Using NASA's Chandra observatory and Europe's XMM-Newton spacecraft, astronomers may have solved a 45-year-old cold case. The team of cosmic homicide detectives determined a strange X-ray signal is the dying scream of a planet destroyed by a superdense stellar corpse called a white dwarf.
The signal, which originates from the heart of the Helix Nebula, was first detected by X-ray telescopes in 1980. This planetary nebula is the remains of a star similar to the sun, which shed most of its mass at the end of its life, with its core becoming a smoldering white dwarf shrouded by this ex-stellar material.
The signal was a puzzle, because white dwarfs like the one at the heart of the Helix Nebula, designated WD 2226-210, don't normally give off strong X-ray signals.
"We think this X-ray signal could be from planetary debris pulled onto the white dwarf, as the death knell from a planet that was destroyed by the white dwarf in the Helix Nebula," team leader Sandino Estrada-Dorado of the National Autonomous University of Mexico said in a statement. "We might have finally found the cause of a mystery that’s lasted over 40 years."
Related: White dwarfs: Facts about the dense stellar remnants
A cosmic crime scene
Even before this research, there may have been a witness to the planetary homicide committed by the white dwarf WD 2226-210. Astronomers had already suspected that a Neptune-sized planet orbits the destructive dead star so closely that its year lasts just three Earth days.
This new research suggests that this planet once had a Jupiter-sized sibling that orbited even closer to the white dwarf. This doomed planet may have begun life much further away from WD 2226-210, gradually migrating inward toward the dead star as a result of gravitational interactions with other planets in this system.
Once this migrating world was close enough to the white dwarf, the immense gravity of the dead star would have ripped it apart.
"The mysterious signal we’ve been seeing could be caused by the debris from the shattered planet falling onto the white dwarf’s surface and being heated to glow in X-rays," said study team member Martin Guerrero, a researcher at the Institute of Astrophysics of Andalusia, Spain. "If confirmed, this would be the first case of a planet seen to be destroyed by the central star in a planetary nebula."
Using data collected by ROSAT (the ROentgen SATellite), Chandra and XMM-Newton, the team discovered that the X-say signal first spotted in 1980 from the Helix Nebula remained approximately constant in brightness between 1992, 1999 and 2002.
However, what the scientists also discovered was a subtle and regular change in the X-ray signal around every three hours. This served as the "smoking gun" evidence of the wreckage of a planet extremely close to WD 2226-210.
The team then eliminated the possibility that the X-ray signal may have originated from a small companion star destroyed by the white dwarf rather than a planet.
They reasoned that, while a small star could be around the size of Jupiter, it would have a great deal more mass, which would have enabled it to resist complete destruction by the white dwarf.
The X-ray signal from WD 2226-210 is similar to the X-ray emissions of two other white dwarfs that do not sit within cocoons of shed stellar material, known as planetary nebulae (a misleading name, as these nebulae don't usually involve planets).
One of these white dwarfs is thought to be stripping material from a planetary companion, destroying it albeit in a slower fashion than WD 2226-210 did its planetary meal. This means this planet hasn't yet been completely destroyed. The other white dwarf with a similar X-ray profile to WD 2226-210 appears to be feasting on the remains of an already obliterated planet.
Thus, these three white dwarfs feeding on planets could represent a new class of changing or "variable" stars.
"It’s important to find more of these systems, because they can teach us about the survival or destruction of planets around stars like the sun as they enter old age," team member and University of Mexico researcher Jesús Toala said.
The team's research is published as a pre-peer reviewed paper on the research repository site arXiv.
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