An object with a spectral type Y (also called Y dwarf) is either a

brown dwarf Brown dwarfs are substellar objects that have more mass than the biggest gas giant planets, but less than the least massive main sequence, main-sequence stars. Their mass is approximately 13 to 80 Jupiter mass, times that of Jupiter ()not big en ...

or a free-floating planetary-mass object. They have temperatures below around 500

Kelvin The kelvin (symbol: K) is the base unit for temperature in the International System of Units (SI). The Kelvin scale is an absolute temperature scale that starts at the lowest possible temperature (absolute zero), taken to be 0 K. By de ...

(227°C; 440°F) and are colder than T-dwarfs. Y-dwarfs have a similar spectrum when compared to the giant planet

Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...

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 A giant planet, sometimes referred to as a jovian planet (''Jove'' being another name for the Roman god Jupiter), is a diverse type of planet much larger than Earth. Giant planets are usually primarily composed of low-boiling point materials ( vo ...

of the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

. Objects colder than T dwarfs would primarily emit

infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...

thermal radiation Thermal radiation is electromagnetic radiation emitted by the thermal motion of particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation. The emission of energy arises from a combination of electro ...

, so observations and discoveries with

infrared telescope An infrared telescope is a telescope that uses infrared light to detect celestial bodies. Infrared light is one of several types of radiation present in the electromagnetic spectrum. All celestial objects with a temperature above absolute zero ...

s such as WISE, Spitzer and

James Webb Space Telescope The James Webb Space Telescope (JWST) is a space telescope designed to conduct infrared astronomy. As the largest telescope in space, it is equipped with high-resolution and high-sensitivity instruments, allowing it to view objects too old, Lis ...

were anticipated. Modelling of such cold objects predicted the disappearance of the

sodium Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...

(Na D) and

potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...

(K I) features at around 500 K and the appearance of water clouds at around 400–500 K.

Ammonia Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...

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 0855−0714 WISE 0855−0714 (full designation WISE J085510.83−071442.5, or W0855 for short) is a sub-brown dwarf of Y dwarf, spectral class Y4, located from the Sun in the constellation Hydra (constellation), Hydra. It is the fourth-List of ne ...

, 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

The first full spectral energy distribution of a Y-dwarf

A Y-dwarf is characterized by its deep

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...

(CH₄) and

water vapor Water vapor, water vapour, or aqueous vapor is the gaseous phase of Properties of water, water. It is one Phase (matter), state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from th ...

(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 Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...

(NH₃) in the near-infrared spectrum. Modern observations with JWST detect CH₄, H₂O, NH₃,

carbon monoxide Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...

(CO) and

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

(CO₂) in the atmosphere of Y-dwarfs.

Phosphine Phosphine (IUPAC name: phosphane) is a colorless, flammable, highly toxic compound with the chemical formula , classed as a pnictogen hydride. Pure phosphine is odorless, but technical grade samples have a highly unpleasant odor like rotting ...

(PH₃) was missing from the atmosphere, despite being predicted to be present. Later observations found low amounts of phosphine in WISE 0855−0714. 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 Ammonium dihydrogen phosphate (ADP), also known as monoammonium phosphate (MAP) is a chemical compound with the chemical formula (NH4)(H2PO4). ADP is a major ingredient of agricultural fertilizers and dry chemical fire extinguishers. It also has ...

(NH₄H₂PO₄), an incomplete understanding of

phosphorus Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...

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 The surface gravity, ''g'', of an astronomical object is the gravitational acceleration experienced at its surface at the equator, including the effects of rotation. The surface gravity may be thought of as the acceleration due to gravity experi ...

or due to differences in

metallicity In astronomy, metallicity is the Abundance of the chemical elements, abundance of Chemical element, elements present in an object that are heavier than hydrogen and helium. Most of the normal currently detectable (i.e. non-Dark matter, dark) matt ...

. CO₂ was noted to slightly decrease from T dwarfs to Y dwarfs, but not CO.

Hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is toxic, corrosive, and flammable. Trace amounts in ambient atmosphere have a characteristic foul odor of rotten eggs. Swedish chemist ...

(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. Some

isotopes Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), but ...

were found in WISE 0855−0714. One study found deuterated methane (CH₃D) and another study found ¹⁵NH₃.

Colder lower atmosphere

Usually brown dwarfs have a

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...

–

temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

(P–T) profile in an adiabatic form, which means that the pressure and temperature increase with depth. JWST spectroscopy and

photometry Photometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an astronomical object's electr ...

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 Rotation or rotational/rotary motion is the circular movement of an object around a central line, known as an ''axis of rotation''. A plane figure can rotate in either a clockwise or counterclockwise sense around a perpendicular axis intersect ...

of these isolated objects. The rapid rotation leads to dynamical, thermal, and chemical changes, which disrupt the

convective Convection is single or multiphase fluid flow that occurs spontaneously through the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...

