A ring of planetary debris dotted with moon-sized structures has been observed orbiting near a white dwarf star, hinting at a nearby planet in the “habitable zone” where water and life might exist, according to a new study by researchers at UCL.
White dwarfs are glowing embers of stars that have burned up all their hydrogen fuel. Almost all stars, including the Sun, will eventually become white dwarfs, but very little is known regarding their planetary systems.
In the study published in Royal Astronomical Society Monthly Noticesan international team of researchers has measured the light from a white dwarf in the Milky Way known as WD1054-226, using data from ground and space telescopes.
To their surprise, they found pronounced dips in the light 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 light from the star every 23 minutes – suggests that they are held in such precise arrangement by a nearby planet.
Lead author Professor Jay Farihi (UCL Physics & Astronomy) said: “This is the first time astronomers have detected a planetary body in the habitable zone of a white dwarf.
“The moon-sized structures we observed are irregular and dusty (eg, comet-like) rather than solid, spherical bodies. Their absolute regularity, one pass in front of the star every 23 minutes, is a mystery that we cannot currently explain.
“An exciting possibility is that these bodies are held in such a widely spaced orbital pattern due to the gravitational influence of a nearby planet. Without this influence, friction and collisions would cause the structures to disperse, losing the precise regularity observed. A precedent for this “shepherd” is how the gravitational pull of the moons around Neptune and Saturn helps create stable ring structures orbiting these planets.
“The possibility of a planet in the habitable zone is exciting and also unexpected; we weren’t looking for that. However, it is important to keep in mind that more evidence is needed to confirm the presence of a planet. We cannot observe the planet directly so confirmation can come by comparing computer models with other observations of the star and orbiting debris. »
This orbit around the white dwarf is expected to have been cleared during the giant star phase of its life, and so any planet that might potentially host water and therefore life would be a recent development. The area would be habitable for at least two billion years, including at least a billion years in the future.
Over 95% of all stars will eventually become white dwarfs. Exceptions are larger stars that explode and become either black holes or neutron stars.
Professor Farihi added: “Since our Sun will become a white dwarf in a few billion years, our study provides insight into the future of our own solar system. »
When stars begin to run out of hydrogen, they expand and cool, becoming red giants. The Sun will enter this phase in four to five billion years, swallowing up Mercury, Venus and possibly Earth. After the outer material has been gently blown away and the hydrogen depleted, the hot core of the star remains, slowly cooling over billions of years – this is the star’s white dwarf phase.
Planets orbiting white dwarfs are difficult for astronomers to detect because the stars are much fainter than main sequence stars (like the Sun). So far, astronomers have only found tentative evidence of a gas giant (like Jupiter) orbiting a white dwarf.
For the new study, the 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 attached to the New Technology Telescope (NTT ) of 3.5 m from ESO to La Silla Observatory in Chile. To better interpret changes in light, the researchers also looked at data from NASA’s Transiting Exoplanet Survey Satellite (TESS), which allowed the researchers to confirm that the planetary structures had a 25-hour orbit. .
They found that the light from WD1054-226 was still somewhat obscured by huge clouds of orbiting material passing in front of it, suggesting a ring of planetary debris orbiting the star.
The habitable zone, sometimes called the Goldilocks zone, is the area where the temperature would theoretically allow liquid water to exist on a planet’s surface. Compared to a star like the Sun, the habitable zone of a white dwarf will be smaller and closer to the star because white dwarfs give off less light and therefore heat.
Structures observed in the survey orbit in an area that would have been shrouded by the star when it was a red giant, are therefore likely to have formed or arrived relatively recently, rather than having survived 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, University of Warwick, Lund University, the University of Cambridge, University of St Andrews, Wesleyan University, University of La Laguna, Naresuan University, University of Sheffield and Instituto de Astrofísica de Canarias.
