WASP-18 b and Other Wonderfully Weird WASP Worlds

Posted on June 10, 2023 by Brian H. Gill
K. Miller/IPAC's artist's concept: WASP-18 b, a gas giant exoplanet 10 times more massive than Jupiter, with day side temperatures up to 5,000 degrees Fahrenheit, 2,700 C. (May 31, 2023)
WASP-18 b: a hot Jupiter with an unexpectedly cool twilight zone. An artist’s concept. (2023)

When I started writing this, I’d planned on talking about WASP-18 b, a hot Jupiter: how we’ve found water in its atmosphere, and something odd about the planet’s temperature on the edge of its sunlit side.

Down the Rabbit Hole: Exoplanet Designations and Cosmic Scale

John Tenniel's 'The White Rabbit' from 'Lewis Carroll's 'The Nursery Alice.' (1890) from the British Library, via WikipediaAs I said, my plan for this week was talking about what scientists are learning about WASP-18 b.

Then I typed WASP-18 b into Google Search and noticed Google’s occasionally-helpful “People also ask” snippet feature.

  • People also ask
    • How big is WASP 18b?
    • How was WASP 18b discovered?
    • Why is WASP-17 B odd?
    • Does WASP-12b still exist?

The last two, “Why is WASP-17 B odd? and “Does WASP-12b still exist?”, aren’t about WASP-18 b.

So I started looking up WASP, WASP-12, WASP-17, and related topics.1

My powers of concentration are just fine. Their focus, however, often jumps the tracks, or forgets that there are tracks.

I could complain about Google Search pushing me down a rabbit hole, but I won’t.

Instead, I’ll count them as helpful, since they give me an excuse to talk about weird worlds and exoplanet designations.

Plus, I eventually got around to the “People also ask” list. And, more to the point, a little of what we’re learning about WASP-18 b.

Astronomical Designations: A Discursive Digression

Frederik de Wit's 'Planisphaerium coeleste' star chart. (1670) Frederik de Wit, via Wikimedia Commons, used w/o permission.Back in the day, some of the few thousand visible stars had names.

That worked fine, and still does.

Although things get tricky when a discussion involves Thuban, Zǐ Wēi Yòu Yuán yī and Alpha Draconis. All of which are the same star.

Bayer designations, starting in 1603, helped; and that’s another topic.

The good news is that designations for most stars are fairly consistent: thanks partly to the IAU (International Astronomical Union).

And partly to generations of astronomers, frustrated from dealing with star catalogs assembled by many individuals, with little if any concern for cross-catalog consistency.2

Exoplanet designations, although somewhat standardized, are new. So the designation standards don’t quite fit the reality of some exoplanets.

First Known Exoplanets

NASA/JPL-Caltech/R. Hurt (SSC)'s artist's concept of the PSR B1257-12 planetary system. (2006)We didn’t know for sure that there really were exoplanets until 1992.

That’s when two planets orbiting a pulsar were confirmed as being real.

They’ve got assorted designations, but at least there’s only one name for each:

  • Poltergeist
    • PSR B1257+12 c
    • PSR B1257+12 B
  • Phobetor
    • PSR B1257+12 d
    • PSR B1257+12 C

Some of the pulsar’s planets were first detected in 1988 or maybe 1990. Then the Arecibo Observatory confirmed that the planets were real in 1992.3 Probably.

PSR B1257 et cetera’s muddled designations and jumbled history may be due to their being the first confirmed exoplanets.

We’ve been learning a lot since then.

A Circumbinary Planet’s (allegedly) Impractical and Unworkable Designation

Mengzy's diagram: 'Typical configuration of a circumbinary system, in which A and B are primary and secondary stars, ABb is denoted as the circumbinary planet.' (June 15, 2016) via Wikipedia, used w/o permission.For example, there’s PSR B1620-26 b.

It’s (probably) a gas giant, about two and a half times Jupiter’s mass, orbiting a pulsar and a white dwarf.

The pulsar and white dwarf orbit each other, PSR B1620-26 b orbits both, so it’s a circumbinary planet.

I’m not convinced that the “PSR B1620-26 b” designation is official, although it’s used in both NASA’s Exoplanet Catalog and the planet’s Wikipedia page.

PSR B1620-26 b doesn’t have an official name, but it does have two nicknames: Methuselah and The Genesis Planet.