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 Sulfide (also sulphide in British English) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to large families of ...

potassium chloride Potassium chloride (KCl, or potassium salt) is a metal halide salt composed of potassium and chlorine. It is odorless and has a white or colorless vitreous crystal appearance. The solid dissolves readily in water, and its solutions have a sa ...

(KCl) and possibly

(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 Sodium sulfide is a chemical compound with the formula Na2 S, or more commonly its hydrate Na2S·9 H2O. Both the anhydrous and the hydrated salts are colorless solids, although technical grades of sodium sulfide are generally yellow to brick red ...

(Na₂S) clouds into a patchy cloud cover. WISE 0855−0714 (Y4) was suspected to have

water ice Water ice may refer to: *Ice formed by water (as opposed to other substances) *In ice climbing, ice made from flowing water (as opposed to ice from precipitation) *The alternate term for various similar frozen fruit-flavoured desserts: ** Italian ic ...

clouds, but a later study with

MIRI Miri () is a coastal city in north-eastern Sarawak, Malaysia, located near the border of Brunei, on the island of Borneo. The city covers an area of , located northeast of Kuching and southwest of Kota Kinabalu. Miri is the second largest ...

did not detect any water ice clouds. A study using the

NIRCam NIRCam (Near-InfraRed Camera) is an instrument aboard the James Webb Space Telescope. It has two major tasks, as an imager from 0.6 to 5 micrometre, μm wavelength, and as a wavefront sensor to keep the 18-section mirrors functioning as one ...

photometry of WISE J0336−0143B found a significantly bluer color when compared to WISE 0855−0714, suggesting the presence of water ice clouds in this Y-dwarf.

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

or an unusual

. 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 An aurora ( aurorae or auroras), also commonly known as the northern lights (aurora borealis) or southern lights (aurora australis), is a natural light display in Earth's sky, predominantly observed in high-latitude regions (around the Arc ...

. One of the first suspected Y-type

subdwarfs A subdwarf, sometimes denoted by "sd", is a star with luminosity class VI under the stellar classification#Yerkes spectral classification, Yerkes spectral classification system. They are defined as stars with luminosity 1.5 to 2 Absolute ...

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 A white dwarf is a Compact star, stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very density, dense: in an Earth sized volume, it packs a mass that is comparable to the Sun. No nuclear fusion takes place i ...

, 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 Epsilon Indi Ab is a gas giant exoplanet orbiting the star Epsilon Indi A, about 11.9 light-years away in the constellation of Indus. The planet was confirmed to exist in 2018. It orbits at around 30 AU (almost as far as Neptune from the Sun) ...

(275 K), WD 1202–232b, WD 2105–82b (both low temperature), GALEX J071816.4+373139b (400 K), and WD 0310–688b ( 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: * April 2010: Two newly discovered ultracool

sub-brown dwarf A sub-brown dwarf or planetary-mass brown dwarf is an astronomical object that formed in the same manner as stars and brown dwarfs (i.e. through the collapse of a gas cloud) but that has a planetary mass, therefore by definition below the limi ...

s ( UGPS 0722-05 and SDWFS 1433+35) were proposed as prototypes for spectral class Y0. * February 2011: Luhman et al. reported the discovery of WD 0806−661 B, a brown dwarf companion to a nearby white dwarf, with a temperature of c. and mass of . * February 2011: Shortly after that, Liu et al. published an account of a "very cold" (c. ) brown dwarf orbiting another very-low-mass brown dwarf and noted, "Given its low luminosity, atypical colors and cold temperature, CFBDS J1458+10B is a promising candidate for the hypothesized Y spectral class." * August 2011: Scientists using data from NASA's

Wide-field Infrared Survey Explorer Wide-field Infrared Survey Explorer (WISE, List of observatory codes, observatory code C51, Explorer 92 and MIDEX-6) was a NASA infrared astronomy Space observatory, space telescope in the Explorers Program launched in December 2009.. . WISE L ...

(WISE) discovered six objects that they classified as Y dwarfs with temperatures as cool as . These were published in two papers. * July 2012: Seven new Y-dwarfs were discovered, making the total number of confirmed Y-dwarfs fourteen. One of the Y dwarfs, called

WISE 1828+2650 Wise, as a word, refers to someone with wisdom. Wise may also refer to: People * Wise (surname) * Wise (composer), also known as Wise da' Gangsta, a reggaeton producer/songwriter * Wise (rapper) (born 1979), Japanese hip hop artist * Wise (S ...

