About 650 light-years from here, in the constellation Aquarius, lies the planetary nebula Caldwell 63, commonly known as the Helix Nebula. It’s the slowly cooling corpse of a dead star, a vast cloud of gas and dust stretching more than 3 light-years across.
The center of the nebula is home to the white dwarf WD 2226-210, the dense core of a medium-mass star, which has been the source of an astronomical mystery for decades. Observations from the Einstein X-ray Observatory, the Chandra X-ray telescope, and ROSAT telescopes revealed the presence of mysterious high-energy X-rays coming from the white dwarf’s surface.
Now, astronomers believe the X-rays are created by fragments of a shattered planet falling onto the white dwarf’s surface. The paper describing the discovery is available in preprint and will be published in the Monthly Notices of the Royal Astronomical Society.
A quiet stellar death: How white dwarf stars are made
When a medium-mass star roughly the same size as the Sun reaches the end of its life, it transforms into a tiny, incredibly dense, cosmic monstrosity called a white dwarf. Stars are in a constant battle between the inward pressure of gravity and the outward pressure created by the fusion of light elements in the core. During most of the star’s life, those two pressures remain in balance, but toward the end of a star’s life, gravity wins out and a fatal collapse begins.
Decades of observations combined with new high-tech telescopes are revealing the sort of cosmic drama you can usually only see on SYFY's The Ark. Depending on the mass of the star, it ends its life as a black hole, violently explodes in a supernova, or gently sheds its outer layers and becomes a white dwarf. Any star about eight times the mass of the Sun or less will end its life as a white dwarf. As it nears the end, the outer layers of the star swell and there’s no longer enough gravitational influence to hold them. The outer layers get sloughed off and spread out into deep space, creating a planetary nebula, while the inner core condenses into a hot dense ball.
Your average white dwarf has about half the mass of the Sun packed into an area about the same size of the Earth. They’re some of the densest objects in the universe, roughly 200,000 times the density of the Earth. Only neutron stars and black holes are denser. Surface temperatures are about 100,000 Kelvin (approximately 180,000 Fahrenheit), cooling slowly over the course of about a billion years.
X-ray echoes show a white dwarf eating a planet in the Helix Nebula
White dwarfs don’t usually emit strong X-ray signals, causing astronomers to suspect something unusual was going on at WD 2226-210. Telescope observations confirmed that the X-ray signal from the white dwarf remained constant between 1992 and 2002, with a subtle regular variation every 2.9 hours. That variation suggests the remains of a Jupiter-like planet in an incredibly close orbit.
“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, via Sci News.
The suggested planet may have started its life much farther away from the white dwarf, but migrated inward through interactions with other planets. As the planet neared the intense gravitational influence of the white dwarf, parts of it may have been ripped away or the entire planet torn to shreds. The debris then might have spun up into a tight ring around the star, falling bit by bit onto the surface. If confirmed, this would be the first case of astronomers seeing a planet being consumed by the white dwarf at the center of a planetary nebula.
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