In 1930, Clyde Tombaugh, using the telescopes at Lowell Observatory in my adopted hometown of Flagstaff, AZ, noticed something moving in the outer reaches of the solar system. Newspaper headlines trumpeted the discovery of Pluto, the ninth planet! But while the general public grew accustomed to a new addition to our planetary family, astronomers debated the distant world’s true nature. Pluto’s planetary status was questioned almost immediately. It’s diminutive size and odd orbit made it a black sheep among the other eight worlds. Was this really a planet or was this simply a peek, the first hints, of a grander population of icy debris beyond the orbit of Neptune?
Throughout the 20th century, astronomers hypothesized about the existence of such a debris field left over from the formation of the solar system. At that distance from the sun, the material in the protostellar disk from which the planets coalesced would have been too thinly spread out to form anything of consequence. All that would remain from our solar system’s formative years would be an a icy junkyard of planet building material. Furthermore, the continued existence of comets was a bit of a mystery. Every time a comet comes close to the sun, a fraction of its icy bulk is blown off into space. After many such passages, all the comets should have long ago sublimated into faint whispers of their former selves. The fact that we continue to see comets means that there must be a source from which they are continually replenished. A repository of icy debris beyond Neptune would make an ideal candidate for just such a source.
In 1992, American astronomer David Jewitt, using telescopes on the summit of Mauna Kea on Hawai’i, discovered the first such object. Provisionally named (15760) 1992 QB1 – a mouthful of a moniker that has unfortunately stuck – it suggested that there was indeed a host of bodies beyond Neptune of which Pluto was only one. Since then, over one thousand objects have been found out past Neptune in a region of our solar system now called the Kuiper Belt.
Named after Dutch astronomer Gerard Kuiper, who hypothesized that such a belt formed in the solar sytem’s earliest years, the Kuiper Belt is a doughnut-shaped region of space just beyond the orbit of Neptune filled with detritus composed predominately of water, ammonia, and methane ices. Similar to, though significantly larger than, the asteroid belt between Mars and Jupiter, the Kuiper Belt extends roughly 5 billion miles from the sun – fifty times larger than the orbit of our planet. Out here, the sun is a distant, pale light 1500 times fainter than it appears to us (though still 250 times brighter than a full moon). The temperature barely gets past a chilly -370 degrees Farenheit: just fifty degrees above absolute zero. Astronomers estimate that some 70,000 small icy bodies live out here of which Pluto is the largest and most famous. Despite being spread over a region 1.5 billion miles across, the total amount of mass in the Kuiper Belt is roughly 1/25 – 1/10 that of Earth. And yet, it is out here that we see the remnant scars of a much more chaotic time in the solar system’s past.
How the Kuiper belt came to its current configuration is still hotly debated amongst astronomers though there seems to be wide agreement that Neptune is to blame. Both Uranus and Neptune have always been oddities in our understanding of the solar system’s formation. At roughly 14 and 17 times the mass of Earth, respectively, the primordial disk of ice, rocks, and gas would not have been a dense enough source of material from which these two sentries of the outer solar system could have formed. Therefore, they most likely did not form in their current orbits but migrated there from a denser part of the protosolar nebula.
Extensive computer simulations suggest a chaotic development of the outer solar system, one where all four of the giant planets actually formed much closer to the sun. Over time, interactions with the debris that did not get locked into planets forces the giants to slowly drift out away from the Sun. At some point roughly 4 billion years ago, Jupiter and Saturn get locked into what is called an orbital resonance: Saturn orbits the sun exactly once for every time Jupiter goes around twice. Such a resonance essentially creates a recurring gravitational pulse on Uranus and Neptune that sends them careening into the outer solar system. As Neptune’s orbit widens, it plows through the icy debris field, scattering objects to and fro. Some of them fall in towards the sun, pummeling the young Earth along the way and some are ejected from the solar system entirely. One gets captured by Neptune and becomes its largest moon, Triton. What survives gets locked into a stable population of orbits that make up today’s Kuiper belt.
While the Kuiper belt was once hypothesized to be the source of so-called short period comets, those which visit the sun once every 200 years or less, astronomers have since realized that this can not be the case. The Kuiper Belt Objects – or “KBOs” if you want to sound like someone “in the know” – are what astronomers call “dynamically stable”; their orbits will not change over the remaining lifetime of the solar system. They orbit in such a way that Neptune does not affect them. There does exist, however, beyond the Kuiper Belt a second population of icy bodies called the Scattered Disk. While chemically similar to KBOs, the orbits of so-called SDOs are much different. While KBOs all orbit roughly in the plane of our solar system, SDOs orbit well above and below the ecliptic – some 40 degrees or so – and are in highly elongated orbits that take them some 100 times further from the sun than Earth. These do get perturbed by Neptune every so often and it is from here where the comets most likely originate.
Once thought to be a region devoid of anything interesting, the past couple of decades have revealed the edge of our solar sytem to be a complex, dynamic space filled with fossils from a more primitive time. Beyond the Kuiper belt and Scattered Disk, it is not entirely clear what remains. Astronomers hypothesize that well beyond the edge of the Scattered Disk, some 500 times further still or nearly one quarter the distance to the nearest star, Proxima Centauri, is the Oort Cloud: a spherical wall of icy refuse completely enveloping our sun. The Oort Cloud is thought to be the source of long period comets like Hale-Bopp, comets which take thousands of years to orbit the sun just once. While not directly observed, there are a handful of objects known to orbit well past the edge of the Scattered Disk. These may come from the Oort cloud or be part of a whole other family known as the Detached Objects which fill the space between the Scattered Disk and the Oort Cloud.
We now know that Kuiper-like belts are not uncommon, with many nearby stars showing evidence of a similar debris disk. Does a debris disk necessarily entail the existence of planets? Do these stars house giant planets of their own? We don’t know. Our own Kuiper Belt has locked within it tales from the earliest days of our solar system. It’s sobering to wonder what other secrets the Kuiper Belt can reveal about our own origins and whether it can tell us anything about the development of planetary systems across the galaxy.