Luhman 16

Luhman 16 (also designated WISE 1049−5319 or WISE J104915.57−531906.1) is a binary brown-dwarf system in the southern constellation Vela at a distance of 6.51 ly from the Sun. These are the closest-known brown dwarfs and the closest system found since the measurement of the proper motion of Barnard's Star in 1916, and the third-closest-known system to the Sun (after the Alpha Centauri system and Barnard's Star). The primary is of spectral type L7.5 and the secondary of type T0.5 ± 1 (and is hence near the L–T transition). The masses of Luhman 16 A and B are 35.4 and 29.4 Jupiter masses, respectively, and their ages are estimated to be 400–800 million years. Luhman 16 A and B orbit each other at a distance of about 3.5 astronomical units with an orbital period of approximately 26.6 years.

Discovery
This system was discovered by Kevin Luhman, astronomer from Pennsylvania State University and a researcher at Penn State's Center for Exoplanets and Habitable Worlds, from images made by the Wide-field Infrared Survey Explorer (WISE) Earth-orbiting satellite—NASA infrared-wavelength 40 cm space telescope, a mission that lasted from December 2009 to February 2011; the discovery images were taken from January 2010 to January 2011, and the discovery was announced in 2013 (the pair are the only two objects announced in the discovery paper). The system was found by comparing WISE images at different epochs to reveal objects that have high proper motions.

Luhman 16 appears in the sky close to the galactic plane, which is densely populated by stars; the abundance of light sources makes it difficult to spot faint objects. This explains why an object so near to the Sun was not discovered in earlier searches.

Discovery of companion
The second component of the system was also discovered by Luhman in 2013, and was announced in the same article as the primary. Its discovery image in the i-band was taken on the night of 23 February 2013 with the Gemini Multi-Object Spectrograph (GMOS) at the Gemini South telescope, Chile. The components of the system were resolved with an angular distance of 1.5 arcseconds, corresponding to a projected separation of 3 AU, and a magnitude difference of 0.45 mag.

Precovery
Although the system was first found on images taken by WISE in 2010–2011, afterwards it was precovered from the Digitized Sky Survey (DSS, 1978 (IR) & 1992 (red)), Infrared Astronomical Satellite (IRAS, 1983), ESO Schmidt telescope (1984 (red)), Guide Star Catalog (GSC, 1995), Deep Near Infrared Survey of the Southern Sky (DENIS, 1999), Two Micron All-Sky Survey (2MASS, 1999), and the AKARI satellite (2007).

On the ESO Schmidt telescope image, taken in 1984, the source looks elongated with a position angle of 138°. The similarity of this position angle with that of the resolved pair in the GMOS image (epoch 2013) in Fig. 1 of Luhman (2013) suggests that the time period between 1984 and 2013 may be close to the orbital period of the system (not far from original orbital period estimate by Luhman (2013)).

Name
Eric E. Mamajek proposed the name Luhman 16 for the system, with the components called Luhman 16A and Luhman 16B. The name originates from the frequently updated Washington Double Star Catalog (WDS). Kevin Luhman had already published several new discoveries of binary stars that have been compiled in the WDS with discovery identifier "LUH". The WDS catalog now lists this system with the identifier 10493−5319 and discoverer designation LUH 16.

The rationale is that Luhman 16 is easier to remember than WISE J104915.57−531906.1 and that "it seems silly to call this object by a 24-character name (space included)". The "phone number names" also include WISE J1049−5319 and WISE 1049−5319. Luhman–WISE 1 was proposed as another alternative.

As a binary object it is also called Luhman 16AB.

Position in the sky
Luhman 16 is located in the southern celestial hemisphere in the constellation Vela. As of July 2015, its components are the nearest-known celestial objects in this constellation outside the Solar System. Its celestial coordinates: RA = 10:49:18.723, Dec = −53°19′09.86″.

Distance


The trigonometric parallax of Luhman 16 as published by Sahlmann & Lazorenko (2015) is $14.94$ arcsec, corresponding to a distance of 1.998 ±. Subsequent observations with Hubble and Gaia improved the parallax to $10.73$ ±0.050 mas, corresponding to a distance of $10.68$ ±0.0002 parsec, which is accurate to about 50 astronomical units.

