The decades-long mystery of a never-ending explosion of X-rays around the remains of a dead star may have finally been solved. The radiation probably originates from the scorching-hot wreckage left behind by a giant planet’s annihilation.
This discovery stems from four decades of X-ray observations of the Helix Nebula, located 650 light-years from Earth. The stream of X-ray radiation remained effectively constant over at least 20 years, researchers report in the January Monthly Notices of the Royal Astronomical Society. The best explanation, the scientists say, is that the ruins of a Jupiter-sized world continuously fall onto the nebula’s white dwarf star, getting frazzled and glowing in X-rays.
“We don’t know very much about how planetary systems behave after their star transitions from a red giant to a super long-lived white dwarf,” says Paul Byrne, a planetary scientist at Washington University in St. Louis who was not involved in the research. This study, he says, offers “a potential glimpse of the far, far-off future of the solar system.”
The Helix Nebula resembles a technicolor explosion frozen in time. It is a planetary nebula, a halo of gas jettisoned by a star that ran out of nuclear fuel. At its center lies a white dwarf — the leftover heart of that once-mighty star.
The white dwarf is not a quiet object. In fact, it appears to be screaming in X-rays. This emission was detected by two space-based observatories: NASA’s Einstein Observatory in the early 1980s and the internationally operated ROSAT in the 1990s.
“It is very unusual to find single white dwarfs with an X-ray emission,” says astrophysicist Sandino Estrada-Dorado of the National Autonomous University of Mexico in Mexico City. To try and crack the case, Estrada-Dorado and his colleagues examined more recent observations of the nebula taken by NASA’s Chandra X-Ray Observatory in 1999 and the European Space Agency’s XMM-Newton mission in 2002.
The team found that the X-ray emissions aren’t a fluke but rather a constant — from 1992 to 2002, but probably up to the present day — indicating a significant fuel source. One possibility is that matter from an obliterated companion has been raining down onto the white dwarf; this debris is heated so dramatically that it shines brightly in X-rays.
Calculations based on the intensity of the X-ray emissions suggest that a Jupiter-sized world is the most likely culprit. Long ago, such a planet might have drifted too close to the white dwarf, allowing the stellar remnant’s intense gravity to tear it to shreds, leaving behind a disk of debris that powers the X-ray conflagration.
Using X-rays to detect signs of a world’s destruction could offer researchers a new way to explore the apocalyptic final chapters of planets — and to get a look under the geologic hood.
“We have precious little data about the deep interiors of giant planets,” Byrne says. “If, through similar observations of this and other white dwarfs, we can better distinguish the signals of the star from the infalling planet, then we might be able to tease out information about the planet’s composition, too.”
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