Colliding Dwarf Planets Created a 'Cat's Tail' 1,000 Astronomical Units Long

Beta Pictoris (β Pictoris), a young main sequence star about 63 light years away, is known for having the first directly-imaged debris disk. Three planets in that disk have also been directly-imaged. One of them, Beta Pictoris c, is also the closest exoplanet to its star that has ever been directly imaged.

This is a composite image of of Beta Pictoris as seen in near infrared light. Beta Pictoris B is shown, the closest exoplanet to its star that's ever been directly imaged. Due to its proximity and other factors, it's an object of much observation and research. Image Credit: By ESO, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=5268432

β Pictoris is the subject of lots of research because of its youth, its proximity, its resolved debris-disk, and because its geometry makes it open to detailed observations. In particular, telescopes can get high-resolution side-views of its disk's structure, including its warped nature and its asymmetries. Astronomers have even found evidence of exocomets in the system.

In a paper published in 2024, researchers presented the JWST's first images of β Pictoris and its debris disk. "We report for the first time the presence of an extended secondary disk in thermal emission, with a curved extension bent away from the plane of the disk," the authors wrote. They called it a “cat’s tail,” and suggested that it was connected to other features found in the disk: a clump of carbon monoxide, a warp in the disk, and a "mid-infrared asymmetry detected on the southwest side of the disk."

Unusual features like the Cat's Tail, which extends for about 1,000 astronomical units, always pique the interest of researchers, who want to know how it was created. New research led by a University of Arizona scientist has figured out what created the unusual feature. The research is titled "Deep HST/STIS Coronagraphic Imaging of the β Pictoris Debris Disk: On the Scattered Light Component of the Cat’s Tail," and is published in The Astrophysical Journal Letters. The lead author is Arin Avsar, a PhD Candidate in planetary science in the Lunar and Planetary Laboratory at the UofA.

"The β Pictoris debris disk is a unique system where mid-infrared coronagraphic imaging with JWST/Mid-Infrared Imager (MIRI) revealed a new spatially resolved substructure known as the Cat’s Tail," the authors write. "We present deep Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS) coronagraphic imaging of β Pic in scattered light," the authors add. Their Hubble observations were obtained in six epochs between 2012 and 2025, which they used to search for the cat's tail's scattered light.

The β Pictoris disk can be seen in scattered light, precisely because it's a dusty disk, and the dust scatters the light. The colour and the brightness of the scattered light reveals the properties of the dust, including the size of its grains, their porosity, and the dust's overall composition. Identifying those properties can explain where the dust came from.

"The Cat's Tail is thought to be the aftermath of a collision between large bodies – think something like Pluto," lead author Avsar said in a press release. "Oddly, this feature had previously only been seen with the James Webb Space Telescope, which detects infrared light, but remained invisible at shorter wavelengths, which has puzzled the community for a while."

"We think the Cat's Tail has eluded our observations in visible light until now because of what it's made of," Avsar explained. "It consists almost entirely of what we call organic refractory material, which is heavily carbon-based. These long chains of carbon atoms absorb visible light, so they're quite black."

These panels show the cat's tail feature (white arrow) in visible light from the Hubble on the left, and in mid-infrared light from the JWST on the right. Image Credit: Arin M. Avsar et al 2026 ApJL 1010 L10

The Cat Tail's composition of organic refractory material is a clue to the objects that collided and created it. Both this research and the 2024 research show that the Cat's Tail is predominantly composed of porous organic refractory material. "In the solar system, organic refractory material best reproduces the red surface spectra of Kuiper Belt objects, most notably Pluto," the authors write. NASA's New Horizons found tholins, a type of refractory organic compound, on Pluto's surface, and on the surface of its companion, Charon. But tholins make up only a small proportion of Pluto's mass, not enough to create the Cat's Tail. "This points to organic material from the interior of the disrupted planetesimals as the likely origin of the organic refractory material seen in the Cat’s Tail."

"This invites the question: do dwarf planets in exo-Kuiper Belts (like that of β Pic) contain enough organic material to create the Cat’s Tail?" the researchers write. To answer that, they examined the amount of organics in the local ISM, which placed constraints in the amount available in the Beta Pictoris disk, where the colliding objects formed. They also examined the amount present in comet 67P/Churyumov-Gerasimenko, an icy body visited by the ESA's Rosetta mission.

These measurements let the researchers calculate what the combined mass of the colliding objects must have been, in order to supply enough refractory material needed to create the tail.

"... we find that each parent body must have had a mass of ∼1–3 × 10²¹ kg to produce 2 × 10¹⁹ kg of organic refractory debris in the Cat’s Tail," the authors write. That equates to a few percent of the mass of Earth's Moon, or about 1 to 3 times the mass of the dwarf planet Ceres. These amounts agree with the estimates in the 2024 paper that initially found the Cat's Tail.

This figure helps show how the JWST and the Hubble see things differently, and how their combined observations have a powerful symmetry. The top panel is an RGB image of the HST/STIS image presented in this work overlaid with the deconvolved F1550C image from the 2024 work. The bottom two images show a grayscale version of each dataset in the RGB image. They highlight the smooth/axisymmetric halo in the HST/STIS image in contrast to the complex substructures seen in the F1550C image. Image Credit: Arin M. Avsar et al 2026 ApJL 1010 L10

The researchers also calculated the sizes of the objects that collided. Assuming that they had bulk densities in line with the dwarf planets in our Solar System, the objects must have had radii between about 600km and 800 km. "The mass and radius range is similar to that of dwarf planets in the Kuiper Belt, like Charon and Makemake," the researchers explain.

One puzzling feature of the Cat's Tail is the size and porosity of its dust particles. They're micron-sized and very porous. "How you can get dust grains that small that are as porous as we see here, no one really understands, and that's part of why we study this feature," Avsar said.

"It's possible that the velocities involved in the collisions were so high that the carbon grains vaporized, turned into gas, and then recondensed as they cooled, and that's how they became so porous," said co-author Kevin Wagner, an assistant research professor at the U of A Steward Observatory.

Aside from the Cat's Tail, the new Hubble observations also revealed a strange warp in Beta Pictoris' dust halo. "We also find a convex/concave morphological asymmetry in the outer halo of β Pic with HST/STIS," the authors write. This could be because of interactions with Beta Pictoris B, a giant gas planet that's about ten times as massive as Jupiter. "However, dedicated follow-up dynamical modeling is needed to confirm this," the authors conclude.

One of the reasons that planetary scientists are so intrigued by Beta Pictoris is because it holds clues to our own Solar System's past. It's like an analogue of our system, only much younger. While our system is about 4.6 billion years old, Beta Pictoris is only about 23 million years old. Early systems are far more chaotic than mature systems like ours, so looking at Beta Pictoris gives researchers a window into our own Solar System's past, when collisions were much more common.

Beta Pictoris, its disk, and its exoplanets are too fascinating to ignore. It will continue to the the subject of ongoing observations. When new, powerful telescopes like the ELT and the Giant Magellan Telescope come online in the next few years, it's certain that at some point they'll examine the β Pictoris system. Both telescopes will have the powerful capability of directly imaging exoplanets in greater detail.

These powerful telescopes may also dive deeper into the Cat's Tail, and determine why such an impact created such small, porous dust grains.