T dwarf

Spectral type T

The spectral type "T" was first proposed in 1999 with Gliese 229B as its only representative at the time. Next came the discovery of the first field T dwarf SDSS 1624+00 , four T dwarfs from the Two Micron All-Sky Survey, and the cold companion Gliese 570D,. These were primarily mid- to late T-dwarfs, with the first early T-dwarfs (SDSS 0837, SDSS 1254, and SDSS 1021) discovered in data of the Sloan Digital Sky Survey by Sandy Leggett et al. 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 sodium (Na I) and potassium (K I) features in the red optical broaden in L- and T-dwarfs, while narrower features at 1-1.2 micron strengthen then weaken across this sequence. Iron hydride (FeH) briefly emerges in the early T dwarfs and then fades toward the mid-types. The behavior of these features were synthesized into a unified near-infrared classification scheme in 2006.

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. The molecules silane (SiH₄) and phosphine (PH₃) were reported in a pair of metal poor brown dwarfs in 2026.

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, most notably a T dwarf companion to the nearby metal-poor M dwarf Wolf 1130. 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 by Nicolas Lodieu et al. showed that it only shows absorption due to H₂O and H₂ in its optical and near-infrared spectrum. CH₄ was found to be missing completely, in contrast to the definition of T-dwarfs as "methane dwarfs", and WISEA 1810−1010 was labeled a "water vapor dwarf". In 2024, Adam 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 induced 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 many thousands. JWST has already discovered the first distant T subdwarfs such as Nonino's Dwarf and UNCOVER-BD-1

Brown dwarfs

Most T-dwarfs are brown dwarfs. Brown dwarfs have a mass lower than the hydrogen burning minimum mass (0.075 M_☉ or 78.5 M_J). There are currently 920 objects in the UltracoolSheet with an infrared spectral type of T. The table of ultracool fundamental parameters lists objects with an infrared spectral type of T that have masses between 2 and 58 M_J. Additional T-type brown dwarfs that orbit stars or white dwarfs are known and the age of the primary can help to determine the mass of the T-dwarfs. One of the oldest known T-dwarfs is Wolf 1130C, which is around 10 billion years old.

Planetary-mass objects and exoplanets

One of the first objects that was conclusively determined to be a young isolated planetary-mass object with spectral type T was SDSS J1110+0116 (T5.5), which is a member of the 120 Myr old AB Doradus moving group. Another significant discovery is one of the closest planetary-mass objects, called SIMP J013656.5+093347 (T2.5, 12.7 ±1.0 M_J), which is part of the 200 Myr old Carina-Near moving group. This object is also variable with a period of 2.4 hours, likely due to clouds. It also shows radio emission due to aurorae. Additional young T-dwarf candidates are known from other young stellar associations and these objects show red colors compared to field T-dwarfs. Young directly imaged exoplanets and planetary-mass companions sometimes show a T spectral type, such as 51 Eridani b (T4.5-T6).

Clouds and variability

Two of the most variable brown dwarfs are the T-dwarfs Luhman 16B, showing a variation up to 20% and 2MASS J2139+02, which varies with an amplitude as high as 26%. T-dwarfs, especially young early-type T-dwarfs are often variable. The variability has been connected to the presence of clouds, but other explanations were proposed, such as hot spots and aurorae. These early T-dwarfs are thought to have an iron cloud deck and a patchy silicate cloud layer on top of it. The silicate clouds are thought to dissipate near the L/T transition, resulting into the patchy silicate cloud layer and high amplitude variability for late L and early T dwarfs. The disruption of clouds make deeper layers accessible for observations. These deeper layers are warmer and contain FeH. This explains the reappearance and strengthening of FeH and the blue near-infrared color for early to mid T-dwarfs. Late T-dwarfs should also have cloud layers made of chromium, potassium chloride and different sulfides. These cloud layers are thin and exist above the silicate clouds. A few late T-dwarfs are known to be variable, such as Ross 458C (T8), WISE 0458+6434 (T8.5), and 2MASS J09393548-2448279 (T8).

Magnetic field and aurorae

The first T-dwarf detected in radio emission was 2MASS J1047+21 (T6.5), which was discovered with the Arecibo radio telescope. Since then several other T-dwarfs with radio emission were discovered, including the planetary-mass object SIMP J013656.5+093347 (T2.5) and the discovery of the T-dwarf BDR J1750+3809 with the help of radio emission. The coldest T-dwarf with a radio emission is WISEPA J062309.94-045624.6 (T8). Radio emission in T-dwarfs is thought to be generated by an aurora, similar to late L-dwarfs. Additionally H-alpha emission is often connected to radio emission in L4-T8 dwarfs and is thought to come from aurorae. 2MASS 1237+6526 (T6.5) is an unusual strong H-alpha emitting T-dwarf that was discovered in 2000. It was theorized that the H-alpha emission, UV emission and radio emission come either from a cold companion (1-2.8 R_🜨; <500 K) or from an aurora.

Binaries

Late T dwarf binaries are less common than L-type binaries. Only 8±6% systems with a T5–Y0 primary are binaries and these systems usually have a separation of a few astronomical units (AU). One well-known T dwarf binary is Epsilon Indi B. This binary consists of a T1 and a T6 dwarf that orbit each other with a separation of 2.65 AU. T dwarf triple systems also exist, with 2M0838+15 being the first fully resolved triple T dwarf that was discovered.