Those monikers reflect one of the three things making this exoplanet stand out from the five-thousand-plus others we’ve found so far. It’s:

  • The first circumbinary planet confirmed
  • The first planet found in a globular cluster
  • One of the oldest known extrasolar planets

About the PSR B1620-26 system’s age: it’s just outside the core of the M 41 globular cluster, so the PSR B1620-26 stars almost certainly are part of M41’s set.

We figure all stars in a globular cluster form at about the same time. Since M 41 is probably around 12,700,000,000 years old — that’s a lot of zeroes — the PSRB1620-26 system should be, too.

As for how “The Genesis Planet” ended up orbiting a white dwarf and a pulsar? That’s yet another topic, for another time.

The best-known circumbinary planet is probably Tatooine, fictional setting for parts of the Star Wars saga.4 And that wraps up this bit about one planet orbiting two stars.

Exoplanet Designations: A Work in Progress

Visual overview of input catalogs and methodology used in constructing the TESS Input Catalog (TIC). (2019) From Stassun et al. 2019, via NASA, used w/o permission.Right now, basically, often, an exoplanet’s designation is [star’s designation] [letter of the alphabet, starting with “b”].

This system’s based on Harvard’s Washington Multiplicity Catalog (HWMC), and has the IAU stamp of approval.

By the way: don’t bother trying to memorize these names. That goes double for the designations. There won’t be a test on this.

Again, with this sort-of-HWMC system, [star’s designation] [a] is the star. Unless it’s upper case A for the star. I’ll get back to that.

And if you think “PSR B1620-26 b”, that circumbinary planet’s designation, doesn’t quite line up with the designation system, you’re not alone.

Back in 2010, some researchers said that the then-current exoplanet designation system didn’t work — at all — for circumbinary planets. I think they have a point, and that’s yet again another topic.

The sort-of-HWMC system isn’t the only one in play. Researchers working with data from TESS, for example, use their own designation system for a catalog of some 1.5 billion (mostly stellar) objects.5

My guess is that we’ll never have a standard, one-size-fits-all designation system for each and every star, planet, asteroid and pebble. Different researchers have different needs. There’s also the matter defining “planet” and “asteroid”, and I’m drifting off-topic.

I’m forgetting something. Designations, numbers and letters, being consistent. Right.

Designations and Alphanumeric Alternatives: a Hypothetical Hodgepodge

Patrick Moore's sky chart of Caldwell Objects. (September 3, 2006)) via Wikipedia, used w/o permission. Moore used his other surname, Caldwell, to avoid confusion with the Messier Catalog.Let’s say there’s a star, designation Prufrock 918273645; with a planet, Prufrock 918273645 b.

Nice and simple, right?

Based on how I’ve seen different folks interpret exoplanet designations, I very strongly suspect that the planet would be called:

  • Prufrock 918273645 b
  • Prufrock 918273645 B
  • Prufrock 918273645-b
  • Prufrock 918273645-B
  • Prufrock 918273645b
  • Prufrock 918273645B
  • Prufrock-918273645 b
  • Prufrock-918273645 B
  • Prufrock-918273645-b
  • Prufrock-918273645-B
  • Prufrock-918273645b
  • Prufrock-918273645B

There are more possible variations, but you get the idea.

The first one, in bold, follows the IAU convention, with a lower-case “b”.6 The other versions might be close enough, at least in context, to avoid confusion.

The good news here is that things like Google Search generally recognize alternatively-accurate designations and yield usable results.

I don’t like discombobulated designations, but maybe that comes from having two librarians as parents.

“People Also Ask”: Strange Worlds and Cosmic Scale

Pixabay's photo of marshmallows, via Smithsonian Magazine Sarah Kuta's 'Puffy, Marshmallow-Like Planet Could Float in a Bathtub:' about exoplanet TOI-3757 b which has the same average density as a marshmallow. (October 25, 2022)Finally, I’m back to this list:

  • People also ask
    • How big is WASP 18b?
    • How was WASP 18b discovered?
    • Why is WASP-17 B odd?
    • Does WASP-12b still exist?

First of all, WASP stands for Wide Angle Search for Planets: a bunch of academic outfits looking for exoplanets. They’re using the transit method, with two robotic telescopes. I’m not talking about that (them?) today: so if you’re curious, check out footnote seven.7

WASP-18 b is about as big as a gas giant can get before being a brown dwarf. It’s roughly 10 times as massive as Jupiter. It’s a gas giant, but more tightly packed than the Solar System’s largest world: with a diameter about 1.1 times Jupiter’s.

WASP-18 b was discovered in 2009.

WASP-17 b has several weird attributes.

