WISE 0855−0714

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 stellar or substellar 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).

Physical characteristics

Mass and age

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 and 10 M_Jup. The mass could also be obtained with measurements of the surface gravity and radius of the brown dwarf. One 2024 study obtained two values of 10^3.93 and 10^4.03 cgs for the surface gravity from two different data sets. Assuming a radius of one Jupiter radius (R_Jup) for the brown dwarf, they obtained mass values of 3.44 and 4.33 M_Jup, respectively. All these mass values are in the planetary mass range.

Classification

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. WISE 0855–0714, being a free-floating object with a mass lower than the deuterium burning limit, would be called a free-floating planetary mass object, although the literature classifies it as a brown dwarf. 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.

Temperature, size and luminosity

WISE 0855–0714 is the coldest-known brown dwarf, with an effective temperature of 276 K (3 °C; 37 °F), as estimated from evolutionary models based on its bolometric luminosity of 6.03×10⁻⁸ L_☉ and an assumed age between 0.5 and 10 billion years. The luminosity and temperature imply a radius of 0.107 solar radii (R_☉), or 1.045 R_Jup. Atmospheric models matching the NIRSpec spectrum are well fitted with a temperature of 285 K (12 °C; 53 °F), somewhat higher than that estimated from evolution models.

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 its luminosity is very low and it primarily emits infrared radiation 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.

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 bands 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 (H₂O) 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 (PH₃), 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 (NH₄)(H₂PO₄), 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 (CH₄), water vapour (H₂O), ammonia (NH₃) and carbon monoxide (CO) in the atmosphere, but was not able to confirm any phosphine (PH₃) or carbon dioxide (CO₂) 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 vapour 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 rigorous mixing. Future variable studies might resolve if upward mixing or settling is the dominant process. Cloud models, however, potentially detected deep ammonium dihydrogen phosphate (NH₄)(H₂PO₄) clouds. The observations also detected ¹⁵NH₃ for the first time in WISE 0855. The atmosphere has a mass fraction of ¹⁴NH₃/¹⁵NH₃ = 332+63
−43, meaning it has about 99.7% ¹⁴N and about 0.3% ¹⁵N. Compared to solar values and the ratio of WISE 1828, the atmosphere of WISE 0855 is enriched in ¹⁵N. The nitrogen isotope ratio is closer to today's ¹⁵N-enriched interstellar medium. This could mean that WISE 0855 formed from a younger cloud, but more measurements of ¹⁵N 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 (CH₃D) and about one part per billion PH₃ in WISE 0855. This detection of deuterium showed that WISE 0855 has a mass below the deuterium-burning limit. The low amount of PH₃ is, on the other hand, in disagreement with predictions, showing incomplete knowledge of phosphorus chemistry. In September 2026 a team used NIRSpec time-series to detect patchy water ice clouds within WISE 0855. This is based on a better fit to the spectrum and to explain some of the variation. Based on the variation the researchers determine an edge-on orientation.

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. JWST NIRSpec time-series showed variable carbonmonoxide and phosphine emission. The CO absorption is variable up to 10% at some wavelengths. Both variations are correlate with each other and are mainly explained with deep temperature modulations in the convective zone. The rest of the variations are explained with patchy water ice clouds at lower pressures (higher altitudes).

Search for satellites

A team of researchers used 11 hours of JWST observations to search for transits caused by exomoons around WISE 0855. The researchers did not find any transits. By injecting transits into the light curve, the researchers found that they could have detected ≥0.5% deep transits with a detection rate of 96%. This transit depth would be seen for an object about twice as large as Saturn's moon Titan.