Astronomers have found a planet that returned from the dead, orbiting a white dwarf star that was once a red giant. The finding, described as “completely unexpected,” suggests that the universe may be full of worlds built from the ashes of dead suns — including, perhaps, a version of our own solar system.
A planet without a name orbits the white dwarf HS 0209+0832, found roughly 270 light-years from Earth. After exhausting their fuel, massive stars collapse into shriveled husks known as white dwarfs, remnants that are sometimes referred to as “zombie stars.”.
The Dead Star’s Strange Spectrum
HS 0209+0832 is young for a white dwarf, still burning at around 63,000 degrees Fahrenheit, or 35,000 degrees Celsius. That suggests it made the transition from red giant to white dwarf within the past few million years.
The Hubble Space Telescope, which first began making observations of the star in the late 1990s, has revealed that its visible spectrum contains unusual heavy elements. These elements, which do not normally appear inside white dwarfs, include:
- Aluminium
- Titanium
- Nickel
New Instruments, New Details
The research, released Oct. 5 in Nature Astronomy, drew on upgrades to Hubble’s instruments along with data from other observatories to produce a more detailed spectrum. That spectrum showed a high concentration of heavy elements — zinc, copper, and niobium — with niobium appearing for the first time in a white dwarf.
The lead writer of the research, Jamie Williams, a doctoral candidate at the University of Warwick studying astronomy and astrophysics, said the finding came as a surprise.
“Second-generation planets are worlds that form out of the material a star casts off as it dies,” Williams said in a statement. “They’re incredibly rare, and finding one around a white dwarf was completely unexpected. It’s a bit like finding a planet that has risen from the ashes of the very star it once orbited.”
How the Elements Point to a Second Generation
The scientists believe that these misplaced fragments came to be in the last moments of the once swollen star’s life, and that they then gathered together into a “second-generation” world circling the freshly formed white dwarf.
The planet’s atmosphere is likely being stripped by the white dwarf’s stellar winds, causing the rare elements to fall back onto the zombie star. That is why they are visible in the star’s spectrum.
Later observations showed that the brightness of HS 0209+0832 dipped at regular intervals, suggesting an exoplanet was repeatedly passing in front of the undead star. From the alien world’s orbit, which lasts about 4.4 Earth days, and the length of its transits across the star, the team estimated the planet to be roughly the size of Jupiter.
What Sets This Planet Apart
Nicholas Stone, an astronomer at the University of Wisconsin-Madison, identifies the “s-process,” as evidence of the nuclear reaction that builds heavy elements inside dying stars during their bloated red giant phase. The telltale sign is the pattern of elements in the star’s spectrum.
“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,” Stone said in the statement. “It’s a chemical signature no ordinary, ‘first-generation’ planet should carry, which told us that this new planet was something different.”
The shortage of common first-generation planetary materials, including silicon and iron, backs up this conclusion. Planets from the first generation form out of the material present when a system was born, not from the material a star sheds as it ages.
A Second Generation of Worlds
This is not the first time astronomers have spotted an exoplanet around a white dwarf. In 2019, some of the authors of the new study previously spotted an alien world surrounding the white dwarf WDJ0914+1914. In that case, they were confident that it was a first-generation world that somehow survived its star’s fiery end.
What sets this fresh find apart is the planet orbiting HS 0209+0832, a world that appears to have formed from the very debris its own star shed when it expired.
The Star’s Past and Future
When HS 0209+0832 was a red giant, it may have been orbited by other exoplanets. If such planets existed, however, they probably were destroyed as the star expanded. This fate mirrors what experts anticipate will befall Earth when the sun finishes its fuel and shifts into a red giant within roughly 5 billion years.
The sun’s period of swelling will eventually give way to its transformation into a white dwarf. A recent observation suggests that our star might spawn entirely new worlds after that change, building a new kind of solar system 2.0.
The Search Continues
More of these reborn alien worlds may be waiting to be found throughout the cosmos, and this finding raises hopes that they will turn up. They may also give us a look at what our solar system’s own fate has in store.
What stands out about the world orbiting HS 0209+0832 is that it did not originate with the system, according to Boris Gänsicke, the study co-author and astrophysicist at the University of Warwick who also helped to discover the exoplanet around WDJ0914+1914 in 2019.
“It looks like it was built from the very material its own star cast off as it died,” is a reference to something Gänsicke said in the statement, “In a sense, this system has given birth to a new world using the foundations of the old one.”, but the exact wording of that statement is not provided.
Other stellar remnants, including pulsars, which form when stars explode as supernovas, have been spotted alongside planets like those in Phoenix. Yet a white dwarf had never before been found to hold such a world.
This discovery provides a view of what our own solar system may one day resemble.
Source material: “'Completely unexpected': Astronomers spy never-before-seen 'phoenix planet' rising from the ashes of a dead star,” Live Science.
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