Astronomers may finally solve a decades-old mystery: A strange X-ray signal from a dying star at the end of its life may indicate that the white dwarf destroyed a nearby planet.
A decades-old cosmic mystery may finally be solved. Scientists now suspect that the unusual X-ray glow coming from a distant white dwarf is due to a planet being torn apart and swallowed by it.
Astronomers may finally have solved a decades-old mystery: A strange X-ray signal from a dying star at the end of its life may indicate that the white dwarf destroyed a nearby planet.
Since 1980, X-ray telescopes have detected unusual emission from the center of the Helix Nebula. Thanks to advanced telescopes – NASA's Chandra and the European Space Agency's XMM-Newton – scientists can now understand this mysterious phenomenon more clearly.
The Helix Nebula It is a stellar nebula, the final stage in the life of a dead star that has shed its outer layers, leaving behind a small, dense core called a white dwarf.
The mystery of the white dwarf in the Helix Nebula
Over the past decades, the Einstein Telescope and the ROSAT telescope have detected high-energy X-rays emitted by the white dwarf at the center of the Helix Nebula, WD 2226-210, which is only 650 light-years from Earth. White dwarfs like WD 2226-210 do not typically emit powerful X-rays.
A new study, using data from Chandra and XMM-Newton, seems to finally be able to pinpoint the source of the X-rays coming from WD 2226-210.
"We believe that the X-ray signal could be due to the remnants of a planet being pulled into the white dwarf," said lead author Sandino Estrada-Dorado of the Autonomous University of Mexico. "We may have finally found the cause of a mystery that has lasted for over 40 years."
An in-depth look at the engulfed star
Previously, scientists had determined that there was a Neptune-sized planet orbiting very close to the white dwarf, completing a full rotation in less than three days. The researchers in this study conclude that there may also have been a Jupiter-sized planet orbiting even closer to the star.
The engulfed planet may have started out at a considerable distance from the dwarf, but wandered inward due to gravitational interactions with other planets in the system. When it got close enough, the dwarf's gravity partially or completely tore the planet apart.
Luminous Residues: X-ray Effects
"It is possible that the mysterious signal we saw comes from the remnants of the planet, which fall on the dwarf planet and heat up to the point of emitting X-rays," explained Martin Guerrero, a co-author of the study from the Institute of Astrophysics of Andalusia, Spain. "If confirmed, this would be the first time that a planet has been observed being swallowed by a dead star at the center of a stellar nebula."
The study shows that the intensity of X-rays from the white dwarf remained approximately constant between 1992, 1999 and 2002 (based on observations by ROSAT, Chandra and XMM respectively). The data, however, suggest a subtle and constant change in the signal every 2.9 hours, indicating the presence of a remnant planet in a very close orbit to the dwarf.
Was it a star or a planet??
The researchers also considered the possibility that it was a low-mass star, rather than a planet. Such stars are similar in size to a Jupiter-type planet but have a higher mass, making their chances of being torn apart by a dwarf much lower.
WD 2226-210 has similar X-ray behavior to two other non-nebular white dwarfs. One of them may be sucking material from a satellite planet, but in a slower process without destroying the star. The other white dwarf is likely pulling material from the remnants of a planet onto its surface. This group of white dwarfs may constitute a new class of variable bodies.
"It is important to find more systems of this type, because they can teach us about the survival or destruction of planets around stars like the Sun, as they reach the end of their lives," said Jesús Tavela, a co-author of the study from the Autonomous University of Mexico.
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