Scientists discover a small, dense planet orbiting a white dwarf

A new study reports discovery of one of the first small, intact planetary bodies orbiting a white dwarf star, a finding that sheds light into the twilight years of planetary systems. To date, scientists have identified about 3,000 stars that host one or more planets, most of which are main-sequence stars similar to the Sun. When such main sequence stars run out of fuel after billions of years, they transition into smaller white dwarfs, which researchers suspect could host surviving planets. Some white dwarfs show signatures of debris around them, probably from planets that were destroyed during the transition period, but evidence of intact planets around these stellar remnants has been lacking. In this study, Christopher Manser and colleagues used spectroscopy to study the gas in a debris disc surrounding the white dwarf SDSS J122859.93+104032.9. Their analysis revealed a periodic variation in gas emission lines that they concluded must be caused by a small planetary body (called a planetesimal) orbiting within the disc. The scientists calculated that the planetesimal, which orbits close to the white dwarf every two hours, must be unusually dense and no larger than 600 kilometers in diameter to avoid being ripped apart by the star’s gravitational forces. Based on the object’s high density, Manser et al. speculate that the planetesimal may be the remnant core of a planet whose outer layers were stripped away by the star’s tidal forces. It is thus among the first detections of an intact planetesimal orbiting a white dwarf. In a related Perspective, Luca Fossati notes that the study’s methods could help identify other planetesimals orbiting white dwarfs that cannot be detected with traditional planet-hunting techniques.

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http://dx.doi.org/10.1126/science.aat5330

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