Collisions in a gas-rich white dwarf planetary debris disc.

WD 0145+234 is a white dwarf that is accreting metals from a circumstellar disc of planetary material. It has exhibited a substantial and sustained increase in 3-5 [Formula: see text]m flux since 2018. Follow-up Spitzer photometry reveals that emission from the disc had begun to decrease by late 2019. Stochastic brightening events superimposed on the decline in brightness suggest the liberation of dust during collisional evolution of the circumstellar solids. A simple model is used to show that the observations are indeed consistent with ongoing collisions. Rare emission lines from circumstellar gas have been detected at this system, supporting the emerging picture of white dwarf debris discs as sites of collisional gas and dust production.

A giant planet candidate transiting a white dwarf.

Astronomers have discovered thousands of planets outside the Solar System1, most of which orbit stars that will eventually evolve into red giants and then into white dwarfs. During the red giant phase, any close-orbiting planets will be engulfed by the star2, but more distant planets can survive this phase and remain in orbit around the white dwarf3,4. Some white dwarfs show evidence for rocky material floating in their atmospheres5, in warm debris disks6-9 or orbiting very closely10-12, which has been interpreted as the debris of rocky planets that were scattered inwards and tidally disrupted13. Recently, the discovery of a gaseous debris disk with a composition similar to that of ice giant planets14 demonstrated that massive planets might also find their way into tight orbits around white dwarfs, but it is unclear whether these planets can survive the journey. So far, no intact planets have been detected in close orbits around white dwarfs. Here we report the observation of a giant planet candidate transiting the white dwarf WD 1856+534 (TIC 267574918) every 1.4 days. We observed and modelled the periodic dimming of the white dwarf caused by the planet candidate passing in front of the star in its orbit. The planet candidate is roughly the same size as Jupiter and is no more than 14 times as massive (with 95 per cent confidence). Other cases of white dwarfs with close brown dwarf or stellar companions are explained as the consequence of common-envelope evolution, wherein the original orbit is enveloped during the red giant phase and shrinks owing to friction. In this case, however, the long orbital period (compared with other white dwarfs with close brown dwarf or stellar companions) and low mass of the planet candidate make common-envelope evolution less likely. Instead, our findings for the WD 1856+534 system indicate that giant planets can be scattered into tight orbits without being tidally disrupted, motivating the search for smaller transiting planets around white dwarfs.

A triple protostar system formed via fragmentation of a gravitationally unstable disk.

Binary and multiple star systems are a frequent outcome of the star formation process and as a result almost half of all stars with masses similar to that of the Sun have at least one companion star. Theoretical studies indicate that there are two main pathways that can operate concurrently to form binary/multiple star systems: large-scale fragmentation of turbulent gas cores and filaments or smaller-scale fragmentation of a massive protostellar disk due to gravitational instability. Observational evidence for turbulent fragmentation on scales of more than 1,000 astronomical units has recently emerged. Previous evidence for disk fragmentation was limited to inferences based on the separations of more-evolved pre-main sequence and protostellar multiple systems. The triple protostar system L1448 IRS3B is an ideal system with which to search for evidence of disk fragmentation as it is in an early phase of the star formation process, it is likely to be less than 150,000 years old and all of the protostars in the system are separated by less than 200 astronomical units. Here we report observations of dust and molecular gas emission that reveal a disk with a spiral structure surrounding the three protostars. Two protostars near the centre of the disk are separated by 61 astronomical units and a tertiary protostar is coincident with a spiral arm in the outer disk at a separation of 183 astronomical units. The inferred mass of the central pair of protostellar objects is approximately one solar mass, while the disk surrounding the three protostars has a total mass of around 0.30 solar masses. The tertiary protostar itself has a minimum mass of about 0.085 solar masses. We demonstrate that the disk around L1448 IRS3B appears susceptible to disk fragmentation at radii between 150 and 320 astronomical units, overlapping with the location of the tertiary protostar. This is consistent with models for a protostellar disk that has recently undergone gravitational instability, spawning one or two companion stars.

A VLBI resolution of the Pleiades distance controversy.

Because of its proximity and its youth, the Pleiades open cluster of stars has been extensively studied and serves as a cornerstone for our understanding of the physical properties of young stars. This role is called into question by the "Pleiades distance controversy," wherein the cluster distance of 120.2 ± 1.5 parsecs (pc) as measured by the optical space astrometry mission Hipparcos is significantly different from the distance of 133.5 ± 1.2 pc derived with other techniques. We present an absolute trigonometric parallax distance measurement to the Pleiades cluster that uses very long baseline radio interferometry (VLBI). This distance of 136.2 ± 1.2 pc is the most accurate and precise yet presented for the cluster and is incompatible with the Hipparcos distance determination. Our results cement existing astrophysical models for Pleiades-age stars.

Rapid disappearance of a warm, dusty circumstellar disk.

Stars form with gaseous and dusty circumstellar envelopes, which rapidly settle into disks that eventually give rise to planetary systems. Understanding the process by which these disks evolve is paramount in developing an accurate theory of planet formation that can account for the variety of planetary systems discovered so far. The formation of Earth-like planets through collisional accumulation of rocky objects within a disk has mainly been explored in theoretical and computational work in which post-collision ejecta evolution typically is ignored, although recent work has considered the fate of such material. Here we report observations of a young, Sun-like star (TYC 8241 2652 1) where infrared flux from post-collisional ejecta has decreased drastically, by a factor of about 30, over a period of less than two years. The star seems to have gone from hosting substantial quantities of dusty ejecta, in a region analogous to where the rocky planets orbit in the Solar System, to retaining at most a meagre amount of cooler dust. Such a phase of rapid ejecta evolution has not been previously predicted or observed, and no currently available physical model satisfactorily explains the observations.