Colliding Planets Near ASASSN-21qj: Maybe

Matthew Kenworthy, Simon Lock, Grant Kennedy, Richelle van Capelleveen's sketch, showing their hypothesis for the observations of ASASSN-21qj (2MASS J08152329-3859234). Matthew Kenworthy et al., Nature. (October 2023)They were looking for supernovae.

What they found may become a double planet, like the Earth-Moon system, once it cools down. Or a planet with a giant moon, again like the Earth-Moon system.

Then again, an oddly-uneven dusty disk may be orbiting this young, very “Sun-like” star. Either way, ASASSN-2qj is much more interesting than it was a few years back.

Barycenters and Binaries: Briefly

Apollo 11/NASA photo: Earth rising over the Moon's horizon (Smyth's Sea on the Lunar nearside). (July 1969)I suspect one reason scientists generally call Earth and Earth’s moon a planet and its satellite is that we live on Earth and have been thinking of the Moon as — well, as the moon — since long before we started thinking about the planētai, planets, as other worlds.

Another reason may be that the center of mass for the Earth-Moon system is about a thousand miles below Earth’s surface.

“Barycenter” is a fancy word for center of mass. It’s the point that two objects, like Earth and Moon or Jupiter and our Sun, orbit around.

The Jupiter-Sun barycenter is a little more than 28,000 miles above the Sun’s surface. But we don’t call the Sun-Jupiter system a binary star. Mostly because Jupiter isn’t a star: although if it was thirteen times more massive, it would be: a brown dwarf.1

Rabbit Holes and an ‘Assassin Star’

(“A planetary collision: ASASSN-21qj“, Las Cumbres Observatory, YouTube (October 2023)

An advantage and disadvantage of being my own editor is that I can go chasing after information, even if there’s a looming deadline.

I’d noticed a piece in the Sky and Telescope February 2024 issue’s News Notes section. That’s right: February 2024, it’s only the first week of January, and that’s another topic.

Anyway, the short article mentioned a “young, sun-like star” and colliding exoplanets. The situation sounded a great deal like a recent explanation for how the Earth-Moon system formed, which caught my attention.

The article didn’t mention the star’s designation, but did include several names. That’s more than I sometimes have to start with. So by Tuesday, I’d learned that the star’s designation was, for the scientists who’d noticed its odd behavior, ASASSN-21qj.

And from that point on, every time I saw the star’s designation, I read it as “Assassin-Twenty-One-Que-Jay”. Why, I don’t know. Although my reading has included a fair fraction of pulp fiction, most of it was of the science fiction variety.

Getting back to ASASSN-21qj and a probable planetary pileup.

Before it was labeled ASASSN-21qj, it’d been 2MASS J08152329-3859234: and still is, for that matter. Its ASSASSN designation is (probably) mostly for folks working in and with the All-Sky Automated Survey for Supernovae (ASAS-SN) network.

The ASAS-SN network collects and organizes data from 20 robotic telescopes,2 and if I go off on that tangent I’ll never get this ready by Saturday.

Professional Scientists, Amateur Astronomers, Teamwork and Twitter/X

Dan Caselden, NASA's animation: NASA Volunteer Arttu Sainio saw the star Asassn-21qj brightening, possibly due to crashing planets.After I learned that I was looking for ASASSN-21qj, I found a NASA article that told how professional scientists, amateur researchers, and social media, went from speculation about something “weird” in a database to — oh, never mind.

Sharing an excerpt will be easier than paraphrasing the article.

But first, about that animation with what I assume are years and months displayed at the top.

It’s from the NASA Science Editorial Team article, and I suspect it’s a series of images taken between 2010 and 2021. If that’s so, the black blotch in the middle is very probably ASASSN-21qj.

And although the animated GIF’s caption says who made it, —

“NASA Volunteer Arttu Sainio saw the star Asassn-21qj brightening, possibly due to crashing planets.
“Credit: Dan Caselden, NASA”

— I’ve yet to learn when it was made, or where the data came from. Thursday afternoon I finally decided that its origins would remain a mystery.

