Using the ULTRACAM instrument on ESO’s 3.5-m New Technology Telescope and NASA’s Transiting Exoplanet Survey Satellite (TESS), astronomers have observed a ring of planetary debris studded with moon-sized structures in the habitable zone of WD 1054-226.
WD 1054-226 is a 16-magnitude relatively cool white dwarf star with a hydrogen atmosphere.
Otherwise known as LP 849-31, it resides approximately 118 light-years away in the small southern constellation of Crater.
University College London’s Professor Jay Farihi and colleagues recorded changes in light from WD 1054-226 over 18 nights using the ULTRACAM high-speed camera on ESO’s 3.5-m New Technology Telescope at the La Silla Observatory in Chile.
In order to better interpret the changes in light, the astronomers looked at the TESS data.
They found pronounced dips in light corresponding to 65 evenly spaced clouds of planetary debris orbiting the star every 25 hours.
They concluded that the regularity of the transiting structures suggests they are kept in such a precise arrangement by a nearby planet.
The planet is thought to be similar in size to that of the terrestrial planets in our Solar System.
The approximate distance between the planet and WD 1054-226 is roughly 2.5 million km (1.55 million miles), around 1.7% of the Earth-Sun distance.
They found that the light from WD 1054-226 was always somewhat obscured by enormous clouds of orbiting material passing in front of it, suggesting a ring of planetary debris orbiting the white dwarf.
The structures 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.
It is expected that this orbit around the white dwarf was swept clear during the giant star phase of its life, and thus any planet that can potentially host water and hence, 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.
“This is the first time astronomers have detected any kind of planetary body in the habitable zone of a white dwarf,” Professor Farihi said.
“The moon-sized structures we have observed are irregular and dusty (e.g. comet-like) rather than solid, spherical bodies. 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 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.”
A paper on the findings was published in the Monthly Notices of the Royal Astronomical Society.
J. Farihi et al. 2022. Relentless and complex transits from a planetesimal debris disc. MNRAS 511 (2); doi: 10.1093/mnras/stab3475
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