An object possibly twice the size of Pluto has been found - hiding in plain sight. The discovery could be the biggest world in the Kuiper belt of rocky objects that orbit the outer reaches of the solar system.
The find suggests more such objects are waiting to be discovered and is likely to reignite the fierce debate about what constitutes a planet.
On Thursday, an email with the subject, "Big TNO discovery, urgent" was sent to a popular astronomy mailing list. The message described the discovery of a "very bright" object that was creeping along slowly beyond the orbit of Neptune - making it a Trans-Neptunian Object, or TNO.
Its exact size cannot be determined because the reflectivity of its surface is not known. But if the reflectivity is as dim as most other distant, rocky objects that have been studied, it could be twice as wide as Pluto, which is about 2300 kilometres across.
Jose-Luis Ortiz, an astronomer at the Sierra Nevada Observatory in Spain, and colleagues discovered the object when they re-analysed observations they had made in 2003. Then, they scoured older archives and found the object in images dating back to 1955.
Based on these so-called "precoveries", they calculated the object's orbit and sent urgent emails asking people around the globe to observe the new find.
Amateur observers Salvador Sanchez, Reiner Stoss, and Jaime Nomen found it on Thursday using a 30-centimetre telescope in Mallorca, Spain. "I am not going to sleep tonight," said Stoss, a mechanical engineering student in Darmstadt, Germany. "To find an object bigger than Pluto - it's like the X Prize," he said, referring to the $10 million prize for private spaceflight won in 2004.
The observations were then verified by the International Astronomical Union's Minor Planet Center (MPC) in Cambridge, Massachusetts, US, which designated the object 2003 EL61.
Time to move
The MPC reports the object is about 51 Astronomical Units from the Sun - 1 AU is the distance between the Earth and the Sun. Its orbit brings it comes as close to the Sun as 35 AU, while Pluto maintains an average distance of about 39 AU. "Someone should have found this before," Brian Marsden, director of the MPC, told New Scientist.
One reason they did not is the object's speed, suggests Stoss. Many surveys of Near Earth Objects take a trio of images spaced 20 minutes apart to search for telltale movement in relation to background stars.
But 2003 EL61 is too far away to detect its progress in that time. Ortiz's survey compares images taken a day apart. "They give the object time to move," Stoss says.
Another reason may be the plane of the object's orbit, says Tommy Grav, an astronomer at the University of Hawaii in Manoa, US. That plane is tilted by 28° with respect to the orbital plane of most planets, where surveys tend to scan the skies for Near Earth Objects.
2003 EL61 is even more off-kilter than Pluto, which orbits in a plane tilted by 17°. "Pluto was pushed out of the plane of the solar system when Neptune moved outwards" soon after the solar system formed, Grav told New Scientist. "It's possible this object has suffered something similar."
The discovery, coupled with other recent finds such as Sedna and Quaoar, suggests other large objects may lurk in the murky region beyond Neptune.
"Some people have claimed we'd never find something as bright as this out there," says Grav. "But there may be something even further out that's moving so slowly we haven't seen it yet."
And the discovery is likely to revive previous fierce debates about what constitutes a planet and even how astronomical objects are named. "But don't even start that discussion," Stoss jokes. He says future observations of the object's colour and brightness could reveal its true size, shape, rotation period, and any companion moons.