Here’s an overly-long excerpt from that NASA article. I’ve highlighted a few bits:

Amateur Astronomers Help Discover Cosmic Crash
NASA Science Editorial Team, Citizen Science, Get Involved With NASA, NASA (December 8, 2023)

“…A recent paper in Nature describes how an international group of professional and amateur astronomers teamed up to measure the heat glow of two ice giant planets colliding and see the resultant dust cloud moving in front of the parent star several years later.

The story began back in 2021, when the All-Sky Automated Survey for Supernovae (ASAS-SN) network noticed that a Sun-like star 1800 light years away was rapidly fading. Some 30 days later, NASA volunteer Arttu Sainio was reading X (formerly Twitter), and caught professional astronomers Dr. Matthew Kenworthy and Dr. Eric Mamajek speculating about this weird event. Arttu decided to further investigate this star, called Asassn-21qj, on his own, using data from NASA’s NEOWISE mission. Arttu was surprised to find that the star had demonstrated an unexpected brightening in infrared light two years before the optical dimming event. So he joined the talk on social media and shared his finding with the two astronomers.

‘Out of the blue, amateur astronomer Arttu Sainio on social media pointed out that the star brightened up in the infrared over a thousand days before the optical fading,’ said Kenworthy. ‘I knew then that this was an unusual event.’

“More contributions from amateurs helped determine the nature of the star. Amateur spectroscopist Hamish Barker tried to capture a spectrum of Asassn-21qj in late July, 2022. A spectrum spreads out the colors of the starlight, revealing the star’s temperature. However, the star turned out to be too dim, so Hamish asked Olivier Garde from a French amateur astronomy team if they could add ASASSN-21q to their target list. The team, called the Southern Spectroscopic project Observatory Team (or ‘2SPOT’), succeeded in collecting the needed spectrum in early September, 2022 and forwarded it Kenworthy. The 2SPOT team members are Stéphane Charbonnel, Pascal Le Dû, Olivier Garde, Lionel Mulato and Thomas Petit.

“Two more amateur astronomers also independently observed the star and contributed their data to the study….”
[emphasis mine]

The point, one point at any rate, is that today’s information technology lets folks living on different continents share data and ideas as easily as folks living in the same neighborhood could, a generation or so back.

I see that as a good thing.

“…So Slow Smart”

I would have saved myself a lot of time, if I’d thought to check Sky and Telescope’s website. There’s another version of the “February 2024” article there, posted in October of last year: with a link to one of the scientists’ Twitter/X posts.3

Since Matthew Kenworthy’s Twitter/X post included the star’s designation (ASASSN-21qj), if I’d gone there first, I’d have saved most of a day’s search.

As an old Norwegian-American said, “I get so quick old, and so slow smart”.

ASASSN-21qj: Once Obscure, Now Intriguing

Matthew Kenworthy, Simon Lock, Grant Kennedy, Richelle van Capelleveen's sketch, showing their hypothesis for the observations of ASASSN-21qj (2MASS J08152329-3859234). Collision happens at t = 0, producing a cooling and expanding cloud of debris. Material close to the (now hot) remnant is heated, generating the 1,000 K infrared emission. About 1,000 days later, the expanding cloud crosses the line of sight between the star and the Earth, generating the observed optical light curve. Matthew Kenworthy et al., Nature. (October 2023)
Matthew Kenworthy et al.’s sketch, showing how a synestia may have formed near ASASSN-21qj. (2023)

Kenworthy et al.'s Extended Data Table, stellar properties of ASASSN-21qj (Gaia DR3 5539970601632026752, 2MASS J08152329-3859234). Nature (October 2023) via arXivAfter I had one of the star’s designations, ASASSN-21qj, I could start checking out just what various discussions of it meant by ‘sun-like’.

Along the way, I learned that ASASSN-21qj is Gaia DR3 5539970601632026752 in the Gaia Data Data Release 3, and I’m drifting off-topic again.

