Astronomers believe they have found a planet that differs from all previous ones: a “second-generation” planet formed from the remains of the very dead star it now orbits. If confirmed, this would be the first example of its kind around a white dwarf.
Jamie Williams, a PhD student at the University of Warwick and lead of the study, explains that second-generation planets form from material a star ejects when it dies. According to him, these planets are extremely rare, and finding one around a white dwarf was completely unexpected. This discovery is somewhat like finding a planet that rose from the ashes of the same star that once orbited it.
Previously, candidate “reborn” planets of this type had been seen around pulsars, but this is the first time a specimen has been reported around a white dwarf. Because white dwarfs are far more common in the cosmos, the number of such planets may be larger than previously thought.
White dwarfs are the collapsed cores that remain after a star exhausts its fuel. They accrete material from nearby planets and leave a chemical trace that astronomers can use to identify orbiting planets.
In this study, scientists examined a white dwarf named HS 0209+0832 and found an unusual chemical signature showing a very large amount of niobium. This element is produced when a star dies, but it had not previously been reported in any white dwarf. The researchers believe this signal comes from the white dwarf accreting material from a freshly formed giant planet nearby - a planet that itself was built from the star’s death remnants.
Data from NASA’s TESS telescope lent further credence to the finding. Those data showed a weak signal repeating every 4.4 days that can be explained by a Jupiter-sized planet in a very close orbit around the white dwarf. The researchers say that because of this proximity, the planet’s outer atmospheric material boils and is lost under the radiation of the dead star. When this material strikes the white dwarf’s surface, it produces the same unusual chemical signature.
If confirmed, this planet would not only be the first of its kind but, by searching for the same chemical signature in the light of other dead stars, would provide a new way to find more examples. This could also be an opportunity to search for habitable planets and extraterrestrial life. Understanding how such planets form will help us determine whether planets around white dwarfs are habitable, and these planets are relatively easy to identify.
The study, titled “Discovery of a second-generation planetary candidate currently accreting onto a white dwarf,” was published in the journal Nature Astronomy.
In the same issue, a separate study examined another type of dwarf star. The researchers found a binary system in which a brown dwarf and a red dwarf orbit each other, and observations and modeling indicate material is flowing from the brown dwarf to the red dwarf - a process never before observed. That study, titled “Stable mass transfer from a substellar object to an M dwarf,” was published in the same journal.