Astronomers have extended the range of our Solar System with the discovery of a new ‘dwarf planet’ orbiting our Sun. The planet, currently technically labeled ‘2012 VP113’, has an elliptical orbit that brings it to within 80AU of the Sun (an ‘AU’ is the unit of distance from the Sun to Earth) at perihelion (closest point in its orbit to the Sun) – some three times the distance from the Sun to Neptune – while it gets as far as 450AU away at the other end of its orbit. The object is not unique: astronomers have previously discovered another similar dwarf planet, named ‘Sedna‘. Indeed, the new discovery, by astronomers Scott Sheppard and Chad Trujillo, hints that there are a multitude of dwarf planets orbiting the Sun in a location known as the ‘inner Oort cloud‘.
An intriguing coincidence is that the current positions of both Sedna and 2012 VP113 – which are also near both their perihelions – are very similar, leading to some speculation of an unknown, massive body orbiting our Sun well beyond our current sensing abilities – a ‘Planet X’:
The latest work has already thrown up an intriguing possibility. The angle of the body’s orbit and that of Sedna’s are strikingly similar, an effect most likely caused by the gravitational tug of another, unseen body. One possibility is a “Super Earth” that traces so large an orbit around the sun that it has never been seen.
“If you took a Super Earth and put it a few hundred astronomical units out, the gravity could shepherd Sedna and this new object into the orbits they have,” said Sheppard.
Over at her Planetary Society blog, Emily Lakdawalla has a more detailed breakdown of the discovery, the coincidences, and what it might mean for the Planet X theory. She notes that for a an object of that size to form (‘accrete’), it needs to be in a circular orbit – so the elliptical orbit of these two dwarf planets suggests that they were ‘scattered’ by something at some point after they formed. But the ‘Planet X’ theory isn’t the only possible explanation – the planets may have been thrown into their current orbits by a star that passed within “several hundred AU of the Sun and disturbed orbits of objects it passed near”, or it may be due to the fact that Earth was born in a star cluster.
It’s really quite striking how close Sedna and 2012 VP113 are to each other right now, both close to their perihelia, both at around 80 AU. Here’s an illustration that I put together using the JPL Small-Body Database Browser that shows you just how close they are.
Their current proximity is mostly a coincidence, given the fact that they have different orbital periods; there’s nothing about their orbits that says they should be in the same place at the same time, except for the fact that their orbits happen to take them to similar spots in the sky when they are close to the Sun. But we are more likely to discover such objects when they are near perihelion (hence brighter and moving faster), so given the proximity of their perihelia they would’ve been somewhat close to each other in the sky because that’s where we could see them both.
The fact that they have perihelia at similar locations is an interesting observation, though. And it’s one that Trujillo and Sheppard noticed, too… They went on to hypothesize that the clustering of argument of perihelia resulted from “a massive outer Solar System perturber” [and] showed that it works for a super-Earth at 250 AU, but “This configuration is not unique and there are many possibilities for such an unseen perturber.”
…I have confess to a bias here: I really wanted this coincidence in argument of perihelion to be strong evidence of a planet X. I would love for there to be a planet X. So would Trujillo and Sheppard, evidently, because they spent quite a bit of space showing it could work. And so would Nature, because then the first clear indication of a planet X would be in an article published in their journal.
But Hal [planetary scientist A second Sedna! What does it mean?