# WISE 0855−0714

WISE 0855−0714 (full designation WISE J085510.83−071442.5, or W0855 for short) is a sub-brown dwarf of spectral class Y4, located 7.4 light-years (2.3 parsecs) from the Sun in the constellation Hydra. It is the fourth-closest star or (sub-) brown dwarf system to the Sun and was discovered by Kevin Luhman in 2013 using data from the Wide-field Infrared Survey Explorer (WISE). It is the coldest brown dwarf found in interstellar space, having a temperature of about 285 K (12 °C; 53 °F). It has an estimated mass between 3–10 Jupiter masses, which makes it a planetary-mass object below the 13 Jupiter mass limit for deuterium fusion.

## Characterization
### Observations

WISE 0855−0714 was first imaged by the WISE telescope on 4 May 2010 during its primary mission of surveying the entire sky. It was later discovered by Kevin Luhman in March 2013, who noticed the object's unusually high proper motion while searching for potential binary companions of the Sun in WISE images. In the interest of confirming the object's spectral properties and nearby distance to the Sun, Luhman made follow-up observations with the Spitzer Space Telescope and the Gemini North telescope in 2013–2014. The discovery of the object was announced in a NASA press release in April 2014.
Since WISE 0855−0714 is an isolated object, its luminosity primarily comes from thermal radiation. WISE 0855−0714's temperature is low enough that it roughly matches room temperature, which means WISE 0855−0714's luminosity is very low and primarily emits infrared light as thermal radiation. Hence, it is best observed with infrared telescopes such as WISE and the James Webb Space Telescope (JWST). WISE 0855−0714 has been detected in spectral wavelengths as short as 1.15 μm—in this near-infrared wavelength, the object appears extremely dim with an apparent magnitude of 26.3. WISE 0855−0714's brightness decreases with decreasing wavelength, so the object is practically invisible in visible light.

### Distance and proper motion

Based on direct observations, WISE 0855−0714 has a large parallax of 439.0±2.4 mas, which corresponds to a distance of around 2.28±0.01 parsecs (7.43±0.04 light-years). This makes WISE 0855−0714 the fourth-closest star or (sub-) brown dwarf system to the Sun. WISE 0855−0714 also has an exceptionally high proper motion of 8,151.6±1.8 mas/yr, the third-highest after Barnard's Star (10,300 mas/yr) and Kapteyn's Star (8,600 mas/yr)

### Spectrometry

Its luminosity in different bands of the thermal infrared in combination with its absolute magnitude—because of its known distance—was used to place it in context of different models; the best characterization of its brightness was in the W2 band of 4.6 μm at an apparent magnitude of 13.89±0.05, though it was brighter into the deeper infrared. Infrared images taken with the Magellan Baade Telescope suggest evidence of sulfide clouds below water ice clouds.
Near- and mid-infrared spectra in the L- and M-band were taken with the GNIRS instrument on the Gemini North Telescope. The M-band (4.5–5.1 μm) spectrum is dominated by water vapour (H$_2$O) absorption. The L-band (3.4–4.14 μm) spectrum is dominated by methane absorption. Both the M- and L-band surprisingly have no detection of phosphine (PH$_3$), which appears in the atmosphere of Jupiter. The M-band spectrum shows evidence for water ice clouds and the near-infrared photometry WISE 0855 is faint compared to models, suggesting an additional absorber, probably clouds made of ammonium dihydrogen phosphate (NH$_4$)(H$_2$PO$_4$), which are below the water ice clouds. An approved JWST proposal describes how the team is planning to use a near-infrared time-series to study the hydrological cycle in the atmosphere of WISE 0855 with NIRSpec.
Observations with NIRSpec detected methane (CH$_4$), water vapor (H$_2$O), ammonia (NH$_3$) and carbon monoxide (CO) in the atmosphere, but was not able to confirm any phosphine (PH$_3$) or carbon dioxide (CO$_2$) in the atmosphere. Water ice clouds are also not confirmed and the spectrum is well matched with a cloudless model. Observations with MIRI showed a water vapor depletion and a water abundance that is variable with pressure. This is consistent with water condensing out in the upper atmosphere. The observations did however not detect any water ice clouds, which were predicted in previous studies. This discrepancy is explained with the rainout of the water: Water condenses into particles in the upper atmosphere, which quickly sink into the lower atmosphere. Clouds only form if upward mixing is present. A similar process is present for alkali metals in L- and T-dwarfs. A direct rainout would suggest weak mixing, but disequilibrium chemistry suggest rigours mixing. Future variable studies might resolve if upward mixing or settling is the dominant process. Cloud models however potentially detected deep ammonium dihydrogen phosphate (NH$_4$)(H$_2$PO$_4$) clouds. The observations also detected $^{15}$NH$_3$ for the first time in WISE 0855. The atmosphere has a mass fraction of $^{14}$NH$_3$/$^{15}$NH$_3$ = 332+63 −43, meaning it has about 99.7% $^{14}$N and about 0.3% $^{15}$N. Compared to solar values and the ratio of WISE 1828, the atmosphere of WISE 0855 is enriched in $^{15}$N. The nitrogen isotope ratio is closer to today's $^{15}$N-enriched interstellar medium. This could mean that WISE 0855 formed from a younger cloud, but more measurements of $^{15}$N in other brown dwarfs are needed to establish evolutionary trends. In November 2024 a team used archived and new NIRSpec data to detect deuterated methane (CH$_3$D) and about one part per billion PH$_3$ in WISE 0855. This detection of deuterium showed that WISE 0855 has a mass below the deuterium-burning-limit. The low amount of PH$_3$ is on the other hand in disagreement with predictions, showing incomplete knowledge of phosphorus chemistry.

