# WD 1856+534

WD 1856+534 is a white dwarf located in the constellation of Draco. At a distance of about 25 parsecs (80 ly) from Earth, it is the outer component of a visual triple star system consisting of an inner pair of red dwarf stars, named G 229-20. The white dwarf displays a featureless absorption spectrum, lacking strong optical absorption or emission features in its atmosphere. It has an effective temperature of 4,700 K (4,430 °C; 8,000 °F), corresponding to an age of approximately 5.8 billion years. WD 1856+534 is approximately half as massive as the Sun, while its radius is much smaller, being 40% larger than Earth.

## Planetary system

The white dwarf is known to host one exoplanet, WD 1856+534 b, in orbit around it. The exoplanet was detected through the transit method by the Transiting Exoplanet Survey Satellite (TESS) between July and August 2019. An analysis of the transit data in 2020 revealed that it is a Jupiter-like giant planet with a radius over ten times that of Earth's, and orbits its host star closely at a distance of 0.02 astronomical units (AU), with an orbital period 60 times shorter than that of Mercury around the Sun.
The unexpectedly close distance of the exoplanet to the white dwarf implies that it must have migrated inward after its host star evolved from a red giant to a white dwarf, otherwise it would have been engulfed by its star. This migration may be related to the fact that WD 1856+534 belongs to a hierarchical triple-star system: the white dwarf and its planet are gravitationally bound to a distant companion, G 229–20, which itself is a binary system of two red dwarf stars. Gravitational interactions with the companion stars may have triggered the planet's migration through the Lidov–Kozai mechanism in a manner similar to some hot Jupiters. An alternative hypothesis is that the planet instead has survived a common envelope phase. In the latter scenario, other planets engulfed before may have contributed to the expulsion of the stellar envelope. JWST observations seem to disfavour the formation via common envelope and instead favour high eccentricity migration.
The planetary transmission spectrum obtained with GTC OSIRIS is gray and featureless, likely because of the high level of hazes. The transmission spectrum was also obtained with Gemini GMOS. It does not show any features beside a possible dip at 0.55 μm. This feature could be caused be auroral emission at the nightside of the planet. The research find a minimum mass of 0.84 M$_J$ by accounting for the transit geometry of a grazing transit. The researchers also revised the white dwarf parameters and found a total age of 8-10 billion years, in agreement with the system belonging to the thin disk.
A search with transit timing variations found no additional planets. The search exclude planets with a mass more than 2 M$_J$ with orbital periods as long as 500 days and planets with >10 M$_J$ with orbital periods as long as 1000 days.
The WD 1856+534 planetary system :
| Companion (in order from star) | Mass | Semimajor axis (AU) | Orbital period (days) | Eccentricity | Inclination | Radius |
| b | 2.4-13.8 or >0.84 M$_J$ | 0.0204±0.0012 | 1.40793925±0.00000004 | ~0 | 88.778±0.059° | 0.966+0.040 −0.039 R$_J$ |