Trans Neptunian Space: Pushing the Boundaries of the Solar System
The breathtaking images taken of Pluto by NASA's New Horizons probe are giving Earthbound eyes an historic look at this mysterious little world – and reminding us just how little we know about the outer reaches of the Solar System.
Until fairly recently, every school child learned that the Solar System consisted of just nine planets, ending with Pluto. But advances in earthbound imaging and the launch of probes such as New Horizons reveal that the Solar System stretches far beyond the orbit of Pluto – and these regions, called Trans-Neptunian Space, are dynamic and densely populated.
Neptune represents the boundary of major planetary development in the early days of the solar system. Out past this blue gas giant, a variety of objects large and small circle the Sun in three distinct but overlapping "bands" of space. The dwarf planets, asteroids and other bodies that populate the zones of Trans-Neptunian Space are mostly remnants of protoplanetary material that failed to coalesce. Because these strange contain some of the most ancient matter in the Solar System, astronomers believe they may hold the keys to understanding the origins of the System and of life on Earth.
The International Astronomical Union's Minor Planets Center calculates that Trans-Neptunian Space is home to over 700,000 objects of various kinds, many of which are large enough to have names. The best known of these is of course Pluto, that distant body that was famously first a full-fledged planet, then a dwarf planet, then a planetoid. Pluto and a few other dwarf planets are large enough to have moons of their own, and nearly all of them orbit the sun in the Kuiper Belt, the region closest to Neptune.
The Kuiper Belt: Our Nearest Neighbor
Closest to the sun, the Kuiper Belt is home to a large number of small objects, which are divided into two different populations. Astronomers think that the Belt's "dynamically cold' objects probably formed where they are, and their orbits remain stable and circular. The "dynamically hot" Kuiper Belt Objects, though, are thought to have been formed closer to the inner planets and were forced out into the Belt by the gravitational effects of Jupiter.
The Scattered Disk: Home of Short Period Comets
Farther out, the Scattered Disk overlaps the Kuiper Belt and stretches far beyond it – a region populated with (as the name implies) scattered icy objects with unstable, elliptical orbits that can carry them far into the inner solar system as short period comets. These ancient objects include the famous Halley's Comet and C67P-Churyumov-Gerasimenko, the target of the European Space Agency's recent Rosetta Comet Mission.
The Oort Cloud: Gateway to the Galaxy
But the outer regions of the Solar System stretch even farther. Beyond the Scattered Disk, the Oort Cloud hangs between the Solar System and interstellar space. The Oort Cloud contains a population of small icy objects believed to be remnants of the Solar System's original protoplanetary disk, forced outward as the inner planets formed. The Cloud is home to long period comets, those whose orbits bring them into the inner Solar System on cycles of many hundreds to thousands of years.
This far from the Sun, the Oort Cloud is subject to influence from the gravitational pull of nearby stars and the core of the Milky Way galaxy itself. Recent research indicates that the largest impact craters on Earth were probably caused by long period comets, pushed out of the Oort Cloud by the "galactic tide" – the gravitational pull of the Milky Way's disk as the Solar System travels across it.
Trans-Neptunian Space looks very different than it did in the days when telescopes on Earth could catch no more than blurry glimpses of Pluto alone against the blackness of space. New imaging technology and real-space exploration of the outer reaches of the Solar System reveal a rich, dynamic region whose ancient objects link our Earth to the very heart of the Milky Way.