Perhaps the luckiest planet in the Milky Way lives about 6,500 light-years away, toward the center of our galaxy: a large, gassy world that narrowly escaped being obliterated by its dying home star.
Astronomers spotted the system, described today in the science journal Nature, when the planet and its star distorted background starlight. The faraway, fortunate Jupiter-size world is circling a tiny stellar corpse—a dim white dwarf star about the size of Earth that was once much like the sun. As that star aged, it expanded into a red giant before collapsing into the dense white dwarf—a process that can easily destroy orbiting planets.
“It would have been very easy to lose this planet,” says Caltech’s Juliette Becker, who was not involved in the detection. “Most likely it barely avoided destruction.”
Had the planet lived a bit closer to its star, it could have suffered a number of grisly fates, from incineration to shredding—one of which will eventually befall Earth when the dying sun swells into a red giant star. Scientists suspect this alien planetary system could be analogous to what our own solar neighborhood will look like after the sun has burned down to an ember.
“This system is very similar to what we expect for the end state of our solar system,” says study author David Bennett of NASA’s Goddard Space Flight Center.
The system also helps scientists understand how frequently planets might survive the violent deaths of their home stars. If it turns out that intact planets are common around white dwarfs, then there are “more planets out there than we thought,” says astronomer Scott Gaudi of the Ohio State University.
“That means that our counting of planets in the galaxy has likely been underestimated until now,” he says. “In the exoplanet field, the joke is, every rock you turn over, you find a new planet.”
Going out in a blaze of glory
As stars age, they run out of hydrogen to feed the nuclear furnaces in their bellies, triggering a sequence of events that can be very dangerous for any orbiting planets. In about five billion years, this will happen to the sun—and when it does, Earth is in trouble.
Slowly, the hydrogen-starved sun will balloon into a red giant star. As it inflates, it will engulf and incinerate Mercury and then Venus. Earth, if it escapes incineration, will almost certainly be ripped apart by the star’s gravity. Mars is probably far enough away to survive. In the outer solar system, the four giant planets will get pushed around, most likely into more distant orbits—although in some special cases, they could be tossed out of the solar system entirely or even hurled into the sun.
“There’s a lot of really weird things that can happen to the planets in a system as the star is evolving,” Becker says. “The process is fairly violent, especially for planets in the inner part of the system.”
About a billion years after expanding into a red giant, the sun will collapse into a dense, stellar corpse—a white dwarf star with roughly half its original mass, squeezed into a sphere about the size of Earth. That process can also wreak havoc on nearby planets. Any worlds that are left behind will find themselves in a neighborhood that looks very different than before. And even if the four giant planets make it through, there’s a good chance that all of them will be lost over billions of years through encounters with passing stars.
Surviving the inferno
Although astronomers suspect planets can survive the chaotic deaths of their host stars, they haven’t found many examples of planets that made it through the maelstrom. The newly described system was first spotted in 2010 by scientists with the Microlensing Observations in Astrophysics collaboration, when the surviving planet and its white dwarf star moved directly in front of a more distant background star.
That alignment allowed the pair’s gravity to magnify and distort the faraway starlight, producing what astronomers refer to as a microlensing event. So far, microlensing has revealed the presence of about 90 worlds, including some free-floating planets, or rogue worlds that have slipped away from their home stars and roam the galaxy alone.
The precise way in which the giant planet and white dwarf bent the background starlight revealed several crucial characteristics of the system, including its motion on the sky, the presence of both a star and a planet, and the planet’s large orbit. The observations also helped astronomers calculate the relative masses of the two objects. Called MOA-2010-BLG-477Lb, the system intrigued astronomers—but they’d have to wait several years to try to get a closer look.
“It’s hard to distinguish the foreground star from the background star when the [microlensing] event occurs because they have to be right on top of each other,” Bennett says. “So we wait for them to separate.”
In 2015, Bennett and his colleagues used the powerful Keck-II telescope atop Mauna Kea in Hawaii to go hunting for the star. They knew how far the system must have traveled over those five years, so they aimed Keck at their target, peered into the darkness, and found nothing resembling the star they were looking for—just a different star, moving in the wrong direction.
The team repeated the observations in 2016, again in 2018, and both times came up empty. But they knew the system had to be there, based on the distorted starlight. The inability to see it told Bennett and his colleagues that whatever they were seeking was so dim that not even Keck could spot it.
“We knew it had to be a dark star that was a bit less massive than the sun, and white dwarfs are the obvious choice,” Bennett says.
After running a few more calculations, the team concluded that the system involved a Jupiter-mass world and a white dwarf star with roughly half the mass of the sun. The planet’s orbit carries it at least 2.8 times farther from its star than Earth orbits the sun, putting it in roughly the same place as our solar system’s asteroid belt.
“This planet is out where we expect giant planets to form,” Gaudi says. “And it shows that these Jovian analogs can survive the evolution of a sunlike star.”
Somehow, that massive world grew up and lived in the exact right place to avoid the lethal consequences of its star’s transformation—a distance that depends not only on the dying star, but on the characteristics of the planet and the movements of any siblings it might have.
Searching for more planets circling white dwarfs
Astronomers have found evidence for planets orbiting white dwarfs before, but none of those detections are quite like this new one. In 2019, an international team of astronomers spotted a gassy debris ring around a white dwarf and surmised that a small, dense planetary remnant may be embedded in the debris—a destroyed world that may resemble Earth’s inevitable fate. Several other debris disks have also been identified, thought to be the shredded remains of unfortunate planets and asteroids.
Last year, another team using NASA’s exoplanet-hunting TESS instrument identified a candidate planet—a giant world—orbiting a white dwarf in a mere 34 hours. That planet is so close to its host star, it “would have definitely been engulfed during the red giant phase,” Becker says. “Which means it had to migrate into its location after the star became a white dwarf.”
And over the last two decades, scientists have seen smears of chemical elements on white dwarf stars’ faces—the crumbs of chewed up, rocky worlds.
All of these observations, in addition to the newly discovered system, suggest that some planets can survive their host stars’ evolutionary transformations—at least for a time. But the processes that determine whether a planet survives or meets its doom are still fuzzy.
NASA’s Nancy Grace Roman Space Telescope, slated to launch in the mid-2020s, should reveal plenty of planets orbiting white dwarfs. And as astronomers spot more planets circling these stellar corpses, they’ll learn more about how a star’s death throes alter the architecture of planetary systems, allowing us to gaze into our own solar system’s future.
The newly discovered system may even have additional planets circling the white dwarf, Bennett says. While our own planet will not survive the sun’s dramatic death—at least not in any recognizable state—perhaps other worlds will, and the solar system will live on transformed.