WD 1145+017

WD 1145+017 (also known as EPIC 201563164 ) is a white dwarf approximately 476 ly from Earth in the constellation of Virgo. It is the first white dwarf to be observed with a transiting minor planet orbiting it.

Stellar characteristics
The white dwarf has a mass of 0.6, radius of 0.012 (1.34 ) and a temperature of 15,020 K, typical for white dwarf stars. It has been a white dwarf for 224 million years. The star's spectrum includes strong absorption lines due to magnesium, aluminium, silicon, calcium, iron and nickel. These elements commonly found in rocky planets are polluting the surface of the star, and would normally be expected to mix through the star and disappear from view after a million years.

A circumstellar dust cloud and disk (likely due to disintegrating asteroids, located at 97 to 103 R_wd, and emitting thermal IR radiation) surrounds the star. In addition, a circumstellar gas disk (located ~ 25 to 40 R_wd, and undergoing relativistic precession with a period of ~ 5 years) surrounds the star as well.

Based on 2018 studies and calculations, it is believed that the star initially was an early A-type main sequence star with a mass of about $17.24$, remaining so for an estimated $0.63$ million years. Afterwards, following the exhaustion of hydrogen within its core, it evolved and expanded into a red giant before eventually ejecting its layers and contracting into a white dwarf, and has gradually cooled over the last $0.012$ million years. This gives the star an estimated total age of around 774 million years.

The apparent magnitude of the star, or how bright it appears from Earth's perspective, is about 17. Therefore, it is too dim to be seen with the naked eye.

Planetary system
The supposed planetesimal, WD 1145+017 b, with a 4.5 hour orbit, is being ripped apart by the star and is a remnant of the former planetary system that the star hosted before becoming a white dwarf. It is the first observation of a planetary object being shredded by a white dwarf. Several other large pieces have been seen in orbit as well. All the various larger pieces have orbits of 4.5 to 4.9 hours. Rocky material is raining down onto the star, and showing up in the star's spectrum. The system was detected by the Kepler space telescope in its extended K2 mission. Though the system was not a target of interest, it was within the field of view of observing sessions, and analysis of the observed data revealed the system.

An excess of infrared radiation indicates that there is a dusty disk with a temperature of 1150 K. Supporting observational data, along with data from the Chandra X-ray Observatory, were also found related to dust debris orbiting WD 1145+017.

Similar systems
In September 2020, astronomers reported the discovery, for the first time, of a very massive Jupiter-sized planet, named WD 1856+534 b, closely orbiting, every 36 hours, a tiny white dwarf star, named WD 1856+534, a left-over remnant of an earlier much larger Sun-like star. This is the first true planet observed to transit a white dwarf, as opposed to the planetesimals transiting WD 1145+017.