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Starts With A Bang

Fomalhaut’s “great dust cloud” isn’t real

A surprising JWST discovery around Fomalhaut has a different, superior explanation: not a great dust cloud, but a mere background object.
Fomalhaut debris system ALMA Keck JWST
This image shows the region of sky closely associated with the Fomalhaut system. Within this field of view, a number of bright point sources can be seen: suspected to be background objects, not part of the foreground Fomalhaut system itself. One such object, shown inset with two separate identifiers (corresponding to ALMA and Keck observations), is likely to be a background galaxy previously mistaken for a "great dust cloud" within the Fomalhaut system.
Credit: G.M. Kennedy et al., MNRAS submitted, 2023
Key Takeaways
  • One of the nearest, bright stars with a debris disk around it, Fomalhaut, has recently been imaged by a series of powerful observatories: Hubble, ALMA, Keck, and JWST.
  • The JWST data revealed a number of striking features: three separate belts, gaps between them, and a “bright spot” just inside one of the belts, identified as a Great Dust Cloud.
  • However, the full suite of observations, going back more than a decade, suggests a different explanation: it’s not a dust cloud at all, but a far more distant background object, like a sub-millimeter galaxy.

One of the nearest, brightest stars to us, Fomalhaut, is a remarkable system.

ALMA view Fomalhaut system
The dust ring around the bright star, Fomalhaut, is shown in ALMA data in orange and from older Hubble Space Telescope data in blue. Even though these observatories differ in wavelength sensitivity by a factor of ~1000, they can both reveal the same dusty ring that’s the analogue of our Kuiper belt.
Credits: ALMA (ESO/NAOJ/NRAO) and NASA/ESA Hubble Space Telescope

Only a few hundred million years old, it’s incredibly dusty.

how many planets
Here in our own Solar System, a single star anchors the system, where inner, rocky planets, an intermediate-distance asteroid belt, and then more distant gas giant planets eventually give way to the Kuiper belt and Oort cloud. This configuration is not universal among stellar and planetary systems, which means our Solar System possesses many properties that are not necessarily common among exoplanetary systems.
Credit: NASA/Dana Berry

No surefire planets were seen.

exoplanets in orbit direct image
This animation shows the four super-Jupiter planets directly imaged in orbit around the star HR 8799, whose light is blocked by a coronagraph. The four exoplanets shown here are among the easiest to directly image owing to their large size and brightness, as well as their huge separation from their parent star. Our ability to directly image exoplanets is constrained to giant exoplanets at great distances from bright stars, but improvements in coronagraph technology will dramatically change that story.
Credit: Jason Wang (Northwestern)/William Thompson (UVic)/Christian Marois (NRC Herzberg)/Quinn Konopacky (UCSD)

This impact-rich system likely undergoes heavy bombardment.

meteors impact early Earth
Artist’s concept of meteors impacting ancient Earth. Some scientists think such impacts may have delivered water, amino acids, and other molecules useful to emerging life on Earth, as the evidence is strong that the impact and cratering rate across the Solar System was much higher than present for the first 0.6-0.7 billion years of our Solar System’s history.
Credit: NASA’s Goddard Space Flight Center Conceptual Image Lab

Earlier Hubble observations revealed a ring with an interior “bright spot.”

Fomalhaut b vs. our solar system
In 2008, when the early Hubble observations showed the bright point of light interior to Fomalhaut’s great dust ring, many leapt to the conclusion that this bright, interior object was a shepherding planet orbiting Fomalhaut. This is no longer a viable explanation for the data.
Credit: NASA, ESA, and A. Feild (STScI)

Originally a planetary candidate, follow-up observations showed peculiar motions and dimming.

Fomalhaut b dust cloud
Although early Hubble observations of Fomalhaut indicated not only a Kuiper belt-like ring around it, but a possible planet, dubbed Fomalhaut b, interior to that ring, follow-up observations have shown this to more likely be an expanding, fading, transient dust cloud.
Credit: NASA, ESA, and A. Gáspár and G. Rieke (University of Arizona)

One explanation was an expanding, fading dust cloud.

bok globule barnard 68 dust wavelength
This animation shows the Bok globule Barnard 68 in a variety of visible and infrared wavelengths. As the longer wavelengths reveal, this is not a hole in the Universe but simply a dusty cloud of gas, where the longer wavelengths of light penetrate and pass through the dust. As dust clouds form and dissipate, the dust density can be revealed by examining the light blocked and transmitted by fixed, background objects.
Credit: ESO

However, new JWST observations revealed unprecedented details within Fomalhaut.

3 wavelength views of Fomalhaut JWST
These three images show the Fomalhaut system as observed by JWST at wavelengths of 15.5, 23.0, and 25.5 microns, respectively. The 23.0 micron observations were made with a coronagraph, while the inner disk is revealed at 15.5 microns. The bottom row shows these images “stretched” to illustrate their true size as they would appear if viewed face-on.
Credit: NASA, ESA, CSA, A. Gáspár (University of Arizona) et al., Nature Astronomy, 2023

Both Kuiper belt and asteroid belt analogues were seen, along with an intermediate belt.

