A planet may be found in the habitable zone around the dead star. It would be a first.

White dwarfs are the smoldering remnants of stars that have used up all the hydrogen—their fuel. Almost all stars, including our sun, will eventually turn into a white dwarf. However, it is still unknown what will happen to planets – such as our Earth – that orbit around these ‘dead stars’ in that case. But a new study offers a glimpse of our own future.

Ring of rubble

In the study, using the ULTRACAM mounted on ESO’s New Technology Telescope (NTT), the researchers examined the white dwarf star WD1054-226, located 117 light-years away from Earth. To their surprise, they found significant dips in brightness, suggesting a ring of planetary debris orbits the star, completing one orbit every 25 hours. “The structures are the size of a moon,” said study leader Jay Farihi. “In addition, they are quite dusty, possibly comet-like. Their pinpoint regularity, dimming the white dwarf’s light every 23 minutes, is a mystery we can’t explain at the moment.”

Planet?

However, the researchers speculate wildly. And in doing so, they come up with an intriguing possibility. “An exciting possibility is that these celestial bodies are held in such an even orbit by the gravity of a nearby planet,” suggests Farihi. “Without this influence, friction and collisions would cause the structures to disintegrate. And then we would not have seen the precise regularity observed.” The researchers compare this with the moons around Neptune and Saturn, which also contribute significantly to the stable rings we find around these planets.

livable zone

Also interesting is that, according to the research team, this suspected planet could be in the habitable zone, meaning it may be harboring liquid water (see box). “This is the first time astronomers may have detected a planetary body in the habitable zone of a white dwarf,” said Farihi.

More about the habitable zone
The habitable zone is an imaginary zone around a star. Planets located in this zone receive enough heat from the star to prevent any water from freezing on their surface. At the same time, they also do not receive so much heat that any water on their surface evaporates. In short, planets in this zone are expected to be able to harbor liquid water. Compared to a star like our sun, the habitable zone around a white dwarf will be smaller and closer to the star because white dwarfs give off less light and therefore less heat.

The discovery of a possible planet in the habitable zone of a dead star is quite surprising. When the dying process of a star has started, it will first turn into a ferocious red giant. This process greatly affects the planets orbiting such a star, as most of the planets will be drawn towards the star and swallowed. So it seems unlikely that planets will survive the dying process of their parent star. And that means, according to the researchers, that any planet around a white dwarf that may contain water and thus life formed and evolved quite recently.

Glimpse of the future

The study also provides more insight into our own fate. “As our sun will also turn into a white dwarf in a few billion years, the study offers a glimpse into the future of our own solar system,” says Farihi. At the moment, astronomers suspect that when our sun dies, it will gobble up Mercury, Venus and possibly Earth as well. However, not all chances for life may have been lost. Because it just so happens that planets orbiting our dead sun again become habitable.

While the discovery of a planet in the habitable zone of white dwarf WD1054-226 is an intriguing possibility, the researchers are skeptical. “It’s important to keep in mind that more evidence is needed to confirm the planet’s presence,” Farihi said. “We can’t directly observe the planet.” That’s because white dwarf stars give off much less light than, say, our sun, making it quite a challenge to detect planets around dead stars. “We will need to compare computer models with further observations of the star and the orbiting debris to confirm our hypotheses,” Farihi concludes.