A string of moon-sized objects have been found orbiting a white dwarf star with orbits that imply a larger, unseen, object is keeping them in place. Astronomers suggest this is the first planet found in the habitable zone of a white dwarf. The announcement is one of three unexpected discoveries about white dwarves to come to light in a big week for dead stars.
When it comes to looking for life in the universe, planets around white dwarves have barely been considered. Having fused their hydrogen and shed their outer layers these stars are a shadow of their former selves. Planets that formed when the star was young would have experienced such dramatic temperature changes as the star evolved, if not consumed entirely, that any life on them would be long since dead.
Nevertheless, white dwarfs do have a close-in “habitable zone” where temperatures suit liquid water, and for the first time evidence for a planet in this zone has been detected, published in a paper in Monthly Notices of the Royal Astronomical Society.
The star in question is WD1054-226, which lies 117 light-years away. Nowhere near as bright as it would have been when still turning hydrogen to helium, WD1054-226 is now at 16th magnitude, slightly fainter than Pluto.
Observations made with the European Southern Observatory’s New Technology Telescope reveal dips in light associated with objects passing in front of the star every 23 minutes. This transit method is one of the ways astronomers discover exoplanets.
However, confirmation with NASA's planet-hunting TESS space telescope revealed a pattern to these dips. This led Professor Jay Farihi of University College, London and co-authors to conclude this is not a single object whipping round at tremendous speed, but 65 clouds of debris spaced out around an orbit that sees each one pass the star every 25 hours. WD1054-226 is so faint, temperatures at an orbit like that are compatible with liquid water. “This is the first time astronomers have detected any kind of planetary body in the habitable zone of a white dwarf,” Farihi said in a statement.
There’s no chance of liquid water, let alone life, on the obscuring objects. Farihi described them as “irregular and dusty (e.g. comet-like) rather than solid, spherical bodies.” However, their apparent stability may indicate the presence of something larger and potentially even more interesting.
“Their absolute regularity, one passing in front of the star every 23 minutes, is a mystery we cannot currently explain,” Farihi said. “An exciting possibility is that these bodies are kept in such an evenly-spaced orbital pattern because of the gravitational influence of a nearby planet, Without this influence, friction and collisions would cause the structures to disperse, losing the precise regularity that is observed.” Something similar occurs in Saturn’s rings, where the gravitational tug of nearby moons creates stable structures.
If the planet exists its orbit does not appear to cross the face of the white dwarf as seen from Earth, so direct confirmation via blips in light from the star will be difficult. Instead, the authors hope to confirm its presence with more detailed analysis of the orbits of the debris, ideally bolstered by JWST observations.
The finding adds to recent evidence white dwarves can have a second set of planets after they have completed their period as a main sequence star, like a retiree raising a second family. White dwarfs can continue to glow for longer than some main sequence stars shine. They fade so slowly over that time that WD1054-226’s habitable zone will last for 2 billion years – half of which is still to come – potentially time for life to evolve and develop on a suitably placed planet.
Like the vast majority of stars, the Sun will eventually become a white dwarf. Before that its red giant stage will end life on Earth, but WD1054-226 raises the possibility something could emerge phoenix-like from the ashes close to our current location.
Many white dwarfs’ spectra reveal metal “pollution”, which has been considered evidence of recently swallowed planets. Last week for the first time this was reported to be seen as it is happening. Just yesterday a white dwarf was announced wrapped in carbon and oxygen, which may be a merger of two white dwarves.