Most stars will end their lives as white dwarfs. White dwarfs are the remnant cores of once-luminous stars like our Sun, but they’ve left their lives of fusion behind and no longer generate heat. They’re destined to glow with only their residual energy for billions of years before they eventually fade to black.

Could life eke out an existence on a planet huddled up to one of these fading spectres?

For life to exist around a white dwarf, the white dwarf would have to have planets in its slowly shrinking habitable zone. Astronomers have found what appear to be planets in the habitable zone of a white dwarf about 117 light-years away. The planet might be nestled in a protoplanetary disk of debris.

A new study led by researchers from the University College of London announces the findings. The title is “Relentless and complex transits from a planetesimal debris disc.” It’s published in the Monthly Notices of the Royal Astronomical Society, and the lead author is Professor Jay Farihi from the UCL Department of Physics & Astronomy.

“This is the first time astronomers have detected any kind of planetary body in the habitable zone of a white dwarf.”

Professor Jay Farihi, UCL Dep’t. of Physics and Astronomy.

The star’s name is WD1054–226. The researchers behind this work observed WD1054-226 for 18 nights with the ESO’s New Technology Telescope (NTT) at their La Silla Observatory, observing dips in starlight as something passed between us and the star. They used the NTT’s ULTRACAM high-speed camera to capture data images of the white dwarf. They also examined data on the same star from NASA’s Transiting Exoplanet Survey Satellite (TESS.)

The New Technology Telescope at the ESO's La Silla Observatory. Image Credit: By Masteruk - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=3235044
The New Technology Telescope at the ESO’s La Silla Observatory. Image Credit: By Masteruk – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=3235044

What were the results of those observations?

The team found dips in light that they interpret as 65 clouds of planetary debris. The clouds are evenly spaced and orbit the white dwarf every 25 hours. What causes such regularity? The researchers say that a planet must be there, which forces these debris clouds into a precise orbital pattern. They say the planet is similar in size to rocky planets in our Solar System and that it’s only about 2.5 million km (1.55 million miles) from the star. That’s about 1.7% of the distance between Earth and the Sun.

Alongside the regular dips in starlight is an ever-present obscuration that the team says is debris in a planetary disk around the star. These structures are in a region that would have been overcome when the white dwarf went through its preceding red giant phase. It’s doubtful that any of these structures could have survived the red giant phase, so they must have formed more recently in the aftermath. If there is a planet in the habitable zone, it can’t be a hold-over from the star’s previous life as a main-sequence star. If all of the circumstances lined up just right—and that’s an enormous if—life would potentially have about two billion years to do its thing on the purported planet, with one of those billions in the future.

“This is the first time astronomers have detected any kind of planetary body in the habitable zone of a white dwarf,” said Professor Farihi. The planet’s presence is inferred from the regular dips in the light of all the other objects in the system.

This figure from the study shows the first three nights of ULTRACAM observations of WD1054-226. The figure shows a notable and easily recognized recurring feature, the double-dip structure that recurs every 25.2 hours, and occurs just before hours 2, 27, and 52. The authors say that Did you miss our previous article…
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