Astronomers are looking for the bones of dead planets in bodies of dead stars – and they may have just found some.
In a paper published Feb. 11 in the journal Nature Astronomy, a team of researchers described how they used data from the Gaia space satellite to look at the atmospheres of four white dwarfs – the shriveled crystalline shells of once massive stars that burned through all their fuel. Swirling through the hot soup of hydrogen and helium surrounding those stars, the team discovered clear traces of lithium, sodium and potassium – metals abundant in planetary crusts – in the exact proportion they would expect to find in a rocky planet.
“By comparing all of these elements with different types of planetary material in the solar system, we found that the composition differed markedly from all types of material except one: continental crust,” said lead researcher Mark Hollands, an astrophysicist at the University of Warwick. in England, Live Science told Live Science in an email.
According to Hollands and his colleagues, the presence of these crunchy metals suggests that each of the old, faded stars they analyzed once resided in the center of a solar system not so different from ours; then, in their dying eons, those stars tore their solar systems to shreds and heaped up the remains.
Our solar system can also share this fate.
When stars die
For billions of years, stars with a mass between about one-tenth and eight times the mass of the sun have been burned by their nuclear fuel. When this happens, those ancient stars drop their fiery outer layers and shrivel into a hot, white, dense core that packs half a sun’s mass into a ball no wider than Earth – a white dwarf.
These smoldering energy balls have extremely strong gravity and are incredibly hot and bright at first. But the older a white dwarf gets, the cooler and duller it gets, and the more wavelengths of light become visible in its atmosphere. By studying those wavelengths, scientists can calculate the elemental composition of that star’s atmosphere.
Most of the white dwarf’s atmosphere is dominated by hydrogen or helium, the researchers said, but they can become “polluted” by other elements if the dead star’s intense gravity pulls material from the space around it. For example, if a white dwarf soaks up the chunks of a broken planet, “all the elements in the destroyed object can emit their own light, producing a spectral fingerprint that astronomers might be able to see,” Hollands said.
In their new paper, Hollands and his colleagues focused on four ancient white dwarfs within 130 light-years of Earth, to see if their atmospheres contain any evidence of planetary remains. Each dead star was between 5 billion and 10 billion years old, and cool enough for the astronomers to detect wavelengths of light emitted by metallic elements shining from their dark atmosphere.
In all four ancient stars, the researchers discovered a combination of lithium and other metals that closely matched the composition of planetary debris. One star, which the team got a particularly clear picture of, contained metals in the atmosphere that “formed a near-perfect match with Earth’s continental crust,” Hollands said.
For the researchers, there is only one logical explanation: the ancient white dwarfs still hold the smoldering remains of the planets on which they once shone their light. To end up in a white dwarf’s atmosphere, those planetary remnants must have been pulled in by the star’s intense gravity millions of years ago, after the star ended its period as a red giant and threw its outer gas layers into space, Hollands said.
All planets close to the star would have been obliterated during the red giant phase (just as Mercury, Venus and possibly Earth will be swallowed up by our sun on its day of death), but any planets that survived long enough to see their sun be a white dwarf would also see the gravity of their solar system confused.
“After the red giant phase ends and the sun becomes a white dwarf, the orbits of the planets can become more chaotic, as the white dwarf sun is only half its former mass and the planets are now farther away,” said Hollands.
This gravitational disruption increases the risk of planetary collisions, he added, which could fill the solar system with broken, rocky remains of dead worlds. Larger planets outside the solar system (like Jupiter, for example) could then exert their own powerful gravity to send those remnants flying out of orbit; some of them can end up close enough to the white dwarf sun to be sucked in and lumped together.
While something along these lines seems to have happened around the four white dwarfs Hollands and his colleagues studied, anyone can guess whether Earth will ever suffer a similar fate. According to co-author Boris Gaensicke, also a professor at the University of Warwick, it is likely that our planet will be swallowed up during the red giant phase of the sun, leaving no elements that alien astronomers can detect.
However, that doesn’t mean those alien telescopes will come up empty-handed.
“I wouldn’t bet those alien astronomers would detect the lithium from all the defunct Teslas in the solar-powered white dwarf,” Gaensicke told Live Science. “But chances are they could see asteroids, comets, moons or even Mars swallow.”
Originally published on Live Science.