One of the most distant icy bodies ever seen in the Solar System has been found, but its strange orbit hints at something even more extraordinary: an undiscovered planet larger than Earth even further away from the Sun, corralling objects within the inner Oort Cloud into looping, elongated orbits.

Artist's concept of Sedna. Image credit: NASA

The discoverers of the confirmed new object, Scott Sheppard of the Carnegie Institution in Washington DC and Chad Trujillo of the Gemini Observatory, are calling it a dwarf planet, although this designation awaits official ratification by the International Astronomical Union.

Called 2012 VP113 - at least until it gets a proper name - it is far smaller than the other dwarf planets at just 450 kilometres across. Nevertheless, this "would be large enough for gravity to make the object spherical and thus a dwarf planet," Sheppard tells Astronomy Now.

Found using the new Dark Energy Camera on the NOAO four-metre telescope in Chile, VP113 is estimated to be about half the size of Sedna, its closest-related object and another dwarf planet wannabe. Discovered in 2003, Sedna’s orbit takes it 937 times further from the Sun than Earth (described as 937 AU, where one AU is the mean distance between Earth and the Sun, 149.6 million kilometres), but also brings it to within only 76 AU. Sedna was like nothing ever seen, until now. VP113's closest point to the Sun is 80 AU, which it would have reached in 1979, and a furthest point of 446 AU. While it does not reach as far out as Sedna, its closest point is the most distant of any known object.

Both VP113 and Sedna exist entirely beyond a feature known as the Kuiper Cliff, which is located at around 50 AU and is a sharp outer boundary to the Kuiper Belt of icy bodies where the number of objects suddenly drops off. Explaining the Kuiper Cliff and the orbits of the bodies that are known to lie beyond it has been troublesome to say the least.

By analysing their orbits, Sheppard and Trujillo propose an extraordinary solution. They suggest that somewhere far beyond the Kuiper Cliff, deep within the spherical Oort Cloud of cometary bodies that surrounds the Solar System, lies another planet up to ten times the mass of Earth. This could be a giant solid planet or a small ice giant similar to Uranus and Neptune and its gravitational field would nudge objects like VP113 into their peculiar orbits. However, before we declare {lanet X discovered, even those proposing its existence are showing caution.

"It is only circumstantial evidence that a large planet exists out there," says Sheppard. "We need to find more of these objects [like VP113] to see if they have similar orbital parameters indicative of a large planet shepherding these objects." Sheppard rates the chances that this undiscovered planet exists as "probable" rather than "likely" and reveals that his team are currently pursuing several more candidates that might have similar orbits to VP113.

If it exists then finding this large planet will not be easy. It is likely to have escaped detection because it is too faint for our current crop of survey telescopes to see. Although NASA's infrared satellite WISE recently failed to detect any Jupiter- or Saturn-sized worlds in the Oort Cloud, a super-earth would be too small to have shown up in its search. Instead, it may be ten years before we have the tools to mount a proper search, as we await the construction of the Large Synoptic Survey Telescope (LSST) in Chile, a planned 8.4-metre telescope dedicated to deep surveys of the night sky.

"The LSST will cover the whole sky every month to faint magnitudes and will likely be able to discover many of these distant inner Oort Cloud objects," says Sheppard.

There could certainly be plenty of objects for the LSST to hunt down: Sheppard and Trujillo estimate that, based on the amount of sky they have searched so far and how distant and faint some of these objects can become, there could be 900 large bodies out there in the inner Oort Cloud (the full extent of the Oort Cloud is suspected to extend out past a light year). Besides the suspected 10 Earth-mass planet, some of these other 900 objects could also be big enough to be fully-fledged planets in their own right, similar in size and mass to Earth and Mars. They may have ended up far away from the Sun by being ejected during the early days of the Solar System, or they could be wandering rogue exoplanets that were captured.

The discovery is reported on in the 27 March edition of the journal Nature.