Phoenix planet found orbiting dead star's ashes

Astronomers have found a rare second-generation planet orbiting a white dwarf, the smoldering remnant of the star that gave it birth. The discovery, reported by Space.com, is the first time a planet of this kind has been confirmed around such a stellar corpse.

Second-generation planets form from the disk of material a star casts off as it swells into a red giant and then collapses. The star at the center of this system, cataloged as HS 0209+0832, lies about 790 light-years from Earth. When it ran out of hydrogen fuel, its outer layers puffed out and then drifted away, leaving behind a white dwarf roughly the size of Earth but incredibly dense.

That expanding shell of gas and dust created a protoplanetary disk, the same kind of nursery that gives birth to ordinary planets. From that disk, a new world assembled. The team behind the finding, led by researchers at the University of Warwick and the University of Wisconsin-Madison, published their work Monday (Oct. 5) in Nature Astronomy.

A chemical fingerprint of rebirth

The white dwarf's atmosphere contains unusually high levels of zinc, copper and niobium, with niobium concentrations around 1,000 times what the sun holds. That pattern, the astronomers say, points to a process called the s-process, a chain of nuclear reactions that builds heavy elements inside dying stars during their red giant phase.

"It's a chemical signature no ordinary, 'first-generation' planet should carry," team member Nicholas Stone of the University of Wisconsin-Madison said in a statement sent to Space.com. "Which told us that this new planet was something different."

The white dwarf appears to be stealing material from a planet that formed in exactly that disk of cast-off stellar material. The stolen material carries the s-process signature into the white dwarf's atmosphere, where telescopes can read it.

A Jupiter-sized world cooked by radiation

The team used NASA's TESS spacecraft to confirm the planet's existence. TESS looks for the tiny dip in light a planet causes as it crosses the face of its star. That dip told the researchers the planet orbits its white dwarf every 4.4 Earth days and is about the size of Jupiter.

Its tight orbit places it close enough to the white dwarf that stellar radiation will eventually boil its atmosphere away. That same radiation, striking the white dwarf, produces the unique chemical signature the team detected.

What it means for our solar system

The discovery matters for understanding the long-term fate of our own solar system. In about 6 billion years, the sun will exhaust its hydrogen, swell into a red giant that swallows the inner planets including Earth, and then collapse into a white dwarf. Whether a second-generation world could form from the sun's cast-off material is an open question.

"Finding this one example raises the question of how many more might be out there," team member Boris Gänsicke of the University of Warwick said. "And might our own solar system host a second-generation planet formed from the ashes of our sun?"

Why such planets are rare

Forming a protoplanetary disk from a dying star is not straightforward. A single, isolated star sheds mass roughly symmetrically, and the material drifts away. To get a disk, the star likely needed a companion to pull the ejected material back into orbit.

"To form a disc of material necessary to birth a planet, HS 0209+0832 likely required a companion star," team member Jamie Williams of the University of Warwick said. The companion would have tugged the cast-off gas and dust into a flat, rotating disk where planets could assemble.

Team members called the finding "completely unexpected." But the chemical signature that revealed this planet, they say, could now help astronomers find others like it.

• HS 0209+0832
• second-generation planet
• TESS
• University of Warwick
• University of Wisconsin-Madison
• white dwarf