Researchers have discovered a ring of planetary bodies in the habitable zone of a white dwarf for the first time, suggesting life could exist around these dead stars.
Moon-sized structures are orbiting the white dwarf in a region where water could exist as a liquid rather than as gas or ice—an area known as the habitable zone. These bodies orbit with a regularity that indicates they are guided by a major planet in the zone.
White dwarfs have ended their main life cycle, existing as barely glowing embers, and had been previously considered barren.
Jay Farihi, professor of Astrophysics at University College London, is the lead author of a paper published in the journal the Monthly Notices of the Royal Astronomical Society.
He told Newsweek: "A white dwarf star is the most common 'retirement' of the billions of suns in the Milky Way. Their dim light, and the spectacular giant phase that precedes their inception, has conventionally taught scientists that they are 'dead' or a 'stellar graveyard.'
"Instead, this finding turns that idea on its head completely. Now we know planets may actually be habitable around these dim stars, and life may either persist or be re-forged across cosmic time."
Farihi called the discovery "a real game-changer."
"White dwarf stars and their planetary systems are far from 'dead,;" he said. "Instead, it's more like a caterpillar transforming into a butterfly, a second phase of life as a star and a planetary system."
The habitable zone around a white dwarf such as the one they studied, WD1054–226 located within the Milky Way about 117 light-years away from Earth, differs from the habitable zone around active stars like the sun, Farihi added.
He said: "The habitable zone is roughly 60 times closer to the white dwarf star than the Earth is to the Sun. That is because the light is dim, it must be closer to be sufficiently warm for water to be liquid on a planetary surface."
A Shepherd in a Stellar Graveyard
Paper co-author Mark Wyatt, of the University of Cambridge Institute of Astronomy Professor, outlined the significance of the discovery.
He said: "We have known for a while that there is debris in proximity to several white dwarfs, and we have even seen it transit in front of a handful.
"But this is the first time we have detected that debris being at the right distance from the white dwarf to be in its habitable zone, as the debris is normally much closer to the star where it is much hotter."
Even more importantly, Wyatt added, the fact that it transits in front of the star allows astronomers to probe structures in the debris that have never been seen before, including a major planet that could be "shepherding" the debris.
The team made their discovery by observing dips in light around WD1054–226 that corresponded to 65 evenly spaced clouds of planetary debris orbiting the star and passing its face every 25 hours.
The regularity of this dimming, which occurred every 23 minutes, implied to the team that there is a nearby planet keeping these dusty moon-sized structures in a precise alignment.
Wyatt continued: "Other white dwarfs known to have orbiting debris have an infrared signature from their hot dust, but this was not present for this star. We must have got lucky with the orientation of the system, with the debris lying directly along the line of sight to the star which allowed us to see it pass in front of the star.
"One of the exciting aspects of this discovery is that it means that the habitable zones of white dwarfs have more material in them than we thought - this material has simply evaded detection before in our infrared surveys because we have been looking for much hotter dust."
It is the gravitational influence of a planet that the team believes is "shepherding" this material around the white dwarf.
Wyatt continued: "There could be real estate in this habitable zone on which life could take hold. We also expect this zone to be bombarded by a rain of debris from further out in the system which could bring some of the building blocks for life.
"So, I would say that the chances of a white dwarf supporting life have increased with this discovery, but there are still significant challenges to the development of life there, and much we don't know about the nature of these habitable zones."
Not only do the findings indicate white dwarfs and their systems may not be the graveyards previously predicted, but they could give us a hint at what the future holds for the solar system.
Future of Our Own Solar System
Astronomers consider a star's "life" to be the period during which it burns hydrogen to produce helium. After this raw material is consumed, some stars are massive enough to burn helium and even heavier elements, stars like our sun however will aren't great enough in mass to achieve this.
The sun is approximately halfway through its reverse of hydrogen and when the fusion of this, the universe's lightest element, ceases its inner layers will undergo gravitational collapse.
The effect is eventually reversed as the sun becomes a smoldering white dwarf, with the outer layers billowing outwards—meaning the sun will first undergo a giant phase.
This phase spells doom for the inner rocky planets, Mercury, Venus, and Earth as our star's radius billows out to the orbit of Mars. When this outer material is blown away it leaves behind a "dead" white dwarf.
Farihi continued: "Our Sun will start to become a white dwarf in approximately 5 billion years. On the one hand, this could be telling us that life can persist across this vast timescale.
"On the other hand, maybe we are looking at a new generation of life-bearing planets, constructed or put into its current position only recently, after the giant phase of its star."
It means life could continue in some form around our now "dead" sun, Farihi said.
He added: "It suggests a way for life to exist in the white dwarf stage, but survival over that long period will be tricky, with additional hurdles.
"If those can be overcome, the white dwarf habitable zone can last for
up to several billion years -- plenty of time to grow a civilization if life remains intelligent."
