For the first time, astronomers have observed a white dwarf that is snacking on asteroids and comets suggesting that this object, located 86 light-years from Earth, is messing up its inner and also its outer star system.
“We have never seen both of these kinds of objects accreting onto a white dwarf at the same time,” lead researcher Ted Johnson, a physics and astronomy major at UCLA who graduated last week, said in a statement. “By studying these white dwarfs, we hope to gain a better understanding of planetary systems that are still intact.”
To appreciate the impact of this discovery, presented at the 240th American Astronomical Society Meeting, we need to look at how a star ends up being a white dwarf.
The life cycle of stars like the Sun is quite straightforward. Eventually, they run out of hydrogen to burn, and they resort to fusing helium in their core. This process happens in the red giant phase, where stars swell up and can engulf their closest planet. As the star continues to age, it loses its outer layer, and eventually having run out of helium, it transforms into a white dwarf.
These objects are dense stellar core remnants. Their light is residual thermal energy and no fusion is happening inside them. The initial period of the white dwarf phase – roughly 100 million years – can be tumultuous for star systems, as this research confirms.
The team was able to observe that the atmosphere of the white dwarf is polluted by several elements such as nitrogen, oxygen, magnesium, silicon, and iron. Some of these elements are the most abundant ones in both the formation of terrestrial planets and asteroids while other are common on ice worlds and comets.
“The best fit for our data was a nearly two-to-one mix of Mercury-like material and comet-like material, which is made up of ice and dust,” Johnson said. “Iron metal and nitrogen ice each suggest wildly different conditions of planetary formation. There is no known Solar System object with so much of both.”
If this were our Solar System, the future Sun white dwarf would be feeding on the remains of the rocky planets and the asteroid belt, while also getting a deluge of material from the Kuiper Belt, the region full of icy objects such as Arrokoth, beyond the orbit of Neptune.
The discovery of white dwarf G238-44 suggests that icy reservoirs might be a common feature in star systems with intriguing implications for planets with the right condition for life, as water might have been brought to Earth by these distant bodies.
“Life as we know it requires a rocky planet covered with a variety of volatile elements like carbon, nitrogen and oxygen,” added co-author Benjamin Zuckerman, a UCLA professor of physics and astronomy. “The abundances of the elements we see on this white dwarf appear to have come from both a rocky parent body and a volatile-rich parent body — the first example we’ve found among studies of hundreds of white dwarfs.”