Astronomers may have finally solved a decades-old mystery: Strange X-ray signals from a dying star could be evidence that it destroyed a nearby planet. Since 1980, X-ray telescopes have detected unusual radiation at the center of the Helix Nebula. Thanks to modern observatories, NASA's Chandra X-ray Observatory and the European Space Agency's XMM-Newton telescope, scientists now have a clearer understanding of this puzzling phenomenon.
The Helix Nebula is a planetary nebula, the final stage of a dying star that sheds its outer layers, leaving behind a small, dense core called a white dwarf.
Over the past few decades, the Einstein X-ray Observatory and the ROSAT telescope have detected high-energy X-rays from the white dwarf star at the center of the Helix Nebula, named WD2226-210, which is only 650 light-years away from Earth. White dwarfs like WD2226-210 typically do not emit strong X-rays.
A new study using Chandra and XMM-Newton data may finally answer the cause of the X-rays emitted by WD2226-210.
"We think this X-ray signal could be from planetary debris being pulled up to the white dwarf, which is the death knell for a planet in the Helix Nebula being destroyed by a white dwarf," said lead author Sandino Estrada-Dorado of the National Autonomous University of Mexico. "We may finally have the answer to a mystery that has persisted for more than 40 years."
Previously, scientists discovered that a Neptune-sized planet is orbiting a white dwarf in a very close orbit - completing an orbit in less than 3 days. In the latest study, researchers concluded that there may be a planet closer to the star, such as Jupiter.
A trapped planet may initially be quite far away from the white dwarf, but then migrate inward through gravitational interactions with other planets in the system. Once it gets close enough to the white dwarf, the star's gravity can tear the planet apart, partially or completely.
"The mysterious signal we see could be caused by fragments of a broken planet falling onto the surface of a white dwarf and being heated and emitting X-rays," said co-author Martin Guerrero of the Institute of Astrophysics of Andalusia in Spain. "If confirmed, this would be the first time a planet has been found destroyed by a star at the center of a planetary nebula."
Research shows that the X-ray signal emitted by this white dwarf star remained basically unchanged in brightness between 1992, 1999 and 2002 (observed by ROSAT, Chandra and XMM respectively). However, the data suggest that the X-ray signal may undergo subtle regular changes every 2.9 hours, providing evidence for the presence of a planetary remnant very close to the white dwarf.
The authors also considered whether less massive stars could be destroyed more easily than planets. Such stars are about the same size as the Jupiter-like planets but are more massive and therefore much less likely to be torn apart by a white dwarf.
The X-ray behavior of WD2226-210 has some similarities to two other white dwarfs that are not inside planetary nebulae. One of them may be sucking material away from the companion planet, but in a more gentle way so that the planet is not destroyed as quickly. Another white dwarf is likely dragging the planet's remnants of material to its surface. These three white dwarfs may constitute a new class of variable or changing objects.
"Discovering more systems like this is important because they can tell us whether planets orbiting stars like our sun will survive or perish as they enter old age," said co-author Jesús Toala of the National Autonomous University of Mexico.
A paper describing the results was published in Monthly Notices of the Royal Astronomical Society.
Compiled from /ScitechDaily