Previous experience told me that ‘sun-like’ could mean anything from a G2V star that’s about four and a half billion years old, to simply a star that’s on the main sequence.

Happily, I found what I needed in a pre-publication copy of that Nature paper. The “2SPOT” researchers, mentioned in that NASA article, found that light from ASASSN-21qj “is consistent with being a G2 type dwarf star”.

Normally, I’d go off on a tangent about stellar evolution: but it’s now Friday afternoon. Long story short, looks like ASASSN-21qj is very much like our Sun, but much younger: 300,000,000 years old. Give or take 92,000,000.

That makes ASASSN-21qj and its probable planets about as old as the Solar System was; when our Sun had settled onto the main sequence, but the planets were still playing bumper cars.4

A Very Sun-Like Star

Matthew Kenworthy et al's extended data: simulations of impacts between super-Earths and mini-Neptunes, which can produce post-impact bodies hundreds of Earth radii across. Except for the lower-right panel, particles are colored by their material (forsterite, water or a H2–He mixture moving outwards in the initial bodies) and whether they came from the impactor or target (see top-left panel). The final two panels show only the mass bound to the primary remnant, which has 48.4 times Earth's mass. In the last panel, particles that are at the simulation's minimum density are colored in green. Nature (October 2023) via arXiv
From Matthew Kenworthy et al.’s simulation of collision between super-Earths and mini-Neptunes. (2023)

What had caught my attention about discussions of this “young, sun-like star” was the probable planetary collision: which sounded like giant-impact hypothesis for how the Earth-Moon system began.

Turns out, ASASSN-21qj might have a terrestrial planet, orbiting in its habitable zone. It might even have a churning filled doughnut of planet-stuff that’ll become Earth-Moon 2.0.

But that’s not what the scientists have been studying.

What’s been making ASASSN-21qj flare in the infrared and dim in visible light may have been two “two ice-giant type exoplanets of several to tens of Earth masses”, as a Wikipedia page says.

What the scientists said was that the two probable planets were massive enough to be ice giants like Uranus and Neptune.

“…an infrared brightening consistent with a blackbody temperature of 1000 K and a luminosity of 4% of that of the star lasting for about 1000 days, partially overlapping in time with a complex and deep wavelength-dependent optical eclipse that lasted for about 500 days. These observations are consistent with a collision between two exoplanets of several to tens of Earth masses at 2-16 au from the central star….”
(“A planetary collision afterglow and transit of the resultant debris cloud“, Summary paragraph; Matthew Kenworthy et al. (2023) via arXiv)

That distance from ASASSN-21qj is two to 16 astronomical units. One au (or AU — capitalization varies) is how far Earth is from our Sun – roughly. It’s complicated, it’s now mid-afternoon Friday, so I’ll move along.

Mars is very roughly one and a half AU out and the asteroid belt goes from 2.3 to 3.3 AU, so even if ASASSN-21qj’s probable planetary collision involved something like what became the Earth-Moon system, habitable it won’t become.5

If it’s 16 AU out, that’d make it only slightly less distant from ASASSN-21qj than Uranus is from the Sun. So “ice giants” isn’t an unreasonable term for the planets. Probably.

Uncertainty and Science

Matthew Kenworthy et al.: 'Optical and infrared photometry of ASASSN-21qj. a, Normalized optical photometry from ASAS-SN in the V -band and the g′ band. b, Fractional flux increase in brightness of ASASSN-21qj in both the W 1 and W 2 bands, for which a value of 1.0 represents the stellar contribution alone. c,Calculated NEOWISE color temperature estimated from the photometry of the two bands. The color temperature is plotted as zero when there is no infrared excess and is consistent with a temperature of 1000 K while the excess is present. Error bars are shown at 1σ confidence.' Nature. (October 2023) via arXiv
Matthew Kenworthy et al.’s chart of visible and infrared light from ASASSN-21qj. (2023)

I’ve said “probably” a lot, and you’ll find phrases like “observations are consistent with” in a great many scientific papers.

