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Headline: Scientists discover bizarre 'phoenix planet' born from dead star
Caption: BY MARK WORGAN Astronomers have discovered a bizarre ‘phoenix planet’ orbiting the remains of a dead star. The discovery could offer clues about the future of our own Solar System, as the Sun is expected to become a similar white dwarf star in about six billion years. Scientists have long suspected that second-generation planets could form around the remnants of stars, including the much denser objects known as neutron stars. But researchers say this is the first evidence of such a planet orbiting a white dwarf. The finding comes from a fresh analysis of archival observations made by the Hubble Space Telescope. In a study published in Nature Astronomy, researchers report finding an unusual chemical signature around the white dwarf HS 0209+0832 which they say points to the presence of a second-generation planet. A white dwarf is the dense remnant core of a relatively low-mass star that has exhausted its nuclear fuel and shed its outer layers into space. A second-generation or ‘phoenix’ planet, meanwhile, would form from material expelled by a star as it dies, rather than from the material left over from the star's original formation. “Rather than the white dwarf stage being a kind of epilogue to the story of a star and its planets, this research points to the systems we are familiar with only being the first chapter of a potentially much longer tale, with some new characters showing up. That’s a really exciting prospect to pursue,” said Jamie Williams, an astronomer and lead author of the study who is a doctoral candidate at the University of Warwick. Earth and the other planets in the Solar System are considered first-generation planets, having formed from material left over from the Sun's birth. The researchers instead found unusually high levels of the element niobium around HS 0209+0832. “What Hubble is showing us in this white dwarf system is something we haven’t seen before: a high abundance of the element niobium, the signature of which I was unfamiliar with when I first found it in the archival data,” Williams said. Hubble first observed the white dwarf in 1999. At the time, around 100 chemical features in the data could not be identified. Williams revisited the observations using an updated chemical database and found that niobium matched many of the previously unexplained features. The element is also found in the Solar System and has several uses on Earth, including in jewellery and medical imaging equipment. But the researchers say its abundance around HS 0209+0832 suggests the material did not originate during the star's birth. Instead, it may have come from material expelled as the star died. "Niobium and other elements heavier than iron are astronomically special because, unlike many common elements, they are not formed in the cores of stars by thermonuclear fusion,” said Nicholas Stone, a theoretical astrophysicist at the University of Wisconsin – Madison and member of the research team. “Instead, these heavy elements can only be synthesized in the exotic conditions that briefly emerge inside dying stars. The presence of niobium is a signpost of these ‘death’ throes, and the expulsion of the dying star's innards into space.” The researchers suggest that some of this chemically enriched material could have subsequently clumped together to form a gas giant planet. The remaining material would have dispersed into space, leaving the planet behind. “When Jamie asked me about niobium in relation to this study I was truly gobsmacked, as that element had not been reported in any other white dwarf analyzed to date. Once we realized it was there, everything fell into place,” said astronomer and study co-author Boris Gaensicke, also at the University of Warwick. The Hubble observations were supported by data from Nasa's now-retired Far Ultraviolet Spectroscopic Explorer (FUSE) mission, which also detected strong signatures of niobium in the system. Nasa's Transiting Exoplanet Survey Satellite (TESS) observed the white dwarf for four months. The observations detected regular changes in its brightness, which the researchers say are consistent with a planet orbiting the star at a distance of about 3.7 million miles (6 million km). That is considerably closer to its star than Mercury is to the Sun. The researchers estimate that the candidate is a gas giant roughly the size of Jupiter and that it is rapidly losing material from its atmosphere. Because the white dwarf is relatively young and remains extremely hot, it is likely exposing the planet to intense radiation, stripping away material from its outer layers. That material could form a comet-like tail and eventually create a disc around the white dwarf before falling back on to its surface. The researchers suggest this process could explain why Hubble detected niobium while observing the star. Despite the ongoing loss of material, Williams believes the planet could survive for a considerable period. “If the second-generation planet is there, I think it is likely to survive. Eventually the white dwarf will cool and then maintain a consistent temperature, with the planet in its stable habitable zone for millions of years,” Williams said. The researchers stress that more work is needed to establish how phoenix planets form and how common they are. Scientists also want to understand how such worlds evolve as they orbit the remnants of dead stars.
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