Astronomers may finally have settled the question of what is causing highly energetic X-rays from WD 2226-210, a white dwarf star located at the center of the Helix Nebula.
This artist’s impression shows an exoplanet (left) that has approached too close to a white dwarf (right) and been torn apart by tidal forces from the star. Image credit: NASA / CXC / SAO / M. Weiss.
The Helix Nebula is a so-called planetary nebula, which is the late stage of a star that has ejected its outer layers of gas and left behind what is known as a white dwarf.
In previous decades, the Einstein X-ray Observatory and ROSAT telescopes detected highly energetic X-rays coming from Helix Nebula’s white dwarf, WD 2226-210.
White dwarfs like WD 2226-210, which is located only 650 light-years away, do not typically give off strong X-rays.
“We think this X-ray signal could be from planetary debris pulled onto the white dwarf,” said Dr. Sandino Estrada-Dorado, an astronomer at the National Autonomous University of Mexico.
“We might have finally found the cause of a mystery that’s lasted over 40 years.”
Previously astronomers determined that a Neptune-sized planet is in a very close orbit around WD 2226-210 — completing one revolution in less than 3 days.
Dr. Estrada-Dorado and colleagues 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.
“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 Dr. Martin Guerrero, an astronomer at the Institute of Astrophysics of Andalusia.
“If confirmed, this would be the first case of a planet seen to be destroyed by the central star in a planetary nebula.”
WD 2226-210 resides in the center of the Helix Nebula. Image credit: NASA / CXC / SAO / Univ Mexico / Estrada-Dorado et al. / JPL / ESA / STScI / M. Meixner / NRAO / T.A. Rector / ESO / VISTA / J. Emerson / K. Arcand.
The study shows that the X-ray signal from the white dwarf has remained approximately constant in brightness between 1992, 1999, and 2002 — with observations by ROSAT, NASA’s Chandra X-ray Observatory, and ESA’s XMM-Newton X-ray observatory, respectively.
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.
The authors also considered whether a star with a low mass could have been destroyed, rather than a planet.
Such stars are about the same size as a Jupiter-like planet but are more massive, making them much less likely to have been torn apart by the white dwarf.
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.
“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,” said Dr. Jesús Toala, an astronomer at the National Autonomous University of Mexico.
The team’s paper will be published in the Monthly Notices of the Royal Astronomical Society.
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S. Estrada-Dorado et al. 2025. Accretion onto WD 2226-210, the central star of the Helix Nebula. MNRAS, in press; arXiv: 2412.07863
This article is a version of a press-release provided by NASA.
