Essentially, the real size of the solar system depends on which theories you choose to accept, although the most distant known object, the dwarf planet Sedna, may travel as far as 90 billion miles from the Sun before looping back inwards on a highly elliptical orbit. Astronomical distances are usually measured relative to the speed of light: it takes light from our Sun eight minutes to reach Earth, four hours to reach the most distant true planet, Neptune – and four years to reach the next-closest star, Proxima Centauri. Within our own solar system, however, a more striking measure of distance may be the Astronomical Unit (AU): 93 million miles, or the distance between the Sun and the Earth. Just how big the solar system is depends on where you choose to mark the edge.
ONE AU: EARTH
This is our position, located among the rocky or terrestrial planets within the inner solar system. Significantly much closer to the Sun lie a pair of other rocky planets without moons, scarred Mercury and scorched Venus. Another 0.5 AU (or 50 million miles) out lies the red planet, Mars, with its two moons, Deimos and Phobos.
30 AU: NEPTUNE
The last of the gas giants, Neptune, orbits at 30 AU – that is, 30 times as far from the Sun as the Earth does. As far as true planets go, this is the end of the solar system (at least since Pluto was demoted to dwarf planet status several years ago). Humans have never sent a space probe to Neptune, although the Pioneer 10, Pioneer 11, and Voyager probes have travelled well past it now, and New Horizons will also zip by in a few years’ time, en route to Pluto. Light this far out is feeble indeed. Beginning with Jupiter, orbiting at just 5 AU, the gas giants actually give off more heat into space than they receive from the Sun in the form of sunlight (and will presumably continue to do so for billions of years).
29-49 AU: PLUTO
Pluto has a troubled past: only discovered during the twentieth century, this 700-mile-wide sphere was considered a planet until the recent discovery of the even-farther-out Eris, slightly larger than Pluto, forced both objects to be demoted into a separate category, the “dwarf planets.” In several years, the New Horizons space probe will reach Pluto; when it does, this planetoid will become the most distant target to which we have ever sent a spacecraft.
At this distance from the Sun, sunlight is a virtual irrelevancy. Pluto is so cold that as it heads onto the outward leg of its highly elliptical orbit (as it is doing right now), its atmosphere literally freezes and falls to the ground as ice. In a century or so, it will round the curve, head back into the outer solar system, and its atmosphere will reform. We should learn more about this strange phenomenon from the New Horizons probe.
30-50 AU: KUIPER BELT
The Kuiper Belt is similar in composition to the Asteroid Belt, although spread over a much greater area. The vast majority of bodies here are quite small, although the vast size of the belt is sufficient that there are likely hundreds of thousands of bodies of significant size, could we only detect them at this great distance. Several of these are large enough to be classified as dwarf planets, like Pluto; for example, scientists recently discovered Haumea (at 43 AU) and Makemake (at 46 AU). To be a dwarf planet, a Kuiper Belt object must be large enough for its gravity to begin collapsing it into a roughly spheroidal shape. Several other recently discovered objects, like Quaoar and Orcus, may also be large enough to qualify.
No human spacecraft has ever been specifically sent to the Kuiper Belt, although the Pioneer and Voyager probes have passed through it on their trajectories out of the solar system.
35-100 AU: SCATTERED DISK
The scattered disk is another region of varied small objects, except that it is even more spread-out and even less densely filled than the Kuiper Belt. Many comets originate here. However, this part of the solar system is so distant, so cold, and so dark that it is extremely difficult to detect objects, even if they are of significant size. In 2005, the Palomar Observatory announced that it had discovered a new planet-sized object, Eris, in this region. It was this discovery of Eris, over 100 miles wider at its equator than Pluto, which prompted the planet debate and the eventual demotion of both objects to dwarf planet status.
As we speak, Pioneers 10 and 11 are located within the Scattered Disk, with Pioneer 10 poised to leave the region within the next several years. Neither spacecraft is operational, so neither has contributed to our understanding of this region. The Voyager probes have passed through the Scattered Disk entirely, operating with only minimal instrumentation.
80-180 AU: HELIOSHEATH
At some point, the solar wind – the stream of radiation emitted by our Sun – weakens to the point that it can no longer push back the free-floating hydrogen of the interstellar medium. The precise point at which this occurs, theoretically known as the heliopause, has not been specifically identified, and probably varies over time as the Sun grows and shrinks minutely in intensity. The Voyager 1 and Voyager 2 probes are both located within this region and their minimal remaining instrumentation is being used to study the heliosheath.
50,000 AU: OORT CLOUD
Beyond the Scattered Disk lies a theoretical population of small asteroids and other bodies which have been captured by the Sun’s gravity but are too far, too cold, and too dark to be detected by human-operated telescopes. Certain objects, like comets, are believed to have originated here before being gradually pulled into the inner solar system by gravity. Where the Oort Cloud stops is similarly more a theoretical than an observed limit: effectively, the end of the Oort cloud would be the limit beyond which the Sun’s gravity was incapable of pulling physical objects into orbit, perhaps as much as one light-year, or 50,000 times the distance the Earth is from the Sun.
Several extremely distant planetoids classified as Scattered Disk objects may actually be Oort Cloud objects. In particular, Sedna is currently located at a distance of 90 AU, well within the Scattered Disk population; however, scientists think this is close to the innermost point of its elliptical orbit, and that over a Sedna year (about 12,000 Earth years) it may reach as far as 1000 AU from the Sun. This would take it well beyond the boundary of the Scattered Disk, into the area of the theoretical Oort Cloud.
NEMESIS?
Some scientists have postulated that the Sun possesses a companion star, a red dwarf or perhaps a brown dwarf, which orbits at extreme distance but flies through the Oort Cloud every 26 million years or so, perturbing the objects there and sending a cometary shower into the inner solar system. No physical evidence of the existence of Nemesis has ever been found. However, given what we know about other stars in our galaxy, our Sun does seem to be in the minority in that it does not have a known companion star.
If Nemesis exists, it should be bright enough to be located by several upcoming deep-space survey missions, such as Pan-STARRS and WISE.
