A brown dwarf star serves as an ultrahot-Jupiter analogue

A planet that closely orbits a star often experiences tidal forces that are strong enough to manipulate its rotational period until one side gets locked to the star. The planet’s dayside is then continuously exposed to direct radiation. The intense UV exposure can break the molecules in the planet’s atmosphere and vaporize material from its surface.

So far, astronomers have detected just one planet—a so-called hot Jupiter known as KELT-9b—that receives enough UV radiation to dissociate molecules. Its dayside temperature is a scorching 4600 K, so hot that the planet sheds material behind it, much like a comet. The glare from a nearby star, however, makes it difficult to detect and study planets orbiting hot, massive stars. For one thing, the star generally has few spectral lines, which are so broadened by its rapid rotation and the movement of stellar plasma that radial-velocity measurements of the binary system become extremely challenging.

An alternative approach targets irradiated brown dwarfs as analogues of hot Jupiters. Brown dwarf stars have radii similar to those of giant planets, and because they cannot fuse hydrogen to helium, they more closely resemble giant planets than they do stars. Indeed, as suggested here in an artist’s illustration, brown dwarfs may host turbulent atmospheres. And by examining those atmospheres, astronomers may be able to test atmospheric models. Brown dwarfs that orbit close to Earth-sized white dwarfs are especially useful as hot-Jupiter analogues because they can be detected more easily above the glare of the white dwarf stars.

A team led by Na’ama Hallakoun, a postdoc in astrophysics at the Weizmann Institute of Science in Israel, has recently discovered an extreme version of such a hot-Jupiter analogue. The binary system that Hallakoun and her team analyze in their new study—a white dwarf named WD 0032-317, which is orbited by a brown dwarf companion—was observed with the European Southern Observatory’s Very Large Telescope in Chile in the early 2000s during a survey of hundreds of white dwarfs. When Hallakoun analyzed those data in 2017 with her PhD adviser Dan Maoz at Tel Aviv University, they flagged the system, located 1400 light-years from Earth, as a potential binary system with two white dwarfs. But when they looked at other archival data, they noticed an excess in the system’s IR signal, which made them wonder whether the companion was a brown dwarf rather than another white dwarf. Hallakoun and company’s analysis of additional recent data of the system bore out that suspicion.

The researchers calculate that the brown dwarf’s dayside temperature reaches about 8000 K. That’s 2000 K more than the surface temperature of our sun. The nightside temperature, in contrast with the dayside’s, is cooler by about 6000 K. They hope that future IR observations of the system—for example, by NASA’s James Webb Space Telescope—will provide a better understanding of planetary atmospheres under those extreme conditions. (N. Hallakoun et al., Nat. Astron., 2023, doi:10.1038/s41550-023-02048-z .)