### Variability

Variability of WISE 0855 in the infrared was measured with Spitzer IRAC. A relative small amplitude of 4–5% was measured. Water ice cloud models predicted a large amplitude. This small amplitude might suggest that the hemispheres of WISE 0855 have very small deviation in cloud coverage. The light curve is too irregular to produce a good fit and rotation periods between 9.7 and 14 hours were measured.

### Physical parameters

The mass and age of WISE 0855−0714 are neither known with certainty, but can be constrained with its known present-day temperature. The age of WISE 0855−0714 depends on its mass; a lower mass would lead to a faster rate of cooling and thus a younger age for WISE 0855−0714, whereas a higher mass would lead to a slower rate of cooling and thus an older age for WISE 0855−0714. Assuming an age range of 1–10 billion years, evolutionary models for brown dwarfs predict that WISE 0855−0714 should have a mass between 3 to 10 M$_{Jup}$. This mass is in the range of a sub-brown dwarf or planetary-mass object.
As of 2003, the International Astronomical Union considers an object with a mass above 13 M$_{Jup}$, capable of fusing deuterium, to be a brown dwarf. A lighter object and one orbiting another object is considered a planet. However, if the distinction is based on how the object formed then it might be considered a failed star, a theory advanced for the object Cha 110913-773444.
Combining its luminosity, distance, and mass it is estimated to be the coldest-known brown dwarf, with a modeled effective temperature of 225 to 260 K (−48 to −13 °C; −55 to 8 °F), depending on the model. Models matching the NIRSpec spectrum are well fitted with a temperature of 285 K (12 °C; 53 °F).

## Gallery

* Time-lapsed photo sequence of WISE 0855−0714's movement in the sky using captured images from the WISE and the Spitzer telescopes.
* JWST NIRCam observation of W0855 (orange "star" at the center) showing the movement over about half a year.
* Diagram of the closest stars to the Sun within 7.5 ly.
* The position of WISE 0855−0714 on a radar map among all stellar objects or stellar systems within 9 light years (ly) from the map's center, the Sun (Sol). The diamond-shapes are their positions entered according to right ascension in hours angle (indicated at the edge of the map's reference disc), and according to their declination. The second mark shows each's distance from Sol, with the concentric circles indicating the distance in steps of one ly.
* No water ice clouds are detected in WISE 0855, but maybe deep ammonium dihydrogen phosphate clouds exist. This would make it similar to class III (cloudless) planets on the Sudarsky scale