An object with a spectral type Y (also called Y dwarf) is either a brown dwarf or a free-floating planetary-mass object. They have temperatures below around 500 Kelvin (227°C; 440°F) and are colder than T-dwarfs. Y-dwarfs have a similar spectrum when compared to the giant planet Jupiter.

Early theories and discovery

When the spectral classes of L dwarfs and T dwarfs were defined it was mentioned that the letter Y was available for an additional spectral class. In the early 2000s it was already theorized that objects "beyond the T dwarfs" should exist and that these objects would bridge the gap between T dwarfs and the giant planets of the Solar System. Observations and discoveries with WISE, Spitzer and James Webb Space Telescope were anticipated. Modelling of such cold objects predicted the disappearance of the sodium (Na D) and potassium (K I) features at around 500 K and the appearance of water clouds at around 400–500 K. Ammonia clouds were predicted to exist below around 160 K. The formation of these clouds were theorized earlier in the Sudarsky's gas giant classification. After some candidates were proposed in 2010 and 2011, a larger sample of Y-dwarfs were discovered with WISE and the Y-dwarf spectral type was established, using UGPS 0722-05 as the T9 standard and WISE 1738+2732 as the Y0 standard. A significant discovery was the discovery of WISE J0855−0714, which remains the coldest and closest Y-dwarf discovered. It has a temperature of 285 K (12 °C; 53 °F) and has the latest spectral type of Y4.

The Y-class

A Y-dwarf is characterized by its deep methane (CH4) and water vapor (H2O) bands, as well as a narrower J-band peak than the T9 standard. The J-band peak will get narrower with a spectral type later than T8. Early observations also showed evidence of ammonia (NH3) in the near-infrared spectrum. Modern observations with JWST detect CH4, H2O, NH3, carbon monoxide (CO) and carbon dioxide (CO2) in the atmosphere of Y-dwarfs. Phosphine (PH3) is missing from the atmosphere, despite being predicted to be present. JWST observations showed that models under-predict the abundance of CO2 and over-predict PH3 for late T and Y dwarfs. Proposed explanations for the missing PH3 are that it condenses into clouds of ammonium dihydrogen phosphate (NH4H2PO4), an incomplete understanding of phosphorus chemistry or a different mixing of the atmosphere. The overabundance of CO2 is explained with a better understanding of the CO2 chemistry in respect to CO chemistry. CH4, H2O and NH3 absorption features get deeper with lower temperature. The 5 μm peak does not show such a correlation and instead shows a large diversity. This region is influenced by multiple molecules, including CO and CO2, which vary a lot between sources. The reason for this variation could be due to a different surface gravity or due to differences in metallicity. CO2 was noted to slightly decrease from T dwarfs to Y dwarfs, but not CO. Hydrogen sulfide (H2S) is used to improve the spectral fits of T- and Y-dwarfs. Currently the only Y-dwarfs with detected H2S are WISE 1828+26 and WISE 0359−5401.

Colder lower atmosphere

Usually brown dwarfs have a pressure–temperature (P–T) profile in an adiabatic form, which means that the pressure and temperature increase with depth. JWST spectroscopy and photometry suggest that Y-dwarfs have P–T profiles that are not in the standard adiabatic form. This means that upper layers of the atmosphere have a warmer temperature and lower layers of the atmosphere have a colder temperature. This is explained by the rapid rotation of these isolated objects. The rapid rotation leads to dynamical, thermal, and chemical changes, which disrupt the convective transport of heat from the lower to the upper atmosphere. This different P–T profile influences the shape of the spectrum and influences the composition of carbon- and nitrogen-bearing molecules in the atmospheres of Y-dwarfs.

Clouds and variability

Water clouds were theorized since the early 2000s to exist in Y-dwarfs. The Y-dwarfs do however likely also have clouds made of other condensates, such as sulfides, potassium chloride (KCl) and possibly ammonium dihydrogen phosphate (NH4H2PO4). These clouds would exist below any water clouds for colder Y-dwarfs. Some Y-dwarfs are likely too warm to form water clouds, but could have other observable clouds. The first discovered variable Y-dwarf was WISE 1405+5534 (Y0) and its variability is modelled with a single bright spot. Another variable Y dwarf is WISE 1738+2732 (Y0) and its variability is explained with the breakup of KCl and sodium sulfide (Na2S) clouds into a patchy cloud cover. WISE 0855−0714 (Y4) is suspected to have water ice clouds, which should produce large amplitude variations. It does show relatively small variations of 4–5%, which could mean that both hemispheres have a similar cloud cover.

Peculiar Y-dwarfs

Currently only the suffix pec, standing for "peculiar" or unusual, exists for Y-dwarfs. Any spectral peculiarity is denoted this way, such as the Y-band peak and Y-J color of WISE 1639−6847 (Y0pec), which is different from other Y-dwarfs. In some cases the peculiarity is explained with a non-solar metallicity or an unusual surface gravity. An example is CWISE J1055+5443, for which researchers find that low gravity models fit the spectrum better, likely due to a young age. JWST observations found two Y-dwarfs with unusual spectral features of the carbon-bearing molecules. CWISEP J1047+54 showed abnormally strong CO and CO2 and likely weaker CH4. Similar strong CO and CO2 absorption features were found in WISE J1206+8401. WISE J0535−75 on the other hand showed no discernable CO2 and almost undetectable CO, but it also showed stronger NH3 absorption when compared to Y-dwarfs with similar temperature. Another notable spectral discovery with JWST is the emission of methane in CWISEP J1935-1546, which is interpreted with the presence of an aurora. One of the first suspected Y-type subdwarfs is WISEA J1534−1043, which shows an unusual blue color. Spectroscopic observations are however required to confirm this hypothesis.

Exoplanets and companions

Masses estimated for Y-dwarfs are between 3–29, but more typically below 21. This makes them similar to massive exoplanets. There is only one confirmed Y-dwarf that co-moves with a white dwarf, called WD 0806−661 B. Though of planetary mass, Rodriguez et al. suggest it is unlikely to have formed in the same manner as planets. Additionally there is the T/Y companion Ross 19B, which orbits a main-sequence star. A small sample of (candidate) exoplanets exist with a temperature below 500 K, which could be spectroscopically confirmed as Y-dwarfs in the future. These exoplanets are Epsilon Indi Ab (275 K), WD 1202–232 b, WD 2105–82 b (both low temperature), GALEX J071816.4+373139 b (400 K), and WD 0310–688 b (248 K).

Binaries

Binaries consisting of a late T dwarf primary and a Y dwarf secondary are known since the discovery of CFBDS J1458+10 during the early discovery of Y-dwarfs. Other such binaries are WISEPC J1217+1626 and WISE J0146+4234. The first Y+Y binary is WISE J0336−0143, discovered in 2023 with JWST.

Individual discoveries

Timeline of Y-dwarf discoveries: