A ring system has been found around a dwarf planet for the first time – the distant, potato-shaped Haumea, which lies beyond Neptune.
Haumea, first discovered in 2004, is one of five dwarf planets: large objects like Pluto, mostly in the outer solar system, which are not quite significant enough for planetary status.
It is known for its peculiar and rapid rotation, spinning end-over-end once every four hours.
But the discovery of Haumea’s ring, reported in the journal Nature, is another surprise for scientists.
“This is the first time a ring around a dwarf planet has been discovered, so it is a really very peculiar, unexpected and weird finding,” said the paper’s co-author, Dr Pablo Santos-Sanz from the Instituto de Astrofísica de Andalucía.
The ring appears to be dense and dark, blocking out about half of the light that passes through it to Earth. It’s 70 kilometres wide and lies about 1000 kilometres away from Haumea’s surface.
Associate Professor Jonti Horner from the University of Southern Queensland, commenting on the discovery, said rings may be more common than we thought.
Ring systems exist around the giant planets in our solar system: Jupiter, Saturn, Uranus and Neptune.
But the presence of a ring around Haumea, coupled with the recent discovery of rings around the “centaurs” Chiron and Chariklo – even smaller bodies that orbit the Sun in the vicinity of the gas giants – suggests ring systems could be found in a variety of environments, Dr Horner said.
“If we’re finding them round [these] unusual objects, maybe they’re everywhere,” he said.
Chasing shadows to explore a dwarf planet
Until now, little was known about Haumea apart from its odd shape and spin.
To study the mysterious dwarf world in detail, Dr Santos-Sanz and his colleagues took advantage of its passing in front of a distant star.
When it made that pass, known as an occultation, Haumea blocked the light coming from the star and cast a shadow that telescopes on Earth could detect.
This illustration shows Haumea rotating on its axis — it spins once every four hours
Twelve telescopes at 10 different locations observed a dip in that star’s light as Haumea passed in front of it. But they also detected smaller dips just before and after Haumea’s passage, indicating the presence of the rings.
Dr Santos-Sanz said the research shifted our understanding of Haumea.
“We knew that Haumea had to be a highly non-spherical body and the occultation results are completely compatible with that,” he said.
“But that is the only finding that was entirely consistent with previous knowledge on Haumea.”
The researchers found that Haumea is bigger than they previously thought – more than 2300km on its longest axis, or about the same size as Pluto – and less dense too.
“We now have a correct shape, a correct albedo [reflectivity], a correct density and we have discovered a ring around it,” Dr Santos-Sanz said.
The next step is explaining how that ring can form around such a peculiar body, and why the ring spins three times slower than the dwarf planet itself.
“When we have more knowledge about all these open points we will be able to answer the question about where these ring systems can arise in the solar system,” Dr Santos-Sanz said.
Exploring objects a long, long way from Earth
Dr Horner, meanwhile, said the study was a demonstration of how much you could learn about a faraway object with clever astronomical techniques.
“The object we know best of the trans-Neptunian objects is Pluto, and we know so much about it because we sent the New Horizons mission there,” he said.
But he said that mission cost more than half a billion dollars, for about one day’s close observation of the dwarf planet.
“By taking advantage of good fortune, good alignment with Haumea and the background stars, we can do some of that work without it being up close and personal,” he said.