# Capotauro
**Capotauro** (CEERS ID U-100588) is a compact red astronomical source detected in [James Webb Space Telescope](https://www.edgepedia.org/james-webb-space-telescope) (JWST) imaging of the Cosmic Evolution Early Release Science (CEERS) Survey that was proposed as a galaxy at redshift z ≈ 32 and was later confirmed, through the detection of proper motion, to be a cold Y-type [brown dwarf](https://www.edgepedia.org/brown-dwarf) in the [Milky Way](https://www.edgepedia.org/milky-way) at a distance of 730 ± 110 parsecs (about 2,380 light-years)[1](https://arxiv.org/abs/2509.01664) • [2](https://arxiv.org/abs/2608.07461). The name comes from the old name of Monte Corno alle Scale, a peak in the Tuscan-Emilian Apennine, chosen by the astrophysicist Giovanni Gandolfi of INAF Rome, who led the study that first characterized the object[3](https://www.media.inaf.it/2026/02/20/capotauro-nana-bruna-galassia-lontanissima/).
| Key fact | Detail |
|---|---|
| Designation | CEERS U-100588, nicknamed Capotauro after the ancient name of Monte Corno alle Scale[3](https://www.media.inaf.it/2026/02/20/capotauro-nana-bruna-galassia-lontanissima/) |
| Observed properties | F444W AB magnitude ~27.68; a sharp flux drop of more than 3 magnitudes between 3.5 and 4.5 micrometers; undetected below 3.5 micrometers[1](https://arxiv.org/abs/2509.01664) |
| Initial interpretation | Favored as a Lyman-break galaxy at z ~ 32, near the epoch of the first stars and black holes, with only ~0.5% of the posterior volume at z < 25[1](https://arxiv.org/abs/2509.01664) |
| Confirmed nature | Y-type brown dwarf, spectral type Y1.0 ± 0.5, effective temperature ≈ 350 K, at 730 ± 110 pc[2](https://arxiv.org/abs/2608.07461) |
| Proper motion | Moved 132 ± 20 mas over a 3.5-year baseline, ruling out any extragalactic source at more than 6 sigma[2](https://arxiv.org/abs/2608.07461) |
| Physical parameters | Mass 14.0 ± 11.0 Jupiter masses; radius 1.24 ± 0.16 Jupiter radii (catalog values)[4](https://voparis-exoplanet-new.obspm.fr/catalog/capotauro--12427/) |
## Discovery and the z ≈ 32 galaxy hypothesis
Capotauro appeared in deep JWST imaging of the CEERS field, a 100-square-arcmin region of the [Extended Groth Strip](https://www.edgepedia.org/extended-groth-strip) observed with NIRCam, MIRI, and NIRSpec[5](https://stdatu.stsci.edu/hlsp/ceers). The source is an extreme F356W-dropout: it is detected only in the two longest NIRCam bands, with an F444W AB magnitude of about 27.68 and a flux drop of more than 3 magnitudes between 3.5 and 4.5 micrometers[1](https://arxiv.org/abs/2509.01664).
A spectral break of this kind is the signature astronomers use to find the most distant galaxies. Light from a remote galaxy shows a Lyman break, a sharp cutoff caused when neutral hydrogen absorbs high-energy photons, and the expanding universe shifts that cutoff to longer wavelengths in proportion to redshift[6](https://astrobites.org/2026/09/24/capotauro-z32-candidate/). Capotauro's drop at 3.5 micrometers is consistent with a Lyman-break galaxy at z ~ 32, which would place it about 90 million years after the [Big Bang](https://www.edgepedia.org/big-bang), in the era when the first stars and black holes formed[1](https://arxiv.org/abs/2509.01664) • [3](https://www.media.inaf.it/2026/02/20/capotauro-nana-bruna-galassia-lontanissima/). Under this interpretation the object would be enormous, estimated at over a billion solar masses, and would push back the age of the oldest known galaxy by around 200 million years[7](https://www.livescience.com/space/cosmology/puzzling-object-discovered-by-james-webb-telescope-may-be-the-earliest-known-galaxy-in-the-universe).
