The Helix is a planetary nebula, the remnants of a gas envelope dropped by a red giant at the end of its life. The star itself has become a white dwarf. Research shows that this dead star recently destroyed a planet.
Mysterious X-ray signal
After tracking a mysterious X-ray signal from a dying star for decades, astronomers may have finally explained its nature: it may have destroyed a neighboring planet.
Since 1980, X-ray missions have detected an unusual signal from the center of the Helix Nebula. Using the most powerful X-ray missions to date, NASA’s Chandra X-ray Observatory and the European Space Agency’s XMM-Newton, they now have a much clearer picture of this decades-old mystery.
The Helix Nebula is a so-called planetary nebula, which is a late stage star that has ejected its outer layers of gas and left behind the dimmer and smaller ashes of a star known as a white dwarf.
In previous decades, the Einstein X-ray Observatory and ROSAT telescopes have detected high-energy X-ray emission coming from a white dwarf at the center of the Helix Nebula WD 2226-210, located just 650 light-years from Earth. White dwarfs like WD 2226-210 do not normally emit strong X-ray radiation.
Destroyed planet
A new study using data from Chandra and XMM-Newton may have finally resolved the question of what causes such X-ray emission from WD 2226-210. The study is published in the journal Monthly Notices of the Royal Astronomical Society.
“We think this X-ray signal could be from planetary debris pulled onto the white dwarf, as the death knell from a planet that was destroyed by the white dwarf in the Helix Nebula,” said lead study author Sandino Estrada-Dorado of the National Autonomous University of Mexico. “We might have finally found the cause of a mystery that’s lasted over 40 years.”
Earlier, scientists determined that a planet the size of Neptune is in a very close orbit around the white dwarf, making one revolution in less than 3 days. In their latest study, scientists have concluded that a Jupiter-like planet could exist even closer to the star.
The besieged planet could initially be at a considerable distance from the white dwarf, but then migrate inward, interacting with the gravity of other planets in the system. As soon as it got close enough to the white dwarf, the star’s gravity would partially or completely tear the planet apart.
Interpretations of the received signal and similar cases
The study shows that the X-ray signal from the white dwarf remained roughly constant in brightness between 1992, 1999, and 2002 (from ROSAT, Chandra, and XMM observations, respectively). The data suggest, however, that there may be a subtle, regular change in the X-ray signal every 2.9 hours, indicating the remnants of a planet that is extremely close to the white dwarf.
The authors also considered whether a low-mass star, rather than a planet, could have been destroyed. Such stars are about the same size as a planet like Jupiter, but are more massive, making them much less likely to be torn apart by a white dwarf.
WD 2226-210 shares some similarities in X-ray behavior with two other white dwarfs that are not inside planetary nebulae. One of them might be pulling material away from the companion planet, but more slowly, without rapidly destroying the planet. Another white dwarf is probably pulling material from the planet’s remnants onto its surface. These three white dwarfs may constitute a new class of variable objects.
“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 study co-author Jesus Toala of Mexico’s National Autonomous University.
According to phys.org
