Astronomers discover possible planet born from a dead star’s ashes

By Jacopo Prisco, CNN

(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.

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.”

A new class of planet

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.”

Astronomers still don’t know all the details about how first-generation planets form, which means the potential mechanisms behind this newly reported second-generation planet are even more uncertain. The planet-forming process might have derived from a collision between the dying star and a second celestial body, according to Williams. “There could have been another object close to the core of the star, maybe a star around 20% the mass of the sun, or a brown dwarf,” said Williams, referring to a class of cool objects that are an intermediate between a star and a planet.

If a second object fell into the star during its giant phase, it could have prevented all the dust and gas from spreading out into a massive cloud, making some of it spin in a disk. This disk would then have continued to rotate around the white dwarf, birthing a planet. “In a regular white dwarf there’s no disc, because the star’s material will just be ejected outwards during the red giant phase,” Williams added.

The researchers plan to use Hubble and NASA’s Chandra X-ray Observatory to observe the system again over the next year, but they also requested time with the more powerful James Webb Space Telescope, which could provide more clues about the existence or features of the planet.

If confirmed, this distant world would belong to an entirely new planetary class and suggest that more second-generation planets could exist around white dwarfs, Williams said.

The discovery would also have implications for our own cosmic neighborhood, Wilson said. “The sun will eventually become a white dwarf, so we’re also looking at the future of the solar system here — maybe the Sun will get a new planet someday!”

The afterlives of exoplanets

The new discovery of a potential second-generation planet is incredibly exciting, said Sarah Casewell, a lecturer at the School of Physics and Astronomy of the University of Leicester, England, who was not involved with the study.

“We know of a large number of white dwarfs that are polluted by planetary material which is similar in composition to rocks within our solar system,” she wrote in an email. “However, in this case, the white dwarf is polluted by incredibly unusual material and the composition of this material is similar to atoms we see being created as stars end their lives.”

More than 95% of all stars in the universe will eventually become white dwarfs, and astronomers have already cataloged hundreds of thousands of them. Finding a planet that has formed after the white dwarf would mean that second-generation planets around other white dwarfs are possible — and that after our sun dies, new planets could emerge in our own solar system, Casewell said.

Previous observations of white dwarfs have shown that some planetary systems survive a star’s self-destruction, but the recent findings open the possibility of the genesis of new planets, said Amy Bonsor, an associate professor at the University of Cambridge in England, who also did not participate in the work. “This paradigm shift allows us to consider whether in the future we could characterise or investigate the potential habitability of these second-generation planetary systems,” she wrote in an email.

This discovery would also add more evidence to research that has shown planetary systems are varied and complex, even after the host star dies, according to Susan Mullally, a mission scientist at the Space Telescope Science Institute, the science operations center for Hubble.

“Some planets may survive the death of the star, others may be tossed into space or eaten by the star. If second-generation planets are possible, then many more white dwarf stars will have planets than we may otherwise expect,” Mullally, who was not part of the study, wrote in an email. “This evidence of a second-generation exoplanet indicates there may be more to the afterlives of exoplanets than previously expected.”

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