Fomalhaut is one of the brightest stars in the night sky and is about 25 light-years away, making it a galaxy amenable to detailed observations. It’s also a young star, only about 440 million years old. At that age, stars like Fomalhaut are surrounded by active debris disks made of rock and dust from collisions between planetesimals. Exoplanets form in these disks, and one of the hot topics in exoplanet science concerns how planets form in these circumstellar disks.
Finding exoplanets in these disks is challenging. Astronomers use clues found in the shape and morphology of the disks to try to infer the presence of exoplanets. Fomalhaut’s disk is warped in an unusual way, and new research suggests that the warping is caused by a massive planet orbiting the star.
Two separate papers present new observations of Fomalhaut and its disk. One is “ALMA Reveals an Eccentricity Gradient in the Fomalhaut Debris Disk,” published in The Astrophysical Journal. The lead author is Joshua Lovell, from the Harvard & Smithsonian Center for Astrophysics.
The second paper is “High Resolution ALMA Data of the Fomalhaut Debris Disk Confirms Apsidal Width Variation,” published in The Astrophysical Journal Letters. The lead author is Jay Chittidi, from the Department of Physics & Astronomy at John Hopkins University.
“Fomalhaut’s proximity allows observations to resolve its structure at higher resolution than other systems, which continues to make it an ideal target to explore the early evolution of planetary systems,” write Chittidi et al.
The main discovery is that Fomalhaut’s debris disk is eccentric, but its eccentricity isn’t fixed. Instead, the eccentricity changes depending on distance from the star. It has what the researchers call a ‘negative eccentricity gradient.’ That means that the further a part of the disk is from the star, the less eccentric it is.
“Our observations show, for the first time, that the disk’s eccentricity isn’t constant,” said lead author of one of the papers, Joshua Bennett Lovell, a Submillimeter Array Fellow with the Harvard-Smithsonian Center for Astrophysics. “It steadily drops off with distance, a finding that has never before been conclusively demonstrated in any debris disk.”
This figure shows some of the ALMA observations and JWST observations (bottom right) used in the research. Fomalhaut’s debris disk displays a negative eccentricity gradient, where the further a part of the disk is from the star, the less eccentric it is. The SE side of the disk is 4 au wider than the NW side. The fainter dashed curves in the bottom images denote the inner belt (IB) boundaries of the disk. Image Credit: Chittidi et al. 2025. ApJL
In the second paper, the authors write “We use radial profiles to measure the disk at the ansae and find that the southeast (SE) side of the disk is 4 au wider than the northwest (NW) side as observed by ALMA.”
The question is, what causes this?
Planets are theorized to create these kinds of disks, though none have yet been observed. The researchers worked to fit a model to the data and found that planets hidden in the rings can alter disks into a negative eccentricity gradient.
“Since the inference of an eccentricity gradient in Fomalhaut’s disk has important implications for the presence and orbital properties of an internal planet interacting with the disk, we next investigate planet properties plausible with this interpretation,” Lovell and his co-researchers write in the first paper.
Observations with the JWST place some limitations on the exoplanet’s mass and orbit. “Perhaps more important for constraining planetary properties are the JWST MIRI observations,” write Lovell and his co-authors. “In these, the first evidence of an “intermediate belt” is presented, which has inner and outer edges of 83 au and 104 au, respectively.”
In Lovell’s paper, the authors narrow it down to to possible exoplanet scenarios. “One scenario describes a 109–115 au planet that has directly cleared material up to the inner edge of Fomalhaut’s “main belt” (as imaged by ALMA),” the authors write. The second scenario involves a closer planet between 70 to 75 au “interior to the JWST-imaged ‘intermediate belt’.”
The modelling also shows that Fomalhaut’s disk was likely eccentric to begin with, and that a planet is responsible for sculpting the disk’s morphology. “These findings may suggest that planet–disk interactions are primarily responsible for sculpting the disk’s morphology (i.e., its inner-edges, and as-per the JWST observations, gaps in the disk), but not its eccentricity, and thus that Fomalhaut’s eccentric ring was plausibly born eccentric,” explain Lovell and his co-authors in their conclusion.
The eccentricity isn’t the only feature that arouses the curiosity of researchers. There are different brightness features, as well as different substructures in the rings.
In a press release, lead author of the second paper, Jay Chittidi, said “Simply put: we couldn’t find a model with a fixed eccentricity that could explain these peculiar features in Fomalhaut’s disk. Comparing the old and new models, we are now able to better interpret this disk, and reconstruct the history and present state of this dynamic system.”
Unfortunately, astronomers currently have no way to detect the planet directly, if it’s there. “In both cases, the implied planet mass and semimajor axis ranges are below sensitivity thresholds for existing planet detection methods,” the Lovell paper states.
The model that Lowell et al. developed can be further tested by ALMA observations of other eccentric disks. As observations and detection methods improve, there’s the tantalizing possibility of revisiting Fomalhaut and verifying the existence of the planet and the model that predicted it. “And hopefully we’ll find new clues that will help us uncover that planet!” said Lovell.