The monsters are multiplying. Just months after astronomers announced hints of a giant "Planet X" lurking beyond Pluto, a team in Spain says there may actually be two supersized planets hiding in the outer reaches of our solar system.
When potential dwarf planet 2012 VP113,was discovered in March, it joined a handful of unusual rocky objects known to reside beyond the orbit of Pluto. These small objects have curiously aligned orbits, which hints that an unseen planet even further out is influencing their behaviour. Scientists calculated that this world would be about 10 times the mass of Earth and would orbit at roughly 250 times Earth's distance from the sun.
Now Carlos and Raul de la Fuente Marcos at the Complutense University of Madrid in Spain have taken another look at these distant bodies. As well as confirming their bizarre orbital alignment, the pair found additional puzzling patterns. Small groups of the objects have very similar orbital paths. Because they are not massive enough to be tugging on each other, the researchers think the objects are being "shepherded" by a larger object in a pattern known as orbital resonance.
For instance, we know that Neptune and Pluto are in orbital resonance for every two orbits Pluto makes around the sun, Neptune makes three. Similarly, one group of small objects seems to be in lockstep with a much more distant, unseen planet. That world would have a mass between that of Mars and Saturn and would sit about 200 times Earth's distance from the sun.
Some of the smaller objects have very elongated orbits that would take them out to this distance. It is unusual for a large planet to orbit so close to other bodies unless it is dynamically tied to something else, so the researchers suggest that the large planet is itself in resonance with a more massive world at about 250 times the Earth-sun distance just like the one predicted in the previous work.
Observing these putative planets will be tricky. The smaller bodies are on very elliptical orbits and were only spotted when they ventured closest to the sun. But the big planets would have roughly circular orbits and would be slow moving and dim, making them tough for current telescopes to see. "It's not at all surprising that they haven't been found yet," says Carlos.
"As there are only a few of these extremely distant objects known, it's hard to say anything definitive about the number or location of any distant planets," says Scott Sheppard,at the Carnegie Institution for Science in Washington DC, one of the discoverers of 2012 VP113. "However, in the near future we should have more objects to work with to help us determine the structure of the outer solar system."