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Hints of a planet in the habitable zone of a dead star

When you think of searching for habitable planets, you probably think about looking for Earth-like planets orbiting sun-like stars. But the range of possibly habitable planets is larger than that, and astronomers recently discovered indications of a planet in the habitable zone of a dead star.

When stars eventually run out of fuel, they first grow and cool to become red dwarfs, before losing the last of their hydrogen and shrinking and cooling further to become a white dwarf. This is what will eventually happen to our sun, as well as 95% of other stars. It’s rare to find planets orbiting these white dwarfs, but recent research found indications of one such planet in the white dwarf’s narrow habitable zone for the first time.

An artist’s impression of the white dwarf star WD1054–226 orbited by clouds of planetary debris and a major planet in the habitable zone.
An artist’s impression of the white dwarf star WD1054–226 orbited by clouds of planetary debris and a major planet in the habitable zone. Mark A. Garlick / markgarlick.com

The researchers observed the white dwarf WD1054–226 using the European Southern Observatory’s New Technology Telescope in Chile, as well as gathering extra data using NASA’s Transition Exoplanet Survey Satellite (TESS). They found a ring of planetary debris around the white dwarf, including moon-sized structures whose movements suggest the presence of a nearby planet. They estimate the planet would be a comparable size to Earth and orbit very close to the white dwarf, at just 1.7% of the distance between Earth and the sun.

“The moon-sized structures we have observed are irregular and dusty (e.g. comet-like) rather than solid, spherical bodies,” lead author Jay Farihi explained in a statement. “Their absolute regularity is a mystery we cannot currently explain.

“An exciting possibility is that these bodies are kept in such an evenly-spaced orbital pattern because of the gravitational influence of a nearby major planet. Without this influence, friction and collisions would cause the structures to disperse, losing the precise regularity that is observed.  A precedent for this ‘shepherding’ is the way the gravitational pull of moons around Neptune and Saturn help to create stable ring structures orbiting these planets.”

The discovery of indications of a potential planet was a surprise to the researchers, and it would have had to have formed after the planet’s red dwarf phase, meaning it would be a relatively recent development.

“The possibility of a major planet in the habitable zone is exciting and also unexpected; we were not looking for this,” Farihi said. “However, it is important to keep in mind that more evidence is necessary to confirm the presence of a planet. We cannot observe the planet directly so confirmation may come by comparing computer models with further observations of the star and orbiting debris.”

The research is published in Monthly Notices of the Royal Astronomical Society.

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