, was, as of August 2011, the record holder for the coldest brown dwarf—emitting no visible light at all, this type of object resembles a free-floating planet more than a star. WISE 1828+2650 was initially estimated to have an atmospheric temperature cooler than . Its temperature has since been revised, and newer estimates put it in the range of . * November 2012: WISE J1639−6847 was discovered. As of February 2024, it was the second-closest known Y-dwarf to Earth. * April 2014:

was announced, with a temperature profile estimated around and a mass of . It was also unusual in that its observed parallax meant a distance close to light-years from the Solar System. * May 2014: The Y-dwarf

WISE J2209+2711 WISE J2209+2711 (WISE J220905.73+271143.9) is a brown dwarf of spectral type Y0:, located in constellation Pegasus at 22 light-years from Earth. Its discovery was published in 2014 by Cushing ''et al.'' Physical properties WISE J2209+2 ...

was published. * November 2014: The object WISEA J1141−3326 was estimated to be a Y-dwarf and it was later confirmed. * April 2015: The T+Y dwarf binary WISE J0146+4234 AB was discovered. * May 2015: Three Y-dwarfs were discovered with Hubble, bringing the total number of confirmed Y-dwarfs to 21. * June 2018: WISEA J0302−5817 was published as a Y-dwarf, and WISEA J1141−3326 was confirmed as a Y-dwarf. * August 2019: A search of the CatWISE catalog revealed CWISEP J1935-1546, one of the coldest brown dwarfs with an estimated temperature of . In 2023 it was announced that CWISEP J1935-1546 had methane emission due to an aurora. * January 2020: In January 2020 the discovery of

WISE J0830+2837 Wise, as a word, refers to someone with wisdom. Wise may also refer to: People * Wise (surname) * Wise (composer), also known as Wise da' Gangsta, a reggaeton producer/songwriter * Wise (rapper) (born 1979), Japanese hip hop artist * Wise (S ...

, initially discovered by

citizen scientists The term citizen science (synonymous to terms like community science, crowd science, crowd-sourced science, civic science, participatory monitoring, or volunteer monitoring) is research conducted with participation from the general public, or am ...

of the

Backyard Worlds Backyard Worlds: Planet 9 is a NASA-funded citizen science project which is part of the Zooniverse (citizen science project), Zooniverse web portal. It aims to discover new brown dwarfs, faint objects that are less massive than stars, some of whi ...

project, was presented at the 235th meeting of the

American Astronomical Society The American Astronomical Society (AAS, sometimes spoken as "double-A-S") is an American society of professional astronomers and other interested individuals, headquartered in Washington, DC. The primary objective of the AAS is to promote the adv ...

. This Y dwarf is 36.5 light-years distant from the Solar System and has a temperature of about . * February 2020: The CatWISE catalog combined NASA's WISE and

NEOWISE Wide-field Infrared Survey Explorer (WISE, observatory code C51, Explorer 92 and MIDEX-6) was a NASA infrared astronomy space telescope in the Explorers Program launched in December 2009.. . WISE discovered thousands of minor planets and nu ...

surveys. It expanded the number of faint sources and has therefore been used to find the faintest brown dwarfs, including Y dwarfs. Seventeen candidate Y dwarfs were discovered by the CatWISE researchers. Initial color with the

Spitzer Space Telescope The Spitzer Space Telescope, formerly the Space Infrared Telescope Facility (SIRTF), was an infrared space telescope launched in 2003, that was deactivated when operations ended on 30 January 2020. Spitzer was the third space telescope dedicate ...

indicated that CW1446 is one of the reddest and coldest Y dwarfs. Additional data with Spitzer showed that CW1446 is the fifth-reddest brown dwarf, with a temperature of about and a distance of about 10 parsecs. * August 2020: Five candidate Y-dwarfs were discovered via the Backyard Worlds project. * April 2021: New Y-dwarf candidates were published by the CatWISE and Backyard Worlds teams in a collaborative paper. * August 2021: Ross 19B, an old object near the T/Y-boundary orbiting an M-dwarf, was discovered by the Backyard Worlds team. * April 2023: WISE J0336−0143 was confirmed as a Y-dwarf binary with JWST. The B secondary is likely one of the coldest confirmed Y-dwarfs as of December 2023, with an estimated temperature of . * November 2023: CWISE J1055+5443, an object previously classified as a T-dwarf, was confirmed as a nearby Y-dwarf. * December 2023: Three new Y-dwarf candidates were published. The total number of confirmed Y-dwarfs was 27, and 30 additional Y-dwarf candidates existed as of February 2024.

References

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The Astrophysical Journal ''The Astrophysical Journal'' (''ApJ'') is a peer-reviewed scientific journal of astrophysics and astronomy, established in 1895 by American astronomers George Ellery Hale and James Edward Keeler. The journal discontinued its print edition and ...

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NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...

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The Astronomical Journal ''The Astronomical Journal'' (often abbreviated ''AJ'' in scientific papers and references) is a peer-reviewed monthly scientific journal owned by the American Astronomical Society (AAS) and currently published by IOP Publishing. It is one of the p ...

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Nature Nature is an inherent character or constitution, particularly of the Ecosphere (planetary), ecosphere or the universe as a whole. In this general sense nature refers to the Scientific law, laws, elements and phenomenon, phenomena of the physic ...

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The Astrophysical Journal Letters ''The Astrophysical Journal'' (''ApJ'') is a peer-reviewed scientific journal of astrophysics and astronomy, established in 1895 by American astronomers George Ellery Hale and James Edward Keeler. The journal discontinued its print edition and b ...

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