Planetary bodies observed in habitable zone of dead star

A ring of planetary debris studded with moon-sized structures has been observed orbiting a white dwarf star, hinting at a nearby planet in the “habitable zone” where water and thus life could exist, according to a new study involving astronomers at the University of Sheffield.

White dwarfs are glowing embers of stars that have burned through all of their hydrogen fuel. Nearly all stars, including the Sun, will eventually become white dwarfs, but very little is known about their planetary systems.

In the study, published in Monthly Notices of the Royal Astronomical Society, an international team of researchers led by UCL measured light from a white dwarf in the Milky Way known as WD1054-226, using data from ULTRACAM, an ultra-fast, three-colour camera for high-speed astrophysics, developed at the University of Sheffield.

To their surprise, they found pronounced dips in the brightness of the white dwarf corresponding to 65 evenly spaced clouds of planetary debris orbiting the star every 25 hours. The researchers concluded that the precise regularity of the transiting structures – dimming the star’s light every 23 minutes – suggests they are kept in such a precise arrangement by a nearby major planet.

Professor Vik Dhillon, from the University of Sheffield’s Department of Physics and Astronomy, said: “Our Sun will become a red giant and then a white dwarf in a few billion years’ time, and hence our observations provide us with an opportunity to study the possible fate of the planets in our solar system.”

Lead author Professor Jay Farihi, from UCL Physics & Astronomy, said: “This is the first time astronomers have detected any kind of planetary body in the habitable zone of a white dwarf.

“The moon-sized structures we have observed are irregular and dusty (e.g. comet-like) rather than solid, spherical bodies. Their absolute regularity, one passing in front of the star every 23 minutes, 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 the moons around Neptune and Saturn help to create stable ring structures orbiting these planets.

“The possibility of a major planet in the habitable zone is exciting and also unexpected; we were not looking for this. 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.”

It is expected that this orbit around the white dwarf was swept clear during the red giant phase of its life, and thus any planet that can potentially host water and thus life would be a recent development. The area would be habitable for at least two billion years, including at least one billion years into the future.

For the new study, researchers observed WD1054-226, a white dwarf 117 light years away, recording changes in its light over 18 nights using the ULTRACAM high-speed camera mounted on the ESO 3.5m New Technology Telescope (NTT) at the La Silla Observatory in Chile.

This result would not have been possible without the combination of the high-speed, multi-colour capabilities of our camera ULTRACAM in combination with the light-grasp of the NTT.

Professor Vik Dhillon

Professor of Astrophysics at the University of Sheffield

The habitable zone, sometimes called the Goldilocks zone, is the area where the temperature would theoretically allow liquid water to exist on the surface of a planet. Compared to a star like the Sun, the habitable zone of a white dwarf will be smaller and closer to the star as white dwarfs give off less light and thus heat.

The structures observed in the study orbit in an area that would have been enveloped by the star while it was a red giant, so are likely to have formed or arrived relatively recently, rather than survived from the birth of the star and its planetary system.

The study received funding from the UK’s Science and Technology Facilities Council (STFC) and involved a team of researchers from six countries, including Boston University, the University of Warwick, Lund University, the University of Cambridge, the University of St Andrews, Wesleyan University, the University of La Laguna, Naresuan University, the University of Sheffield, and the Instituto de Astrofísica de Canarias.

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