Our solar system has eight planets (again, sorry Pluto). But it has many, many more planetoids — more than 600,000 at last count. It's a broad category. The word planetoid covers anything that isn't a true planet (objects with enough mass that they have taken on a mostly round shape and have become the dominate gravitational force in their orbit) and that also isn't a comet.
Planetoids are big news this week, with two new discoveries that will teach us more about the structure of objects in space and about our solar system, as a whole.
First up, Chariklo, an astroid about the size of Canada's Prince Edward Island, which orbits the Sun on a path out between Saturn and Uranus. Turns out, Chariklo has rings — something a lot of scientists didn't think asteroids could form.
The researchers think the rings formed from debris left over from a collision of an object with Chariklo or between two objects orbiting it. That debris was shaped into rings by one or more undiscovered small "shepherd" moons — about a kilometre across — embedded in the rings themselves.
"What we are witnessing is perhaps the unveiling of an object that is in the middle of the same stage of development as the Earth and the moon 4.5 billion years ago, when there was a giant collision between Earth and another planet," Jorgensen said in a news release from the University of Copenhagen.
He added that if the rings eventually gather up into a single moon, it will be about two kilometres in diameter.
The second bit of news is much bigger, and concerns 2012 VP113 — a chilly ball of ice whose orbit is so big, and so distant from the Sun that a single loop around takes 4,000 years. 2012 VP113 is a big deal because scientists think it's likely part of the Oort Cloud, a hypothesized collection of huge, frozen planetoids that serves as the birthplace of our solar system's comets. We can't observe the Oort Cloud directly, but scientists have been collecting indirect evidence about it for a couple decades and there's reason to think that it exists and that 2012 VP113 is one of the closer objects that's a part of it. Observations of 2012 VP113 over the years to come will help scientists better understand what's going on in the far reaches of the solar system and what role (if any) the Oort Cloud plays in that.
More speculatively, writes Phil Plait at Slate.com, the orbit of 2012 VP113 (as well as that of a nearby planetoid called Sedna) could indicate the presence of an actual ninth planet, somewhere out in Oort Cloud territory.
The orbits of Sedna and VP113 are weird. They are highly elliptical, dipping down into the inner Oort cloud and stretching out to its vast depths. We'd expect their orbits to have started out more circular, so what could have affected them to alter their orbits into the shapes we see today?
… It's possible that a bigger object—a proper planet-sized thing—could be out there in the Oort cloud, hundreds of AU away from the Sun, that could be affecting the orbits of these objects. If it were a giant planet like Jupiter or Saturn we would have detected it by now, so it would have to be something smaller and colder. An object the size of the Earth (or even somewhat bigger) would fit the bill. It's an idea that's been around for a while now.
Mind you, the evidence here is pretty thin, and as much as I'd love for there to be another planet lurking out there for us to find and study, we just don't have enough data here to say anything either way. It's small number statistics; we've found two objects with odd orbits, but it could be coincidence. We need to find a lot more OCOs like Sedna and VP113 so that the gaps in our understanding of their orbits can be filled in.