It’s staggering how much our view of the Solar System has changed over the past few decades. The system I grew up with seemed a stable place. The planets were in well-defined orbits out to Pluto and, even if it were always possible another might be found, it surely couldn’t pose any great surprise in that great emptiness that was the outer system. But today we routinely track trans-Neptunian objects with diameters over 500 kilometers — about 50 of these have now been found, and some 122 TNOs at least 300 kilometers in diameter. We know about well over a thousand objects in that ring of early system debris called the Kuiper Belt.
It’s an increasingly messy place, this outer Solar System, and it has its own terminology. We have centaurs and plutinos, resonance objects, cubewanos, scattered disk objects (SDOs), Neptune trojans, damocloids, apollos and, perhaps, inner Oort cloud objects.
Nope, this isn’t the Solar System I grew up with, and every new discovery adds to the enchantment. Its burgeoning population of outer objects tells us much about its history, assuming we can make the right deductions from what we see. Orbital trajectories are a kind of history written in motion. The reason that a belt of objects beyond Neptune was first suspected was that Jupiter-family comets have orbital inclinations too low to be consistent with an origin in the Oort Cloud, that spherical cloud of comets thought to stretch a light year or more from the Sun. Advances in CCD technology soon made it possible to track down Kuiper Belt objects, and it’s now believed that 100,000 KBOs with diameters larger than 100 kilometers could exist, and perhaps as many as 800 million objects with diameters larger than five kilometers.
**Image**: Views of the Kuiper Belt and the Oort Cloud. Credit: Donald K. Yeoman/NASA/JPL.
**The Outer System Poker Game**
All of which is intriguing in its own right, but sometimes it takes a wild card to drive the story forward. That wild card came in the form of Sedna, discovered in 2003 by Mike Brown (Caltech). Brown has been ruminating over the discovery on his _Mike Brown’s Planets_ site, where he notes the fact that the orbit of every object in the Solar System can be explained, at least in principle, by interactions with the known planets. Every object except Sedna:
> Seven years ago, the moment I first calculated the odd orbit of Sedna and realized it never came anywhere close to any of the planets, it instantly became clear that we astronomers had been missing something all along. Either something large once passed through the outer parts of our solar system and is now long gone, or something large still lurks in a distant corner out there and we haven’t found it yet.
The possibilities are fascinating, one being the existence of an unknown planet of approximately Earth’s size at roughly 60 AU. Another possibility: A star that passed close to the Solar System at some point in the remote past, perhaps as close as 500 or 600 AU. In both cases, gravitational interactions would have interfered with what would otherwise have been a routine Kuiper Belt object, kicking it into its present orbit. Brown pegs the chances of a rogue star encounter at around one percent, but in any case, finding the culprit star would be impossible. The Sun has orbited the Milky Way 18 times in our Solar System’s history. “Everything is now so mixed up,” he adds, “that there is no way to know for sure what was where back when.”
**The View from a Cluster**
The third possibility? A kick from not one passing star but from many relatively nearby stars, a kick dating back to the Sun’s presence in the cluster in which the Sun was born. Brown’s description of the process and the place in which it might have occurred is worth repeating:
> In the cluster of stars in which the sun might have been born there would have been thousands or even tens to hundreds of thousands of stars in this same volume, all held together by the gravitational pull of the massive amounts of gas between the still-forming stars. I firmly believe that the view from the inside of one of these clusters must be one of the most awesome sights in the universe, but I suspect no life form has ever seen it, because it is so short-lived that there might not even be time to make solid planets, much less evolve life.
A striking view indeed, and the poets among us can muse on its transience. Brown continues:
> For as the still-forming stars finally pull in enough of the gas to become massive enough to ignite their nuclear-fusion-powered cores they quickly blow the remaining gas holding everything together away and then drift off solitary into interstellar space. Today we have no way of ever finding our solar siblings again. And, while we see these processes occurring out in space as other stars are being born, we really have no way to see back 4.5 billion years ago and see this happening as the sun itself formed.
But Sedna may help, because its orbit should be a record of what was going on when the Sun and our Solar System were in their infancy, a key to unlocking a 4.5 billion year old puzzle. The problem is that with only a single object of this kind, we wouldn’t have enough information on which to build the bigger picture, which is why researchers like Brown continue to look for other Sednas. It’s also why numerous other theories have sprung up, including the possibility that Sedna once orbited a different star and is actually an extra-solar dwarf planet. Or (an old favorite) that a brown dwarf somewhere in the Oort Cloud could have given it its nudge.
**Of Dust and the Disk**
All this reminds me of Mark Kuchner’s work on Kuiper Belt dust. Kuchner (NASA GSFC) has been running supercomputer simulations tracking the interactions of dust grains, and points to the Kuiper Belt as not only the home of countless small objects, but of dust and debris that model, though in a much older and developed way, the debris disks around Vega and Fomalhaut. At stake is how dust travels through the Solar System, affected by the solar wind and pushed by sunlight, not to mention the effects of collisions between icy grains themselves.
Kuchner’s team has been able to create infrared simulations of the Solar System as it might be seen from another star, using models of dust generation that could reflect what the condition of the Kuiper Belt was in a series of time frames going back in steps to 15 million years ago. The simulations show that a broad dusty disk like today’s collapses into a dense ring as we go back in time, producing something similar to the rings we’ve found around other stars. But today’s belt is still active. “[E}ven in the present-day solar system,” says Christopher Stark (Carnegie Institution for Science), “collisions play an important role in the Kuiper Belt’s structure.”
Interestingly for our model of dust in the outer system, Neptune’s gravitational effects push nearby particles into preferred orbits, creating a clear zone near the planet and dust enhancements that precede and follow it around the Sun. Kuchner calls this ‘carving a little gap in the dust.’ Our picture of dust in planetary systems is developing, but it’s worth noting how much work we have to do to anticipate the effects of dust on fast-moving spacecraft as we push past the heliopause and into true interstellar space. And Sedna’s odd orbit reminds us how much awaits discovery in our own systems’ furthest reaches.