The spectral type "T" was first proposed in 1999 with Gliese 229B as its only representative at the time. Next came the discovery of
Gliese 570D,
SDSS 1624+00 (first field T-dwarf) and
SDSS 1346-00 (second field T-dwarf). These were however mid- to late T-dwarfs and the first early T-dwarfs (
SDSS 0837,
SDSS 1254, and
SDSS 1021) were discovered in data of the
Sloan Digital Sky Survey in 2000. These objects show weaker CH4 absorption than previously discovered T-dwarfs. CH4 appears first in the
K-band in L8 dwarfs and L- and T-dwarfs are distinguished by the appearance of CH4 in the
H-band for T-dwarfs. T-dwarfs show an increasing absorption of H2O and CH4 from T0 to T8. Neutral Na and K features broaden in L- and T-dwarfs and the Na feature increases in depth for L/T-dwarfs with increasing spectral type. One of the coldest T-dwarfs was discovered with
UKIDSS, called
UGPS 0722-05. Researchers used
WISE to discover additional late T-dwarfs and the objects of the newly discovered Y-dwarfs. The transition between T- and Y-dwarfs is defined with the help of UGPS 0722-05 as the T9 standard and
WISE 1738+2732 as the Y0 standard. Late T and early Y-dwarfs show deep H2O and CH4 absorption features and the transition between T- and Y-dwarfs occurs near 500 K. Another important T-dwarf is
Luhman 16B, which is the closest T-dwarf. It has a spectral type of T0.5, near the L/T transition. It shows a hint of FeH in the spectrum, which weakens in late L dwarfs, but strengthens in early to mid T-dwarfs due to cloud disruption. Observations of T-dwarfs in the near- and mid-infrared with
JWST clearly show additional absorption features due to NH3, CH4, H2O, CO and
carbon dioxide (CO2). Observations with
Gemini showed the first detection of
hydrogen sulfide (H2S) and molecular
hydrogen (H2) in the T6 dwarf
DENIS J081730.0−615520. The molecules
hydrogen cyanide (HCN) and
acetylene (C2H2) were found in the T-dwarf binary
WISE 0458+6434 in 2025 with JWST.
Subdwarfs Subdwarfs with a T spectral type are known, with
2MASSI J0937347+293142 being the first T-type
subdwarf. It shows blue near-infrared colors due to suppression of the 2.1 μm peak, likely caused by enhanced
collision induced absorption (CIA) of
hydrogen (H2). Subdwarfs have a low
metallicity and at first only a small sample with moderate low metallicity was known. In 2020 the
backyard worlds citizen science project discovered the first extreme subdwarfs of spectral type T, called
WISEA 0414−5854 and
WISEA 1810−1010. These objects have unusual blue colors, indicative of a lower absorption from CH4. Follow-up observations of WISEA 1810−1010 show that it only shows absorption due to H2O and H2 in the optical and infrared spectra. CH4 is missing completely, which stays in contrast to the definition of T-dwarfs as "methane dwarfs" and WISEA 1810−1010 was instead called a "water vapor dwarf". In 2024 Burgasser et al. introduced a classification system for T subdwarfs, which allows the classification into mild subdwarfs (d/sdT), subdwarfs (sdT) and extreme subdwarfs (esdT). The signature of a low metallicity are a strong
collision induce absorption (CIA) of hydrogen molecules, obscured methane and water features, and weak potassium K I absorption. This work also identified three brown dwarfs that are candidate members of
stellar streams. Future works with
JWST,
Euclid,
Rubin and
Roman will increase the sample of T subdwarfs to thousands. JWST has already discovered the first distant T subdwarfs such as
UNCOVER-BD-1. == Brown dwarfs ==