It’s first planet we’ve found that orbits backwards. Its sun spins in one direction, WASP-17 b orbits in another. The plane of its orbit is at about a 149° angle to its sun’s equator.

WASP-17 b is also a super-puff planet, which sounds like the name of a breakfast cereal to me; and that’s still another topic. The planet’s between one and a half and two times as wide as Jupiter, with only half the mass of our big gas giant.

WASP-12b is still around, but won’t be for long. it’s being ‘eaten’ by its sun at a rate of about 189 quadrillion tons a year.

Back in 2010, NASA figured the planet had maybe another 10,000,000 years before WASP-12 finished its meal. That’s a short time. On a cosmic scale.

One more thing. WASP-12 b is a hot Jupiter, whipping around its star in about 24 hours. And it’s really hot, around 5,225° Fahrenheit.7

WASP-18 b: Discovering Something Odd

R. Hurt/IPAC's illustration: 'The team obtained the thermal emission spectrum of WASP-18 b by measuring the amount of light it emits over NASA's James Webb Space Telescope's NIRISS SOSS 0.85-2.8 um wavelength range, capturing 65% of the total energy emitted by the planet. WASP-18 b is so hot on the day side of this tidally-locked planet (the same side always faces its star, as the Moon to Earth) that water vapor molecules would break apart. The Webb Telescope directly observed water vapor on the planet in even relatively small amounts, indicating the sensitivity of the observatory.' (May 31, 2023)
WASP-18 b’s thermal emission spectrum, from JWST NIRISS SOSS. (May 2023)

WASP-18 is a spectral class F6V star: a bit hotter and more massive than our sun, and quite a bit younger.

One or two — there’s an ongoing debate over that — planets orbit WASP-18.

Gnathan87's chart: results for an archaeological simulation, an example of Bayesian inference. (October 2011) via Wikimedia Commons, used w/o permissionWASP-18 c‘s year is about 51 hours, 44 minutes long. It’s very roughly an eighth as massive as Jupiter, or about twice as massive as Uranus or Neptune.

Kyle A. Pearson found WASP-18 c by applying Bayesian statistics and machine learning to data from TESS. Maybe that’s why some researchers don’t think WASP-18 c is really there. Or maybe the issue is that not enough other folks have crunched numbers and gotten similar results.

Anyway, WASP-18 b‘s year is about 22 hours, 36 minutes long. It’s about 10 times more massive than Jupiter, only somewhat less massive than the lightest brown dwarfs.

And, like WASP-12 b, WASP-18 b will soon be gone. “Soon” on a cosmic scale. By one estimate, the planet will fall into its star in about a million years.

But unlike WASP-12 b, WASP-18 b isn’t being ‘eaten’ by its star. Tidal effects are slowing it down, bringing it closer and closer to WASP-18.

Besides orbiting a bright star, WASP-18 b is nearby: again. on a cosmic scale. The WASP-18 system is just over 400 light-years out, in the constellation Phoenix.

That, and the Webb space telescope’s instruments, are giving scientist a pretty good look at this hot Jupiter.8 And they like what they see. Not that we’ve got images of WASP-18 b. Scientists have been studying light collected by Webb’s mirrors.

“…The discovery: Scientists identified water vapor in the atmosphere of WASP-18 b, and made a temperature map of the planet as it slipped behind, and reappeared from, its star. This event is known as a secondary eclipse. Scientists can read the combined light from star and planet, then refine the measurements from just the star as the planet moves behind it.

“…’It was a great feeling to look at WASP-18 b’s JWST spectrum for the first time and see the subtle but precisely measured signature of water,’ said Louis-Philippe Coulombe, a graduate student at the University of Montreal and lead author of the WASP-18 b paper. ‘Using such measurements, we will be able to detect such molecules for a wide range of planets in the years to come!’

“Researchers looked at WASP-18 b for about six hours with one of Webb’s instruments, the Near-Infrared Imager and Slitless Spectrograph (NIRISS), contributed by the Canadian Space Agency….”
(“Discovery Alert: Webb Maps and Finds Traces of Water in an Ultra-hot Gas Giant’s Atmosphere“; Kristen Walbolt, NASA’s Exoplanet Exploration Program; News; Exoplanet Exploration, NASA (May 31, 2023))

I’ll mention NIRISS and SOSS, acronyms, from that illustration’s caption, after the next bit.

This WASP World’s Winds: Weirdly Warped?

