[BRIEF NOTE] On what the discovery of 2012 VP113 means

[rfmcdonald]

The recently-announced discovery of 2012 VP113, one of the most distant bodies ever discovered and one of the first objects likel to belong to the Oort cloud, is exciting. It was widely covered in the blogs I read: Bad Astronomy and Centauri Dreams and D-Brief and The Dragon's Tales and the Planetary Society Weblog all had features.

Bad Astronomy's Phil Plait did a good job of outlining the import of the discovery. Why is a body likely only a few hundred kilometres in diameter important?

Astronomers have announced the discovery of an amazing object in our solar system: 2012 VP113, an icy body with an orbit so big it never gets closer than 12 billion kilometers (7.4 billion miles) from the Sun! That’s 80 times the distance of the Earth from the Sun. No other solar system object known stays so far from the Sun. And at its most distant, it reaches an incredible 70 billion kilometers (44 billion miles) from the Sun—and it takes well over 4,000 years to circle the Sun once.

It’s not exactly clear yet, but it’s likely that VP113 is a member of the Oort cloud, a huge collection of gigantic frozen ice balls that orbit the Sun way, way past Neptune. Sedna is the only other object known in that part of the remote solar system, and it gets closer to the Sun by a smidge than VP113 ever does. The closest point an object can get to the Sun is called perihelion, and VP113 has the largest perihelion distance of any object known.

Emily Lakdawalla of the Planetary Society, meanwhile, makes a good case that the existence of this body and of the similar Sedna do not at all provide evidence of a "Planet X" influencing their erratic orbits. (It looks like their orbits wee disrupted by nearby stars in the young solar system instead._

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 [discoverers Chad Trujillo and Scott 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 [Levison] dashed my hopes, or at least my certainty. "It's a very weak result," he told me; and indeed the paper spends more column inches on what 2012 VP113 tells us about the inner Oort cloud as a population than it does about this potential "perturber." Meg Schwamb seems to agree; her News & Views piece didn't even mention the possibility of a planet, only that "This result may be the first hint we have of an identifiable signature of the inner Oort cloud’s formation mechanism on the orbits of closer-in Solar System bodies. If true, any formation mechanism proposed for the origin of Sedna and 2012 VP113 will need to explain this orbital structure."

Hal told me that he looked carefully at whether the clustering in argument of perihelion could just be a statistical fluke, but that ultimately, he "believes the data."

The question is whether it's a planet. And then you have to be a little careful. If this population were massive enough, if there were like one or two Earth masses in it, which is possible, because something like a hundred Earth masses was scattered out as Uranus, Neptune, Jupiter, and Saturn were growing, then maybe it's the self-gravity between these objects themselves that's doing it. There may be other explanations for this, rather than the extreme position of, "it's a planet"; but I can believe there's something going on.

Hal was more interested in a different coincidence about Sedna and 2012 VP113. "It's a little odd that the first two objects that have been discovered have perihelia that large. There's not a guy at 55 or 60 AU. It probably means there's an inner edge to this population; that certainly would constrain things." I poked at this, really wanting the distance to tell me that there was a planet X at 80 AU, but unfortunately, it doesn't work that way. Hal said that some solar system formation simulations do turn out to have an inner edge to the inner Oort cloud or first-generation Oort cloud without the help of any interloping planet, "Because you have to get far enough away from the planets to stabilize the orbits for long periods of time. I don't think it's hugely surprising that that edge is there." He pointed me to this 2012 paper, which mentions a 100-AU inner edge in the abstract.