A planet may have formed around its star after the star had already gone through the final stages of its life.
The system is located around HS 0209+0832, a white dwarf. This type of object is the very dense, inactive core left behind by a star after it expels its outer layers. It is still extremely hot, with a surface temperature of about 35,500 °C, and is thought to have been exposed only 5 million years ago.
Artist’s impression of the HS 0209+0832 system. A second-generation planet candidate orbits a white dwarf and is losing part of its atmosphere to the star.
Credit: NASA, ESA, Leah Hustak (STScI).
The story begins with observations made by the Hubble Space Telescope in 1999. Its spectrum, which breaks light down according to its wavelengths, revealed many elements in the white dwarf’s atmosphere. About a hundred spectral lines remained without a satisfactory identification.
More than twenty-five years later, researchers reanalyzed these data using improved information about the signatures of chemical elements. They identified copper and niobium in particular. The star also contains a great deal of zinc, but little of certain elements commonly found in rocks, especially silicon and iron.
This composition does not resemble the dust usually detected around white dwarfs. It more closely matches what the star produces during the final stages of its life. The researchers therefore propose a surprising explanation: a new planet may have formed from the dust and gas expelled by the dying star.
Artist’s impression tracing the evolution of a Sun-like star into a white dwarf surrounded by a disk, followed by the formation of a second-generation planet.
Credit: NASA, ESA, Leah Hustak (STScI).
Such a planet is known as a second-generation planet. Unlike Earth, which formed at the same time as the Sun, it would have appeared after its star transformed into a giant and then a white dwarf. The researchers nevertheless describe it as a planet candidate: the object has not been directly photographed.
The observations provide another clue. Data from NASA’s TESS space telescope reveal a regular variation in brightness every 4.399 days. This could correspond to the day-night cycle of a planet very close to the star, or to the passage of a trail of material from an object in the process of evaporating.
The white dwarf is indeed hot enough to gradually strip gas from this planet candidate. Some of this atmosphere would escape, form material around the star, and then fall back onto its surface.
This discovery provides a method for searching for other comparable systems. The researchers propose looking in the ultraviolet spectra of hot white dwarfs for excess carbon and elements produced at the end of a star’s life. New examples will make it possible to determine how often second-generation planets can form in this way.