X-Ray Clues Unveil Cosmic Catastrophe
A white dwarf at the center of the Helix Nebula may have destroyed a nearby planet, an event astronomers have never observed before. This discovery could explain a mysterious X-ray signal detected from the nebula for over 40 years. The Helix Nebula is the remnant of a dying star, similar to our Sun, that has shed its outer layers, leaving behind a small but dense white dwarf at its core. A paper describing these results appears in The Monthly Notices of the Royal Astronomical Society.
A composite image of the Helix Nebula combines X-ray data from Chandra (magenta), optical light from Hubble (orange, light blue), infrared data from ESO (gold, dark blue), and ultraviolet data from GALEX (purple). Observations from Chandra suggest that the white dwarf has torn apart a planet that was orbiting extremely close.
An artist’s illustration below depicts this scenario: a planet (left), drawn too close to the white dwarf, is being ripped apart by the star’s intense gravitational forces. The white dwarf, surrounded by the glowing remnants of the planetary nebula, is at the heart of a complex planetary system. The doomed planet may have initially been much farther away but drifted inward over time due to gravitational interactions with other planets.
As the shattered remains of the planet spiral inward, they will eventually form a disk around the white dwarf. When this debris falls onto the star’s surface, it produces the persistent X-ray signal astronomers have been detecting for decades.
Dating back to 1980, X-ray missions, such as the Einstein Observatory and ROSAT telescope, have picked up an unusual reading from the center of the Helix Nebula. They detected highly energetic X-rays coming from the white dwarf at the center of the Helix Nebula named WD 2226-210, located only 650 light-years from Earth. White dwarfs like WD 2226-210 do not typically give off strong X-rays.
A new study featuring the data from Chandra and XMM-Newton may finally have settled the question of what is causing these X-rays from WD 2226-210: this X-ray signal could be the debris from a destroyed planet being pulled onto the white dwarf. If confirmed, this would be the first case of a planet seen to be destroyed by the central star in a planetary nebula.
Observations by ROSAT, Chandra, and XMM-Newton between 1992 and 2002 show that the X-ray signal from the white dwarf has remained approximately constant in brightness during that time. The data, however, suggest there may be a subtle, regular change in the X-ray signal every 2.9 hours, providing evidence for the remains of a planet exceptionally close to the white dwarf.
Previously scientists determined that a Neptune-sized planet is in a very close orbit around the white dwarf — completing one revolution in less than three days. The researchers in this latest study conclude that there could have been a planet like Jupiter even closer to the star. The besieged planet could have initially been a considerable distance from the white dwarf but then migrated inwards by interacting with the gravity of other planets in the system. Once it approached close enough to the white dwarf the gravity of the star would have partially or completely torn the planet apart.
WD 2226-210 has some similarities in X-ray behavior to two other white dwarfs that are not inside planetary nebulas. One is possibly pulling material away from a planet companion, but in a more sedate fashion without the planet being quickly destroyed. The other white dwarf is likely dragging material from the vestiges of a planet onto its surface. These three white dwarfs may constitute a new class of variable, or changing, object.
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