Fomalhaut JWST annotated
The structure of the Fomalhaut stellar system is revealed for the first time in this annotated JWST image. A central inner disk, followed by a (likely planet-caused) gap, an intermediate belt, more planets (and another gap), and finally a Kuiper belt analog, complete with what’s been dubbed the “great dust cloud” newly forming inside, are all revealed.
Credit: NASA, ESA, CSA, A. Gáspár (University of Arizona) et al., Nature Astronomy, 2023

A novel bright, dusty feature also appeared, elsewhere than the now-faded old bright spot.

interplanetary dust
Dust grains come in a variety of sizes and compositions, and can form in the aftermath of an energetic collision. As the material expands and cools, dust forms, gets heated, and re-radiates that heat in the infrared, enabling telescopes like JWST to detect its presence. But we must be careful; other, more distant features can mimic dust in this regard.
Credit: E.K. Jessberger et al., in Interplanetary Dust, 2001

Called the “Great Dust Cloud” by JWST scientists, it suggested an independent, dust-generating impact.

multiwavelength Fomalhaut
A wide variety of telescopes have looked at the Fomalhaut system in a variety of wavelengths from both the ground and in space. Only JWST, so far, has been able to resolve the inner regions of the dusty debris present in the Fomalhaut system.
Credit: NASA, ESA, CSA, A. Gáspár (University of Arizona) et al., Nature Astronomy, 2023

But long-term, multiwavelength observations pointed to another explanation.

Fomalhaut deep imaging Keck 2013
7 candidate objects in the Fomalhaut field-of-view were revealed by the Keck telescopes in a 2013 study. All 7 identified objects were inconsistent with a planet-like explanation, and instead appeared to be background objects.
Credit: T. Currie et al., ApJL, 2013

Perhaps it’s always been the same light source: a more distant, serendipitously aligned background object.

apparent motion over time
As Earth orbits the Sun and nearby stars migrate across the sky, a helix-shaped motion appears to emerge. As different background objects remain relatively fixed with respect to Earth, the foreground star system, Fomalhaut in this case, sees these background objects shift relative to it. If the foreground system is dusty, these background objects will have differing amounts of light absorbed and transmitted over time.
Credit: T. Currie et al., ApJL, 2013

As nearby Fomalhaut moves across the sky, background objects appear to shift position.

Fomalhaut debris system ALMA Keck JWST
Multiple background objects, circled in the images here, can be seen using ALMA, Keck, and JWST in the field-of-view of Fomalhaut. With greater sensitivity comes a greater probability of revealing background objects, in addition to faint objects within the foreground (Fomalhaut) system.
Credit: G.M. Kennedy et al., MNRAS submitted, 2023

Intervening dust causes background light to irregularly scatter and be absorbed.

Fomalhaut background objects ALMA Keck
The offset between bright spots in the Fomalhaut system as seen by ALMA (whose names start with “A”) and Keck (whose names start with “K”) provide evidence that they are background objects, with the Fomalhaut system moving relative to them. Because the ground-based data were obtained 6-to-18 years prior to the JWST data, they are unlikely to be common proper motion companions to Fomalhaut.
Credit: G.M. Kennedy et al., MNRAS submitted, 2023

JWST’s mid-infrared eyes are sharp and sensitive enough to reveal background stars and galaxies through Fomalhaut’s dust.

JWST background galaxies Stephan's Quintet
The stellar streams being ripped from one of the interacting member galaxies of Stephan’s Quintet glitters in this image, while background galaxies shine from much farther away. The new stars that form may not remain gravitationally bound and undisturbed for long, but for as long as they persist, will form collections of stars (or galaxies) that have no dark matter within them at all.
Credit: NASA, ESA, CSA, and STScI

Earlier observations support the “background object” explanation.

a black hole in the middle of a blue sky.
The offset between the bright point sources that ALMA observed previously (circles, with “A” as a prefix) versus what JWST observed (color-coded data) show an offset in position between the bright spots seen. This is a potential indication that these are background objects, which the spectrum of “A02” also supports.
Credit: G.M. Kennedy et al., MNRAS submitted, 2023

JWST’s unprecedented capabilities, sadly, bring novel, confounding sources of error.

fomalhaut JWST alma Hubble
This three-observatory view of the debris disk around Fomalhaut was constructed by Adam Block with Hubble, ALMA, and JWST data. The three dusty areas correspond to an inner disk, an intermediate belt, and a Kuiper-like belt, with planets likely persisting in the gaps. Many of these features were unexpected, but bright point sources seen in these images may indicate background objects, rather than point-like features within the system itself.
Credit: Adam Block/Andras Gaspar/Steward Observatory/University of Arizona

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.


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