Yesterday, the journal Nature published an exciting report documenting a new discovery about what lies at the outer limits of our solar system - including a previously unknown dwarf planet, and the possibility of discovering even larger objects hovering in the darkness beyond.
But before I discuss the new discovery, I want to take a very basic look at the layout of our cosmic neighborhood, to get a sense for the region of space we're talking about.
As you head farther and farther out from our sun, you pass through the familiar regions we all know from school: the inner solar system, with warm, rocky planets including Earth; the asteroid belt; and the outer solar system, with gas giants separated by great distances, ending at the planet Neptune. But after Neptune, it's more than 4 light-years to the nearest star, Proxima Centauri (there's not even 1 percent of a light-year between the sun and Neptune). So what takes up all that space in between? Specifically, what lies in the part closest to us - between Neptune and the outer reaches of our sun's gravitational influence?
Astronomers have known for years that there are belts of small, dark, icy things past the orbit of Neptune. First, beyond Neptune lies the Kuiper belt - a region of small objects much like the asteroid belt, but separated from the sun by between 30 and 50 astronomical units (meaning 30 to 50 times the distance between the Earth and the sun), including standouts like the dwarf planet Makemake and the still-mourned ex-planet Pluto. But past the Kuiper belt lies the much larger and more mysterious Oort cloud, which is believed to extend much farther - perhaps 50,000 or 100,000 astronomical units (AU) from the sun. The estimate of 100,000 AU is illustrated in the NASA image below:
It's within the inner region of the Oort cloud that the newly discovered object is believed to travel. Previously, the only known dwarf planet occupying this vast space was the object called Sedna, which was discovered a little more than a decade ago. But this new discovery shows that Sedna is not as lonely as we might have thought. Astronomers Chad Trujillo and Scott Sheppard first spotted a new trans-Neptunian object in November 2012, using an imaging device called the Dark Energy Camera via the Victor M. Blanco 4-m telescope in Chile. The object, currently designated 2012 VP113 ("VP" or "Biden," for short), is believed by its co-discoverers to be a pinkish-red chunk of icy matter, about 280 miles (450 kilometers) in diameter. Its long, elliptical orbit brings it to a distance of 80 AU at its closest point and about 452 AU at its farthest. In other words, it is way out there.
And this brings up a fascinating question for astronomers to answer: How did an object of this size get so far out from the sun, and what gives it such a peculiar elliptical orbit? There are many hypotheses, including the possibility that when our solar system was forming, a nearby massive object like a star or a rogue planet exerted a gravitational influence that tugged some matter away from the inner orbits of our sun.
Another startling possibility (and I want to stress possibility) implied by the behavior of Sedna and VP113 is that there is now a large, dark, currently unknown outer planet, probably larger than Earth, influencing the orbits of these objects. You might ask: How could an object that large be hidden from us if it's so close? It's actually fairly simple: Planets don't usually emit their own light, so we have to see them by the light they reflect from our sun. The farther out from the sun an object is, the less light from the sun it reflects back to us. So it's possible that a large, dark object could be hiding in the wings of our solar system. Of course here we must remember the unfortunate maxim of science hype: The more popular fascination potential there is for a hypothesis, the more skeptically you should regard it. I try not to get my plausibility gauge thrown out of whack simply by how cool an idea is. So for now I think it's important to keep in mind that such a dark planet is just a suggested possibility, and we have no direct evidence of it. To find such an object, we could look for more objects like Sedna and VP113 and observe their orbits to get a fuller picture of what gravitational influencers may lie beyond Neptune. Or, by studying Sedna and VP113 (and discovering new objects like them), we might simply learn more about the history of the cluster that formed our solar system about 4.5 billion years ago - which, in reality, would probably be more scientifically meaningful than discovering a dark planet somewhere out there.
What's amazing to me about discoveries like this is that there's so much to learn about regions of space that are so close to home. We know there are thousands of unanswered questions about exoplanets, distant stars and wandering rogue planets, dense galactic centers and the origins of the universe. But it's such a strange and excellent surprise to learn how many new things there are to discover right in our neighborhood, without even going halfway to the nearest star.