That’s because, however sure they are about their data and analysis, scientists generally acknowledge they don’t know everything. That’s been my experience, at any rate, reading what they’ve written, and looking through their research.

As for me? Well, I enjoy knowing stuff, and like getting my facts straight. But I figure that “observations are consistent with” means that something may be so. But that the “something”, whatever it is, may not be so.

My guess is that scientists will be taking closer looks at ASASSN-21qj for years. Centuries, very likely, given how long conditions in the star’s vicinity may be changing:

“…The team acknowledges that it’s possible the infrared brightening and the starlight blockage were in fact two separate events, but they make the case that two such events would be even more rare than a planetary-scale collision.

“Their calculations show that such a collision would vaporize both worlds, with a relatively small amount escaping to orbit the star. ‘Over the next few orbits (around a few hundred years), the dust will smear into a ring around the star,’ Kenworthy explains. For now, the debris is in a long, giant cloud a quarter of an a.u. in size, and the dust is thick enough to block much of the star’s light as the cloud passes in front of it.

“Most of the mass, though, has remained gravitationally bound, albeit in vaporized form. Team member Simon Lock (University of Bristol, UK) has previously proposed that such remnants might take the shape of a synestia, a donut-shape cloud with a bit of material straddling the middle (perhaps more akin to an extremely puffy Danish). This collision gives the researchers an opportunity to test that idea.

“…Another team, led by Jonathan Marshall (Academia Sinica Institute of Astronomy and Astrophysics, Taiwan), has published a different explanation in the Astrophysical Journal, suggesting instead that an uneven dusty disk surrounds the star, perhaps originating in the breakup of comets. That team notes a marked similarity between this system and the curious Boyajian’s Star….”
Two Worlds Have Ended in a Planetary Collision — and a New One Has Begun
Monica Young, Sky and Telescope (October 12, 2023)
[emphasis mine]

Boyajian’s Star, by the way, is KIC 8462852, informally called Tabby’s Star, and that’s something I talked about back in 2016.6

‘…A Star to Steer By….’

Johannes Hevelius' constellation of Argo Navis, from his 'Uranographia' celestial catalog. (1690)
Argo Navis: a huge constellation in Johannes Hevelius’ “Uranographia”. (1690)

Light from ASASSN-21qj traveled 1,850 years before reaching Earth.

The star isn’t the only more-or-less-Sun-like star orbited by unexpectedly-warm dust, but it’s the only one I know of where scientists have ‘before and after’ observations from what’s probably a planetary collision.

No wonder at least two teams have crunched data and published papers. Despite ASASSN-21qj being so far away.

ASASSN-21qj is in the general direction of Zeta Puppis, Naos, a whacking great overly-hot star that’s been studied a great deal more that the ‘assassin star’ — and that’s yet another topic.

Both stars are in the southern constellation Puppis, which we got when de Lacaille split up the ancient Argo Navis: which may go back to Sumerian times.7 Or may not. Scientists aren’t the only ones who don’t know everything. 😉

And that’s yet again another topic: also my cue to stop writing, start proofing this thing, and add the usual links:

1 More than you need, or may want, to know about:

2 Dust from a probable planetary collision:

3 ASASSN-21q, also known as 2MASS J08152329-3859234; a quick look at star names and designations:

4 A distant star’s light, and Earth’s early days:

5 Distances and other details:

6 Studying stars:

7 Odds and ends:

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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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One Response to Colliding Planets Near ASASSN-21qj: Maybe

  1. About the “BLACK LINE HERE” text you saw in this week’s post.
    Sorry about that.
    The good news is that I got this week’s post online.
    The frustrating, for me, news is that I haven’t been able to get the WordPress editor to update/correct those “BLACK LINE HERE” placeholders.
    I am looking for a solution, and plan on getting help from my son over the weekend.
    Thanks for reading, and have a good day/weekend/week – or whatever seems appropriate!

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