Since the 1980s, a strange X-ray emission has puzzled astronomers. At the heart of the Helix Nebula, a dying star may have destroyed a neighboring planet, revealing a rare and spectacular phenomenon.
This artistic illustration depicts a planet (on the left) that got too close to a white dwarf (on the right) and was torn apart by tidal forces. The white dwarf is at the center of a planetary nebula illustrated by the blue gas in the background. The planetary system also includes a planet at the top left and another at the bottom right. The shredded planet may have initially been distant from the white dwarf before migrating towards it due to gravitational interactions with other planets in the system.
Credit: NASA/CXC/SAO/M. Weiss
This discovery is based on observations made by the Chandra and XMM-Newton space telescopes. The Helix Nebula, located 650 light-years from Earth, is home to a white dwarf, the remnant of a star similar to the Sun. Unlike most white dwarfs, this one emits energetic X-rays, a mystery that may finally be solved.
A planet devoured by its star
Researchers believe that the X-rays come from the debris of a planet destroyed by the white dwarf. This hypothesis is supported by data collected between 1992 and 2002, showing a constant emission with a slight variation every 2.9 hours, suggesting the presence of an object in a very close orbit.
Initially, the planet would have been located farther away, but gravitational interactions with other celestial bodies would have brought it closer to the white dwarf. Once too close, the star's tidal forces would have torn it apart, creating a debris disk that, as it falls onto the white dwarf, would emit X-rays.
If this theory is confirmed, it would be the first direct observation of a planet destroyed by a white dwarf in a planetary nebula. This discovery opens new perspectives for understanding the fate of planetary systems around dying stars.
A new class of celestial objects?
WD 2226-210, the white dwarf at the center of the Helix Nebula, shows similarities with two other white dwarfs. These also exhibit X-ray emissions likely related to the accretion of planetary material, but without rapid destruction.
These three objects could form a new category of variable white dwarfs, characterized by their interaction with planets or their remnants. The study of these systems would help better understand how planets survive or are destroyed as their host star ages.
Researchers hope to identify other similar systems to deepen these observations. These discoveries could reveal previously unknown mechanisms in the evolution of stellar systems.
To go further: What is a white dwarf?
A white dwarf is the final stage in the evolution of low to medium mass stars, like our Sun. When such a star exhausts its nuclear fuel, it expels its outer layers into space, forming a planetary nebula. The remaining core, extremely dense, becomes a white dwarf.
This dead star is mainly composed of carbon and oxygen, compressed to an incredible density: a teaspoon of white dwarf material would weigh several tons on Earth. Despite its small size, comparable to that of Earth, a white dwarf retains a mass close to that of the Sun.
White dwarfs no longer produce energy through nuclear fusion. They shine only due to the residual heat accumulated during their active life. Over billions of years, they slowly cool, transitioning from white to red, then to black, becoming "black dwarfs."
These stellar objects play a key role in the evolution of planetary systems. Their study helps astronomers understand the fate of stars and planets in the Universe.
Article author: Cédric DEPOND