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Enchanted with the Outer System

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.


Comments on this entry are closed.

  • Adam November 1, 2010, 8:00

    Eduard Drobyshevskii’s Close-Binary Cosmogony (The origin of the solar system – Implications for transneptunian planets and the nature of the long-period comets ) has about 1,000 Moon-mass objects being cast into the Oort Cloud by Jupiter’s failed formation as the Sun’s binary companion. Thus the Oort could hold a multitude of potentially terraformable objects – if we could just spot them in the Deep Dark.

  • kurt9 November 1, 2010, 12:11

    The Kuiper belt starts at around 30 AU and has a “continental shelf” at around 50 AU. No one knows why this is so. The Kuiper belt may have 100,000 objects with a 100 km or greater diameter. Contrast this with the asteroid belt with less than 10 objects of such size.

  • Stan Clark November 1, 2010, 13:07

    It occurs to me that we should be able to spot other ‘Oort’ like clouds around other stars. Has this happen yet?
    Also what effect would the Galictic ‘winds’ have on the shape of the Oort Cloud, has modeling this effect been tried?

    The more you learn the more you need to learn.

  • Michael November 1, 2010, 13:39

    Maybe Sedna an orphan world from another star system when ours was still in the birth cluster

  • bigdan201 November 1, 2010, 14:29

    The thought of an earth-sized object just outside the kuiper belt is an exciting one. The perturbation theory sounds good, but it should affect other Sedna like objects too. You mentioned the possibility of a brown dwarf in the Oort Cloud – was this referencing the Nemesis theory? That theory could also explain gravitational anomalies in the Kuiper belt.

    Not to get ahead of things, but if there is a frozen earth outside the Kuiper belt, that would make a very attractive target for a future mission.

    I also can’t help thinking that all these distant bodies and dust are going to make an interstellar mission more complicated. The kuiper belt should be easy enough to avoid, but the Oort cloud totally surrounds the sun and extends out to 1 light year – surely this will make things more difficult. Then again, it might be very diffuse, allowing us to plot a course through it – but there’s still dust to worry about.

  • Scott G November 1, 2010, 14:54

    This article (particularly the parts about the Sun’s birth cluster) reminded me of something I read in Scientific American. Here’s a couple of links — (1) http://www.scientificamerican.com/article.cfm?id=the-long-lost-siblings-of-the-sun (2) http://www.scientificamerican.com/article.cfm?id=solar-twins-search-galaxy — one includes an artist’s impression of what the night sky may have looked like before the cluster dispersed (very cool!). Also, (if I recall the full article correctly) some researchers think it is possible to track down some of the Sun’s siblings based on comparison of chemical properties and simulations describing the dispersal of the cluster as it orbited the galaxy.

  • Adam November 2, 2010, 2:58

    Relative velocities of stars from a common birth nebula are typically ~1 km/s, though that could be significantly perturbed after a few galactic orbits. At that speed once around the Galaxy takes ~49 billion years. A few stars from that same cluster should be travelling quicker – about 11 km/s means the Sun and the other star should be in each other’s company again about now.

  • Michael Spencer November 2, 2010, 7:41

    Thanks for the link to Mike’s site–what an appealing guy! He’s quite enthusiastic and, like our host, possessed of compelling writing skills that are rooted firmly in years of contemplating this fascinating subject.

    And he keeps coming back to the simple notion that if Sedna were bumped at some point, it must keep returning to the scene of the crime. Simple physics, lost down a sea of years.

  • tesh November 2, 2010, 8:32

    I am sorry if this is a lazy/naive (both) question but how many orbits has the Sun made around the Milky Way? A follow on from that, is the Sun likely to “revisit” the place of its birth or will the sun have gone super nova before that?

  • kzb November 2, 2010, 8:48

    I’ve been reading a few papers on planet formation models recently. These assert that the majority of planets formed become ejected from their natal systems by planet-planet interactions. Many are also captured by close encounters with other stars in the star-forming cluster.

    So perhaps Sedna is captured from another system, back when the sun and its fellow cluster members had just formed.

    Alternatively (and I find this fascinating), if the majority of planets are ejected, that means there are a lot of free-floating planets out there. Perhaps there are more encounters with such objects than we think.

  • Adam November 2, 2010, 16:40

    Hi tesh
    Not a lazy question. The Sun takes about 225-250 million years to go around the Galaxy, thus it has completed about 20-18 orbits since it formed. There’s some uncertainty as to just how fast it orbits, which is why I’ve quoted a range of values. The distance to the Galactic centre is uncertain to within about 1,500 light-years, which makes the Sun’s orbit difficult to constrain. The best estimated distance is 8,000 parsecs (26,000 light-years), but it could be up to ~8,500 or as low as 7,500. New techniques for determining the distances are reducing the uncertainty, but that’s still “work in progress”. We might have a better idea in a few years time.

  • tesh November 3, 2010, 3:49

    Thank-you Adam.

  • Adam November 3, 2010, 5:20

    One modifier of what I wrote above is the range of estimates surprisingly doesn’t affect the Sun’s orbital period very much – the ratio of Vo/Ro is from 29.9 – 31.6 km/s/kiloparsec. Pretty narrow range. Thus the orbital period is ~200 million years for all the varying values of Ro. Thus the Sun has made ~ 22-23 galactic orbits during its life.