NASA/JPL-Caltech (R. Hurt/IPAC's infgraphic: 'scientists used the James Webb Space Telescope to observe the exoplanet WASP-18 b and its star before, during and after the planet was eclipsed. By measuring the change in light when the planet travels behind the star, the planet's brightness is revealed. From these measurements, scientists were able to make a temperature map of the planet's day side. Displayed temperature range: 2,800 to 4,800 degrees Fahrenheit.' May 31, 2023)
Exoplanet transits and eclipses: how scientists mapped WASP-18 b’s day side temperatures.

First, getting those acronyms out of the way. NIRISS and SOSS stand for Near Infrared Imager and Slitless Spectrograph and Single-Object Slitless Spectroscopy.

SOSS is what the NIRISS does when scientists have it getting light from a single object: two objects, actually, in this case. WASP-18 and WASP-18b are so near each other that it’ll likely be some time before we can get images of the star and its planet(s).

The infographic you’ve scrolled past — or, better yet, read — outlines how scientists collect light — infrared light in this case — from the star. They collect three samples: the star being slightly eclipsed by the planet, just the star (while the planet is behind the star) and the planet reflecting some of the star’s light.

Basically. That’s an extreme simplification.

Those samples let scientists subtract the star’s light from the star’s and the planet’s light. And that gives them a sample of light from just the planet, which they run through a spectrograph.

Plus, studying changes in the light reflected by the planet lets scientists work out how the light (infrared, again, in this case) near the center of the side of the planet facing its star compares with light near the planet’s edge.

Since WASP-18 b is a hot Jupiter, a large gas giant, winds in the planet’s deep atmosphere should be distributing heat from the day side to the night side.

That’s not what scientists found.

The odds are very good that WASP-18 b rotates in about the same plane as its orbit and its star’s equator. JWST instruments shouldn’t have picked up all that much temperature difference from the planet’s east and west limbs, as it moved behind its star.

But they did.9

Over-the-Top Winds on WASP-18 b?

NASA/ESA illustration: WASP-43 b, a hot Jupiter orbiting its star. 'A team of scientists using NASA’s Hubble Space Telescope has made the most detailed global map yet of the glow from a turbulent planet outside our solar system, revealing its secrets of air temperatures and water vapor....' (last updated December 15, 2022)
An illustration of WASP-43 b’s orbit, and heat distribution on the hot Jupiter. (2022)

If WASP-18 b’s observed disk had been about the same temperature, east-to-west, on its day side; that would have been useful information.

It would have been another example of how winds carry heat around massive planets like WASP-18 b and WASP-43 b. WASP-43 b is the hot Jupiter in that illustration.

Instead, we have a hot Jupiter that’s not acting the way we’d expected. One of the possible explanations for this minor weirdness is that WASP-18 b’s magnetic field is affecting its winds: so that instead of flowing along east-west lines, they’re moving over the poles.

Late this week, I’d tracked down an illustration and discussion of that idea. But, like I said earlier, I got distracted. So I’ll set that aside for another time.

Another item that’s in my ‘to do’ list is what researchers said about air and ocean currents, temperatures and habitability, on an Earth-like world orbiting a red dwarf star.

Living in Vastness

Torben Hansen' photo: the Andromeda Galaxy, in visible light. (September 1, 2011)
M31, the Andromeda Galaxy. (September 2011)

Once, decades back, my father and I were in a location with a really dark night sky.

He tried to show me what, in my youth, was still called the Great Nebula in Andromeda.

It was almost directly overhead. The sky was clear and dark. I could see each star my father pointed out, but not the vast and subtle brightness of the Andromeda Galaxy.

There’s probably a lesson in that experience, involving metaphors and allusions. Allusions, not illusions, and I’m drifting off-topic.

There’s also a more practical lesson from that and similar experiences. My night vision isn’t and hasn’t been good enough for me to get seriously involved in amateur astronomy.

But that hasn’t kept me from enjoying what others report, and trying to keep up with what we’ve been learning about this vast and ancient universe.

All of which has a serious “wow factor”. And helps me remember just how “wow” God is.

“Indeed, before you the whole universe is like a grain from a balance,
or a drop of morning dew come down upon the earth.
“But you have mercy on all, because you can do all things;
and you overlook sins for the sake of repentance.”
(Wisdom 11:22–23)

Somewhat-related posts:

1 Fascinating (to me) distractions:

2 Names and designations for stars and exoplanets:

3 A pulsar and its planets:

4 First confirmed circumbinary planet, and a fictional planet:

5 Catalogs, names and designations:

6 A poem and IAU naming conventions:

7 Finding strange new worlds:

8 Math, numbers and exoplanets:

9 That’s odd:

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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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