Astronomers have discovered a rare second-generation planet orbiting the ashes of the dead star from which it was born, a smoldering white dwarf.
Though scientists have long suspected that reborn planets could orbit more massive stellar remnants called neutron stars, this is the first time astronomers have discovered a second-generation planet orbiting a white dwarf. The discovery matters for our understanding of the future of the solar system, since a the sun will leave behind a white dwarf stellar remnant when it dies in about 6 billion years.
The astronomers behind this discovery say it was a total surprise. "Second-generation planets are worlds that form out of the material a star casts off as it dies. They're incredibly rare, and finding one around a white dwarf was completely unexpected," team member Jamie Williams of the University of Warwick said in a statement sent to Space.com. "It's a bit like finding a planet that has risen from the ashes of the very star it once orbited."
Cosmic death and rebirth
White dwarfs are created when stars around the size of the sun exhaust the hydrogen in their cores needed to fuel nuclear fusion and create helium. The end of this nuclear process also ends the outward pressure that stops stars from collapsing under the inward push of their own gravity.
As the cores of these dead stars collapse, the outer layers where nuclear fusion is still occurring swell out, beginning a short-lived red giant phase. When our sun undergoes this process, it will puff out to around the orbit of Mars, swallowing the inner planets, including Earth.
These puffed-out outer layers continue to expand until they form a disk of material far out from the star called a protoplanetary disk, leaving behind a cosmic ember in the form of a smoldering white dwarf. Some white dwarfs can strip material from the planets that remain orbiting around them, and this process can be investigated by the detection of rock-forming elements like silicon and iron.
The white dwarf at the heart of this research is HS 0209+0832, located around 790 light-years from Earth. The atmosphere of this stellar remnant contains unusually high levels zinc, copper, and, most notably, niobium, with concentrations around 1,000 times greater than the sun. That told this team something remarkable about the planet that this white dwarf has been snacking on.
"This pattern of elements is a telltale sign of the 's-process,' a nuclear reaction that builds heavy elements inside dying stars during their bloated red giant phase," said Nicholas Stone, of the Department of Astronomy at the University of Wisconsin-Madison. "It's a chemical signature no ordinary, 'first-generation' planet should carry, which told us that this new planet was something different."
The team thinks that this unique chemical signature indicates this white dwarf is stealing material from a planet that formed in the protoplanetary disk of material shed by the dying star during its red giant phase.
"Forming the protoplanetary disc in this situation is not easy and helps explain why these planets are so rare. A single, isolated star dies and sheds mass in a roughly symmetrical way," Williams added. "To form a disc of material necessary to birth a planet, HS 0209+0832 likely required a companion star that pulled the ejected material back into orbit, rather than letting it escape."
What else do we know about this Phoenix planet?
The team further investigated this second-generation planet using NASA's exoplanet hunting spacecraft TESS (Transiting Exoplanet Survey Satellite), which searches for planets beyond the solar system by observing the dips in light they cause as they cross the face of their parent stars.
The dip in light from HS 0209+0832 indicated this Phoenix world has an orbit of around 4.4 Earth days and is a Jupiter-sized gas giant. The stellar radiation the planet experiences due to its proximity to its white dwarf will eventually boil its atmosphere away. And as that radiation strikes the white dwarf it gives rise to the unique chemical signature seen by the team.
That chemical signature might be a way forward that could help astronomers hunt for more of these 'rebirth' planets.
"What's remarkable about the planet around HS 0209+0832 is that this isn't a planet from somewhere else, or a survivor from the system's birth; it looks like it was built from the very material its own star cast off as it died. In a sense, this system has given birth to a new world using the foundations of the old one," team member Boris Gänsicke of the University of Warwick said.
"Finding this one example raises the question of how many more might be out there and might our own solar system host a second-generation planet formed from the ashes of our sun."
The team's research was published on Monday (Oct. 5) in Nature Astronomy,
