Y dwarf

[11] 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.

[12][13] 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.

[16] 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.

The rapid rotation leads to dynamical, thermal, and chemical changes, which disrupt the convective transport of heat from the lower to the upper atmosphere.

[6][5] 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).

[19] WISE 0855−0714 (Y4) is suspected to have water ice clouds, which should produce large amplitude variations.

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.

An example is CWISE J1055+5443, for which researchers find that low gravity models fit the spectrum better, likely due to a young age.

[13] Another notable spectral discovery with JWST is the emission of methane in CWISEP J1935-1546, which is interpreted with the presence of an aurora.

[30] A small sample of (candidate) exoplanets exist with a temperature below 500 K, which could be spectroscopically confirmed as Y-dwarfs in the future.