The only spectrum obtained, from a roughly 0.8-hour NIRSpec exposure, had low resolution and signal-to-noise and could not fix a redshift, so the classification rested on photometry (measuring an object's brightness through filters, rather than obtaining a spectrum) alone[6](https://astrobites.org/2026/09/24/capotauro-z32-candidate/) • [8](https://doi.org/10.1051/0004-6361%2F202554009).
## Alternative interpretations
The discovery team's analysis, published in *Astronomy & Astrophysics* in 2026 as "Mysteries of Capotauro", tested galaxy solutions at z ≥ 15 and z < 10, including dusty, Balmer-break, and strong-line options, against the possibility of a Milky Way substellar object[1](https://arxiv.org/abs/2509.01664). Among extragalactic options the z ~ 32 Lyman-break reading was favored, but the data could not determine the object's nature[1](https://arxiv.org/abs/2509.01664).
**A cold brown dwarf.** The same photometry fits either a brown dwarf of the very cool Y2-Y3 variety or an exoplanet not bound to any star, whose pairing of frigid temperature and great distance would set a record, given an effective temperature under 300 K and a distance of no less than 130 parsecs, potentially reaching roughly 2 kiloparsecs[1](https://arxiv.org/abs/2509.01664). Brown dwarfs are objects that collapsed from gas clouds but never reached the mass needed to sustain hydrogen fusion; they radiate residual heat in the infrared, and the coldest class, Y dwarfs, have spectra dominated by water, methane, and ammonia absorption[6](https://astrobites.org/2026/09/24/capotauro-z32-candidate/). Those molecular bands can mimic a Lyman break at z ≈ 30, which is why cold dwarfs in the foreground of deep fields are a contamination risk for ultra-high-redshift galaxy searches[2](https://arxiv.org/abs/2608.07461).
**A pair-instability supernova.** A 2026 preprint by [Andrea Ferrara](https://www.edgepedia.org/andrea-ferrara-cosmologist) of the Scuola Normale Superiore di Pisa and colleagues noted that Capotauro was found to be variable over an approximately 800-day baseline and proposed that it is a [pair-instability supernova](https://www.edgepedia.org/pair-instability-supernova), the explosion of a massive (250 to 260 solar-mass), metal-free star at z ≈ 15[9](https://arxiv.org/abs/2601.07374). In such an explosion, energetic gamma rays in the core convert into electron-positron pairs, removing the radiation pressure that supports the star and triggering a runaway thermonuclear collapse[10](https://normalenews.sns.it/capotauro-oggetto-cosmico-delluniverso-primordiale-potrebbe-essere-una-supernova-esplosa). The authors showed that such a supernova could plausibly reproduce the observed brightness, photometry, and NIRSpec spectrum, while a local Y0 brown dwarf fits the spectrum but not the variability[9](https://arxiv.org/abs/2601.07374).
## Proper motion confirmation
Distant galaxies do not move measurably across the sky, while nearby Galactic objects do. A team led by F.-Y. F. Liu of the Institute for Astronomy, University of Edinburgh, used new medium-band NIRCam imaging obtained about 3.5 years after the discovery data to test this directly[2](https://arxiv.org/abs/2608.07461).
Using filter-dependent point-spread-function fitting, a relative astrometric frame built from 68 compact reference sources, and a joint four-image injection-recovery analysis, they measured a displacement of 132 ± 20 milliarcseconds over the 3.5-year interval, ruling out an extragalactic source (galaxy, active galactic nucleus, or supernova) at more than 6 sigma[2](https://arxiv.org/abs/2608.07461). The apparent proper motion is 37.6 mas per year in total, and after removing the small parallactic contribution of about 2.8 mas, the parallax-corrected motion implies a tangential velocity of 132 ± 28 km/s, consistent with thin- or thick-disk kinematics[2](https://arxiv.org/abs/2608.07461).
Comparing the photometric spectral energy distribution with empirical templates of late-T and Y dwarfs gives a best match to the Y1 dwarf WISEA J235402.79+024014.1, implying a spectral type of Y1.0 ± 0.5, an effective temperature near 350 K, and a distance of 730 ± 110 pc; the existing templates cannot exclude an even colder, later-type object[2](https://arxiv.org/abs/2608.07461). At that distance Capotauro is one of the most distant Y dwarfs found to date, since most known Y dwarfs lie within about 30 parsecs of the Sun[2](https://arxiv.org/abs/2608.07461).
## Significance
The case shows the value of multi-epoch imaging for identifying substellar contaminants among extreme photometric-redshift candidates[2](https://arxiv.org/abs/2608.07461). Capotauro-like Y dwarfs can masquerade as a population of Lyman-break galaxies at z ≈ 30, so robust identification of z > 15 candidates requires repeated imaging and the inclusion of cold stellar templates in color-selection criteria[2](https://arxiv.org/abs/2608.07461). For brown dwarf science, finding an object this cold and faint is itself valuable: its properties probe the low-mass end of the substellar mass function, and its light profile can help calibrate templates used to filter out such interlopers in future surveys[2](https://arxiv.org/abs/2608.07461) • [6](https://astrobites.org/2026/09/24/capotauro-z32-candidate/).
## References
1. [Gandolfi, G., et al. (2026). Mysteries of Capotauro: investigating the puzzling nature of an extreme F356W-dropout. arXiv:2509.01664; A&A 706, A364.](https://arxiv.org/abs/2509.01664)
2. [Liu, F.-Y. F., et al. (2026). A clear detection of proper motion confirms that the claimed z≃32 galaxy candidate, "Capotauro", is a Y-type brown dwarf. arXiv:2608.07461, submitted to MNRAS.](https://arxiv.org/abs/2608.07461)
3. [Nichelli, E. (20 February 2026). Nana bruna o galassia lontana: Capotauro, cosa sei? MEDIA INAF.](https://www.media.inaf.it/2026/02/20/capotauro-nana-bruna-galassia-lontanissima/)
4. [Planet Capotauro, VizieR/VO Paris Exoplanet Catalog.](https://voparis-exoplanet-new.obspm.fr/catalog/capotauro--12427/)
5. [CEERS, MAST, Space Telescope Science Institute.](https://stdatu.stsci.edu/hlsp/ceers)
6. [Collins, B. P. (24 September 2026). All that glitters is not gold: How our universe's oldest galaxy turned into a failed star. astrobites.](https://astrobites.org/2026/09/24/capotauro-z32-candidate/)
7. ['Puzzling' object discovered by James Webb telescope may be the earliest known galaxy in the universe. Live Science.](https://www.livescience.com/space/cosmology/puzzling-object-discovered-by-james-webb-telescope-may-be-the-earliest-known-galaxy-in-the-universe)
8. [Ultrahigh-redshift or closer-by, dust-obscured galaxies? Deciphering the nature of faint, previously missed F200W dropouts in CEERS. Astronomy & Astrophysics.](https://doi.org/10.1051/0004-6361%2F202554009)
9. [Ferrara, A., Carniani, S., Morishita, T., Stiavelli, M. (2026). Possible evidence for a pair-instability supernova nature of ultra-early JWST sources. arXiv:2601.07374.](https://arxiv.org/abs/2601.07374)
10. [Capotauro, oggetto cosmico dell'universo primordiale, potrebbe essere una supernova esplosa. NormaleNews, Scuola Normale Superiore di Pisa.](https://normalenews.sns.it/capotauro-oggetto-cosmico-delluniverso-primordiale-potrebbe-essere-una-supernova-esplosa)
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*Topic: Encyclopedia › Physical world and mathematics › Astronomy › Stars and galaxies › Stellar astrophysics, structure, evolution, and variables › Stellar classification and star types › Brown dwarf and ultracool classes (L, T, Y)*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license