One of the latest members of the Solar System to be discovered, 2012 VP113 is a tiny isolated world in the outer Solar System. It’s attracting attention as it could one day help lead astronomers to discover a whole new planet.
Flying outward from the Sun, away from the planets, lies the Kuiper Belt, billions of small icy and rocky bodies arranged in a great ring 30 to 48 AU from the Sun (where an 1 AU = 150 million km). This belt of tiny frigid worlds dwarfs the more familiar Asteroid Belt between Mars and Jupiter both in extent and population. It is new territory for astronomy, although predicted much earlier, the first Kuiper Belt object was discovered as recently as 1992.
It is easy to think of the Kuiper Belt as marking the edge of the Solar System, but far beyond it is the Oort Cloud, another staggeringly large realm of miniature frozen worlds. The Oort Cloud is a vast reservoir of potential comet nuclei lying in a spherical shell tens of thousands of times further from the Sun than Earth. The majority of the worlds in our Solar System are to be found here, trillions of chilly little planetoids each only tens of kilometres across lit by faint, months-old sunlight. Pour them all on to a gigantic set of scales and you will need several Earths to balance the pans. No Oort Cloud objects have ever been observed in situ.
Between the Kuiper Belt and Oort Cloud lies a ‘desert’, hundreds of AU wide, of apparently empty space. But if we take a closer look we can see there are little worlds out here too, “scattered disc objects” including the recently-announced 2012 VP113, a ball of ice and rock about 250-450 km wide. The discoverers were Chad Trujillo and Scott Sheppard of the Gemini Observatory.
The surface of 2012 VP113 is a pinkish beige, a tone it shares with Neptune’s giant moon Triton and a handful of trans-Neptunian objects, notably dwarf planet Pluto.
2012 VP113 takes around 4300 years to go around the Sun, this highly elliptical orbit never brings it closer to the Sun than 80 AU and takes it much, much farther away, as far as 450 AU. 2012 VP113 lies between the Kuiper Belt and Oort Cloud but it is not all alone in the endless night. The orbit of 2012 VP113 is similar to that of another object, 90377 Sedna. Discovered in 2003, Sedna is a startlingly large world about 1000 km in diameter making it similar in size to the dwarf planet Ceres.
Sedna has the longest orbital period of any known major object in the Solar System, calculated at around 11 400 years. At its closest to the Sun, Sedna is about 76 AU from our star, at furthest some 937 AU. Sedna and VP113 are believed to be escapees from the Oort Cloud. This common origin is suggested by their similar orbits, however there is a significant difference in their colours, as noted earlier VP113 is pinkish but Sedna is a deep red.
If they do share a common origin in the Oort Cloud, why are these icy wanderers now roaming in this deep space interzone between the Kuiper Belt and Oort Cloud? Something may have put them there. Possibly once in the distant past another star passed close by ours, within as close as a light year. This would be close enough to upset the orbits of Oort Cloud objects sending some like Sedna and VP113 into their unusual orbits. Another possibility is that there is an undiscovered planet serenely circling the Sun in the Oort Cloud, hundreds of AU away from the Sun, that could be affecting the orbits of these objects.
There are many people out there who want to believe in a mysterious Planet X in the outer Solar System, but a giant world out there seems unlikely to exist. NASA’s WISE satellite has mapped the entire sky in infrared light. Recently-released data from this mission suggests there are no objects the size of Saturn or larger out to a distance of 10 000 AU and no objects larger than Jupiter out to 26 000 AU. This does not preclude planets up to the size of Neptune.
Almost certainly, in the years to come astronomers will find more large worlds between the Kuiper Belt and Oort Cloud. Just maybe their orbits will be similar to those of VP113 and Sedna. If so there, the race to spot a new planet will be on!
A Sedna-like body with a perihelion of 80 astronomical units (Trujillo & Sheppard, 2014)
(Article by Colin Johnston, Science Education Director)