Proximity to the Solar System
Currently Luhman 16 is the third-closest-known star/brown-dwarf system to the Sun after the triple Alpha Centauri system (4.37 ly) and Barnard's Star (5.98 ly), pushing Wolf 359 (7.78 ly) to the fifth place, along with the discovery of WISE 0855−0714. It also holds several records: the nearest brown dwarf, the nearest L-type dwarf, and possibly the nearest T-type dwarf (if component B is of T-type).

Proximity to Alpha Centauri
Luhman 16 is the nearest-known star/brown-dwarf system to Alpha Centauri, located 3.577 ly from Alpha Centauri AB, and 3.520 ly from Proxima Centauri. Both systems are located in neighboring constellations, in the same part of the sky as seen from Earth, but Luhman 16 is a bit farther away. Before the discovery of Luhman 16, the Solar System was the nearest-known system to Alpha Centauri.

Luhman 16 is closer to Proxima Centauri than to Alpha Centauri AB, just like Earth, even though Luhman 16 is farther from Earth than is the Alpha Centauri system. Therefore Luhman 16 has smaller angular distance to Proxima Centauri than to Alpha Centauri AB in Earth's sky, and this makes more contribution to the distance difference from Luhman 16 to Alpha Centauri than to the distance difference between them and Earth.

Proper motion


The proper motion of Luhman 16 as published by Garcia et al. (2017), is about 2.79″/year, which is relatively large due to the proximity of Luhman 16.

Radial velocity
The radial velocity of component A is 23.1 ±, and the radial velocity of component B is 19.5 ±. Since values of the radial velocity are positive, the system currently is moving away from the Solar System.

Assuming these values for the components, and a mass ratio of Luhman 16 from Sahlmann & Lazorenko (2015) of 0.78, the system's barycentre radial velocity is about 21.5 km/s. This implies that Luhman 16 passed by the Solar System around 36,000 years ago at a minimal distance of about 5.05 ly.

Orbit and masses
In Luhman 16's original discovery paper, Luhman et al. (2013) estimated the orbital period of its components to be about 25 years.

Garcia et al. (2017), using archival observations extending over 31 years, found an orbital period of 27.4 years with a semi-major axis of 3.54 AU. This orbit has an eccentricity of 0.35 and an inclination of 79.5°. The masses of the components were found to be and, respectively, with their mass ratio being about 0.82.

With the data from Gaia DR2 in 2018, their orbit was refined to a period of $9.56$ years, with a semi-major axis of $8.84$, an eccentricity of $8.87$, and an inclination of $-2,768.511$ (facing the opposite direction as the 2017 study found). Their masses were additionally refined to and. In 2024 the distance and orbit was further refined, resulting in a semi-major axis of 3.52 AU (assuming a parallax of 500.993 mas), an eccentricity of $358.472$ and an inclination of $26.55$, bringing the inclination in line with previous measurements. The secondary has a mass which is $0.344$ that of the primary. The individual masses were measured to be 35.2±0.2 and 29.4±0.2 Jupiter masses.

These results are consistent with all previous estimates of the orbit and component masses.

By comparing the rotation periods of the brown dwarfs with the projected rotational velocities, it appears that both brown dwarfs are viewed roughly equator-on, and they are aligned well to their orbits.

Age
A 2013 paper, published shortly after Luhman 16 was discovered, concluded that the brown dwarf belongs to the thin disk of the Milky Way with 96% probability, and therefore does not belong to a young moving group. Based on lithium absorption lines the system has a maximum age of about 3–4.5 Gyr. Observations with the VLT showed that the system is older than 120 Myr.

However, in 2022, Luhman 16 was found to be a member of the newly discovered Oceanus moving group, which has an age of $79.92$ Myr. Age estimates of 400-800 Myr in 2024 is in line with the membership with this group. The age estimates are mismatched for both components, which could be due to different cloud coverage resulting in different cooling efficiency. Alternatively this could be due to inaccurate luminosities or errors in the evolutionary models.

