’re similar phenomena, seen at different times during their development, is an assumption. A reasonable one.

But we don’t know that’s how our Solar System formed. Not the way we know that Abraham Lincoln gave the Gettysburg address.

That sort of knowledge may have to wait until we’ve been making and recording observations of collapsing interstellar clouds for a few million years.

Meanwhile, scientists have been baffled by the conundrum of planetary formation.

Or, looking at it from another angle, they’ve been fine-tuning the nebular hypothesis and uncovering new questions.

It’s There, But How?

Natalie Batalha's and Wendy Stenzel's chart of exoplanet populations found with Kepler data. (NASA and Ames Research Center)One of these new questions is how “a planet that shouldn’t exist” formed.

Based on what we’ve learned so far, scientists figure that most planets when a protoplanetary disk’s dust grains stick together.

Then the dust clump collides with other grains and clumps, eventually getting as big as a planet. Meanwhile, the protoplanetary disk’s gas and ice will have been falling one the growing planet.

That model, along with what we’re learning about how gravity, growing planets and protoplanetary disks interact, looks like a good fit with the Solar System’s planets. And with planets we’ve been finding around other stars.6

Informed Speculations

NASA/Ames Research Center/JPL-Caltech/R. Hurt's 'Assembly Line of Planets' illustration. (June 19, 2017) via NASA, used w/o permissionExcept for b Centauri(AB)b.

It’s huge, roughly 11 times as massive as Jupiter. Maybe it formed closer to b Centauri(AB) and then was flung out to its current orbit.

A problem with that idea is that, although b Centauri(AB) b’s orbit is a bit eccentric — 0.4, give or take a bit — it’s not nearly as eccentric is it should be, in the ‘flung outward’ scenario.

Or maybe b Centauri(AB) b formed where it is now, through gravitational instability. If that’s what happened, part of the b Centauri(AB)’s protoplanetary disk became so dense that the denser part’s gravity pulled into the exoplanet.

A problem with the gravitational collapse model is that our models say that it’s far more likely to work when there’s enough mass in the lump to make a star.

Since astronomers have been finding a fair number of oversize planets in big orbits around massive stars, whatever process or processes are at work: they’re not rare.7

That’s all I have time for this week. Well, almost all.

Living With Reality: And Loving It

Since at least some folks see religion and science as incompatible. Some of them apparently think that science real and religion requires intellectual blinders. And some ‘religious’ folks seem determined to encourage the notion.

I’ve explained why reality doesn’t threaten my faith, and why I think seeking truth is a good idea. Often.

So, instead of repeating that sort of thing, I’m developing a Science AND Religion section on A Catholic Citizen in America.

Finally, the usual links to more-or-less related stuff I’ve written.


1 ESO, VLT, and other deferred topics:

2 Designations and a catalog:

3 More (or less) of the same:

4 Images of other worlds:

5 Learning how planetary systems form:

6 Planetary system formation, more detail:

7 Instability, eccentricity, and all that:

About Brian H. Gill

I was born in 1951. I'm a husband, father and grandfather. One of the kids graduated from college in December, 2008, and is helping her husband run businesses and raise my granddaughter; another is a cartoonist and artist; #3 daughter is a writer; my son is developing a digital game with #3 and #1 daughters. I'm also a writer and artist.
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2 Responses to Big Planet, Wide Orbit, Unsolved Mystery: b Centauri(AB)b

  1. “Fred”, please.

    And maybe we could start calling COVID-19 / SARS-COV-2 and its variants Carl”, “Carl Alexander”, “Carl Brendan”, “Carl Oscar”, etc. 😉

Thanks for taking time to comment!