‘Phoenix planet’ may have formed from a dead star’s ashes - CNN

In a cosmic first, scientists say they have detected a planet that may have formed from the burned remains of its dead host star.

The planet is likely a gas giant orbiting a white dwarf called HS 0209+0832 located about 270 light-years from Earth, according to the authors of a study published Monday in the journal Nature Astronomy. After analyzing observations from NASA’s Hubble Space Telescope and other instruments, the team found chemical evidence that suggests the planet could have condensed from material expelled by the star as it fizzled out of energy and collapsed into a white dwarf.

When a sunlike star dies, it first rapidly expands into a red giant and then sheds its outer layers, leaving behind a much smaller but dense, hot core — known as a white dwarf. Some of the first-generation planets, those that originally formed at the same time as the star, can survive this cataclysmic event if their orbit is wide enough. Astronomers have observed a handful of such first-generation survivors revolving around white dwarfs, but never before have they recorded a so-called second-generation planet — an entirely new world made from a dead star’s debris.

Planet found orbiting a dead star could preview what will happen to our solar system

The telltale sign that HS 0209+0832 might have birthed a new planet is unusual traces of heavy elements on the white dwarf’s surface that the study authors believe is planetary material raining down onto the leftover core. “This planetary material is very rich in an element called niobium,” said lead study author Jamie Williams, a doctoral student in the department of physics of the University of Warwick in England. “It’s the first time that this element is found in a white dwarf, and this implies that the planetary material is made from the ashes of the star as it died.”

Further analysis of observations from NASA’s Transiting Exoplanet Survey Satellite or TESS revealed a faint brightness signal repeating every 4.4 days — evidence that the researchers say is consistent with a giant planet orbiting the white dwarf.

“What’s interesting about planets orbiting close to white dwarfs is that because white dwarfs cool over time, their habitable zone is very stable. A second-generation planet could form and then be in the habitable zone for tens of billions of years,” Williams said.

Such a large period of stability could provide more ideal conditions for life, but more work is required to validate the second-generation planet discovery, he added. “It’s not a confirmed planet,” Williams said. “It’s only a candidate for now.”

After the red giant phase when a dying star quickly puffs up, it becomes a nuclear furnace that can produce various elements. However, once that fuel runs out and the star transforms into a white dwarf, the heavy elements — including nobium — sink downward rapidly, leaving only lighter elements such as hydrogen and helium on the surface. The researchers determined that the niobium detected on HS 0209+0832 must therefore come from the star’s surroundings. “We think these elements fell onto the white dwarf’s surface because the white dwarf is very hot and emitting loads of extreme ultraviolet radiation, which is stripping the atmosphere of a nearby planet,” Williams said.

The idea of second-generation planets has existed for at least as long as astronomers have known about exoplanets, said study coauthor David J. Wilson, a research associate at the Laboratory for Atmospheric and Space Physics at the University of Colorado Boulder.

Scientists detected the very first exoplanets around a pulsar — a type of rapidly rotating dead star — and because pulsars form as a result of massive explosions called supernovas, these planets are likely second-generation, as any original planets would have been destroyed by the immense blast, Wilson explained.

“Although white dwarfs don’t form via supernova, they are surrounded by debris,” he said via email. “You have the planets that were lucky enough to survive the star’s giant phases, the shattered remains of those that weren’t, and leftover gas and dust ejected by the star as it turned from a giant to a white dwarf. So it’s a compelling idea that all that stuff might coalesce into new planets.”