Search for planets
In December 2013, perturbations of the orbital motions in the system were reported, suggesting a third body in the system. The period of this possible companion was a few months, suggesting an orbit around one of the brown dwarfs. Any companion would necessarily be below the brown-dwarf mass limit, as otherwise it would have been detected through direct imaging. Researchers estimated the odds of a false positive as 0.002%, assuming the measurements had not been made in error. If confirmed, this would have been the first exoplanet discovered astrometrically. They estimate the planet to likely have a mass between "a few" and, although they mention that a more massive planet would be brighter and therefore would affect the "photocenter" or measured position of the star. This would make it difficult to measure the astrometric movement of an exoplanet around it.

Subsequent astrometric monitoring of Luhman 16 with the Very Large Telescope has excluded the presence of any third object with a mass greater than orbiting around either brown dwarf with a period between 20 and 300 days. Luhman 16 does not contain any close-in giant planets.

Observations with the Hubble Space Telescope in 2014–2016 confirmed the nonexistence of any additional brown dwarfs in the system. It additionally ruled out any Neptune mass objects with an orbital period of one to two years. This makes the existence of the previously found exoplanet candidate highly unlikely.

Additional observations with Hubble rules out the existence of a planet with >1.5 Neptune masses at an orbit of 400 to 5000 days. This study did however not rule out planets with a mass of less than 3 Neptune masses and a shorter period of 2 to 400 days.

Atmosphere
A study by Gillon et al. (2013) found that Luhman 16B exhibited uneven surface illumination during its rotation. On 5 May 2013, Crossfield et al. (2014) used the European Southern Observatory Very Large Telescope (VLT) to directly observe the Luhman 16 system for five hours, the equivalent of a full rotation of Luhman 16B. Their research confirmed the observation of Gillon et al., finding a large, dark region at the middle latitudes, a bright area near its upper pole, and mottled illumination elsewhere. They suggest this variant illumination indicates "patchy global clouds", where darker areas represent thick clouds and brighter areas are holes in the cloud layer permitting light from the interior. Luhman 16B's illumination patterns change rapidly, on a day-to-day basis. Luhman 16B is one of the most photometrically variable brown dwarfs known, sometimes varying with an amplitude of over 20%. Only 2MASS J21392676+0220226 is known to be more variable.

Heinze et al. (2021) observed variability in spectral lines of alkali metals such as potassium and sodium; they suggested that the variations were caused by changes in cloud cover, which changed the local chemical equilibrium with chlorides. Lightning or aurorae were deemed possible, but less likely.

Luhman 16B's lightcurve shows evidence of differential rotation. There is evidence of equatorial regions and mid-latitude regions with different rotation periods. The main period is 5.28 hours, corresponding to the rotation period of the equatorial region. Meanwhile, the rotation period of Luhman 16A is likely 6.94 hours.

Biller et al. 2024 observed both components with JWST for 8 hours with MIRI LRS and directly followed by an 7 hour observation with NIRSpec. The observations found water vapor, carbon monoxide and methane absorption in both brown dwarfs, which is typical for L/T dwarfs. Luhman 16A shows a flat plateau beyond 8.5 μm, which is indicative of small grain silicates. The lightcurves produced from the observations show that both components are variable, with Luhman 16B being considerable more variable than Luhman 16A. The variability has a complex wavelength dependent trend. The researchers identified changes in behaviour at 2.3 μm and 4.2 μm coincident with the CO band and changes in behaviour at 8.3-8.5 μm coincident with silicate absorption. These changes in behaviour were interpreted as changes of average pressure at three different depths of the atmosphere. The observations also tested if patchy clouds could produce the variability. And while small silicate grains corresponding to high-altitude silicate clouds were found in Luhman 16A, it is unlikely to be a patchy cloud layer. Luhman 16B does not have this small grained silicate feature, but larger grained silicate clouds deeper in the atmosphere are possible. The researchers also tested general circulation models (GCM) and hotspots, but the lightcurves are more complex than these models predict.

Radio and X-ray activity
In a study by Osten et al. (2015), Luhman 16 was observed with the Australia Telescope Compact Array in radio waves and with the Chandra X-ray Observatory in X-rays. No radio or X-ray activity was found at Luhman 16 AB, and constraints on radio and X-ray activity were presented, which are "the strongest constraints obtained so far for the radio and X-ray luminosity of any ultracool dwarf".