WISE 0855−0714
WISE 0855−0714 is a brown dwarf of spectral class Y4, located from the Sun in the constellation Hydra. It is the fourth-List of [nearest stars and brown dwarfs|closest stellar or substellar system to the Sun] and was discovered by Kevin Luhman in 2013 using data from the Wide-field Infrared Survey Explorer. It is the coldest brown dwarf found yet, having a temperature of about. It has an estimated mass between 3 and 10 Jupiter masses, which makes it a planetary-mass object below the 13 Jupiter mass rough 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 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. WISE 0855−0714 has been detected in spectral wavelengths as short as —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, which corresponds to a distance of around . 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, the third-highest after Barnard's Star and Kapteyn's StarSpectrometry
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 at an apparent magnitude of, 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 spectrum is dominated by water vapour absorption. The L-band spectrum is dominated by methane absorption. Both the M- and L-band surprisingly have no detection of phosphine, 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, 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, water vapor, ammonia and carbon monoxide in the atmosphere, but was not able to confirm any phosphine or carbon dioxide 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 clouds. The observations also detected 15NH3 for the first time in WISE 0855. The atmosphere has a mass fraction of 14NH3/15NH3 =, 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 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.
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. This mass is in the planetary mass range.As of 2003, the International Astronomical Union considers an object with a mass above, 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 [rogue planet|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.
WISE 0855-0714 is the coldest-known brown dwarf, with an estimated effective temperature of, as estimated from evolutionary models based on its bolometric luminosity of and an assumed age between 1 and 10 billion years. Atmospheric models matching the NIRSpec spectrum are well fitted with a temperature of, somewhat lower than that estimated from evolution models. The bolometric luminosity and the 285 K effective temperature imply a radius of, smaller than the predicted by evolutionary models.