In 1995, a cosmic question was answered when a ground-based telescope picked up on a faint, wobbly signal coming from hundreds of light-years away. The telescope had detected the first exoplanet orbiting a sun-like star, a breakthrough discovery proving that, yes, there are planets beyond our solar system—and hinting at the potential for many more.
Astrophysicists have since confirmed nearly 4,000 exoplanets orbiting stars across the Milky Way galaxy. Like our sun, these stars are typically in the so-called main sequence phase of their lives, a period that lasts billions of years and during which the stars burn healthy, hot, and bright.
But now, a group of researchers has zeroed in on a planetary body closely orbiting a white dwarf—a burned-out star that is on death’s door. Described this week in the journal Science, the findings are among the first of their kind, and they offer a glimpse at what Earth’s fate could be when our sun begins to die.
Led by University of Warwick astrophysicist Christopher Manser, the team discovered the rocky object using a method called spectroscopy, which involves collecting and analyzing the different wavelengths of light coming from the gas disk surrounding the white dwarf. This is the first time this method has been used to identify a planetary body orbiting a white dwarf.
Using the Gran Telescopio Canarias in La Palma, Spain, the team observed the "color of light emitted by calcium in the disc, and we collected a spectrum every two to three minutes,” Manser says in an email. This technique allowed the team to detect subtle color changes in the disk as it moved closer and further away from Earth. This kind of color shift is called a Doppler wobble, similar to the audible Doppler effect that makes a police siren seem to change pitch as the car races by.
"For our detection, this change in color was used to identify the presence of a planetesimal orbiting in the disc on a period of two hours," Manser says. The team categorizes the object as a planetesimal, because of its relatively small size.
A big part of the reason scientists study exoplanets is to learn more about the evolution of our own solar system. If this planetesimal was once Earth-like, as Manser believes, the outcome is bleak. (Read about the search for exoplanets outside the Milky Way.)
As the planetesimal’s star began running out of fuel and expanding—as most sun-like stars do when they reach the ends of their lives—the intense gravity would have ripped apart any closely orbiting planets, reducing them to their rocky cores and generating disks of debris. Manser suspects Earth will face a similar fate.
“When the sun will eventually run out of fuel and expand in about five billion years, it’s going to engulf Mercury, Venus, and most likely Earth,” he says. “But Mars and the other bodies like Jupiter, Saturn, the asteroid belt, and so on, they should survive the entire process, although they’ll be on a slightly larger orbit, because some will lose mass and the sun will eventually be a white dwarf.”
However, there may be a bright side, says astrophysics professor Lisa Kaltnegger, who is also the director at Cornell University’s Carl Sagan Institute and was not involved with Manser's research. If planetesimals orbiting white dwarfs were to collide, she says, they could eventually coalesce to form new, stable planets. Her studies of this possibility suggest these reconstructed worlds could even be habitable.
“After the white dwarf cools down further, we have shown that such a planet could maintain balmy conditions for billions of years,” she writes in an email. For instance, while the dramatic conditions of this new planet's birth would likley deprive it of surface water at first, the life-giving liquid could be re-delivered by impacts with water-carrying comets, so that "instead of a hot dry zombie planet, you could get a planet where life could potentially start all over again," she says.
“This paper puts the first puzzle piece in place to determine how planets could form around young white dwarfs from planetesimals.”
For now, Manser hopes to apply the spectroscopy method to other star systems where gas disks are present. They may contain more planetesimals that will help fill in our understanding of planetary life cycles, he says, “and we want to hunt for those next.”