The solar system consists of our planet Earth as well as all of the other objects which orbit around our Sun – the other terrestrial or “rocky” planets (Mercury, Venus, and Mars), the asteroid belt, the gas giants and ice giants (Jupiter, Saturn, Uranus, and Neptune), the Kuiper Belt objects (including Pluto and Eris), and, even further out, the small, frozen comets and debris of the scattered disk and Oort cloud. Just how large this system really is has been a matter of debate. Earth is 94 million miles from the Sun; Pluto, at the farthest point in its orbit, is 4.6 billion miles, and other Kuiper belt objects have known orbits twice as great as that.
THE SUN
At the centre of the solar system is our Sun, a stable yellow dwarf star which is approximately 5 billion years old, and probably has about that much time again left before it runs out of fuel and puffs up into a red giant. As with all stars, the Sun generates its massive quantities of heat and light through the fusion of hydrogen atoms into helium in its core. In one way or another, the climates and features of all other planets ultimately depend upon how much of this solar radiation they receive and absorb.
Despite this brief description, the Sun is actually extremely complex. All of the hydrogen fusion occurs in the core, a ball of plasma heated to many millions of degrees; almost a million miles farther out, the surface is actually comparatively cool and even prone to dark, cool areas, or “sunspots.” A variety of scientific research missions track various facets of the life of this star, including NASA’s Solar Dynamics Observatory, an orbiting probe launched in early 2010.
ROCKY (“TERRESTRIAL”) PLANETS
The first four planets share one feature in common: they are mostly composed of rock. Mercury, Venus, Earth and Mars are also relatively close to the sun: Mars is 1.5 times Earth’s distance from the Sun, but the next planet, Jupiter, is over 5 times the same distance. Beyond this, however, they vary considerably. Sometimes the rocky planets are referred to as terrestrial planets, referring to their similarity to Earth.
Mercury is so close to the Sun that it is tidally locked, the same process our own Moon has undergone relative to Earth (no matter where it is in its orbit, it always presents the same face to us, and Mercury always presents the same surface to the sun). Moreover, most of its marginal atmosphere has also been drawn away by the Sun, leaving its surface with only tenuous protection from solar radiation and from meteroids. As a result, the surface of Mercury looks more like the surface of the Moon than it does any other planet. NASA’s Messenger space probe will reach Mercury in several years’ time and give us a better idea of this strange inner planet.
Venus and Earth each possess thick atmospheres, although the results have been dramatically different. Venus is permanently encircled by thick, toxic clouds, and its surface temperatures are usually measured in hundreds of degrees. It also turns extremely slowly, perhaps due to some cataclysmic collision in the past: a day on Venus lasts 243 Earth-days, but a year on Venus is only 225 Earth-days. Climate and surface conditions on Venus are akin to something well beyond a worst-case scenario for greenhouse effect-based global warming here on Earth. Relatively few space probes have been sent to Venus, and a series of landers successfully deployed by the Soviet Union typically survived less than an hour on the harsh surface.
For this reason, we often find Mars friendlier than Venus, even though the fabled “red planet” is farther from the Sun, smaller than Earth or Venus, has a thinner atmosphere, and is prone both to extreme highs and extreme lows in terms of temperature. Helpfully, the Martian day is approximately the same as an Earth day (at about 24 hours and 37 minutes), although its year is twice as long. Many NASA probes are currently in orbit or meandering along the dry, desert-like surface, like the successful Spirit and Opportunity probes. At the moment, Mars holds such fascination for scientists because research indicates it used to have much more water on its surface, and with that water may have come life, in the form of primitive micro-organisms at least.
MOONS IN THE INNER SOLAR SYSTEM
Among the rocky planets, only Earth and Mars have moons – orbiting objects of their own. Earth’s lunar companion is the sixth-largest moon in the solar system, and its meteor-scarred surface has greeted nighttime observers for the entirety of human history. Right now and for the foreseeable future, the moon is the only other celestial body which has ever been visited by human beings – NASA’s Apollo astronauts, during the late 1960s and early 1970s. Our moon – like most moons – has no substantial atmosphere of its own, and therefore nothing to protect it from scores of meteorite impacts.
Mars, by contrast, has two moons, but both are very small: Deimos and Phobos. Neither is large enough to have compressed into a spherical shape (which results from gravitational forces when objects grow to sufficient size). A Russian space probe is currently expected to visit Phobos within the next decade and return with a soil sample.
THE ASTEROID BELT
Between Mars and Jupiter lies a massive, scattered population of asteroids and other small bodies. Early theories suggested that a planet once lay in this orbit, which was destroyed through some sort of catastrophic event several million years ago. Today, the more generally accepted theory is that a planet simply never formed in the first place, perhaps due to orbital disruptions caused by Jupiter. Indeed, despite the massive gray belt of particles usually depicted in maps of the solar system, the majority of space in the Asteroid Belt is actually empty at any given time, and most of the mass of the objects in this region can be found in four large bodies known as “minor planets”: Ceres, Vesta, Pallas, and Hygiea.
The largest of these, Ceres, was first discovered by astronomers in 1801 (before Neptune and Venus), and originally classified as a planet in its own right. NASA’s Dawn probe is scheduled to visit the 600-mile-wide Ceres in 2015, after first travelling by Vesta. The inner rocky planets, during their own formation, probably once resembled spheres similar to Ceres today, except that whereas they would have been frozen, this minor planet has long since frozen solid.
JUPITER
Jupiter is the innermost of the enormous gas giants, and the largest planet in the solar system. Although obviously much smaller than the Sun, it weighs more than all of the other planets put together and has a diameter 11 times that of Earth. Like the sun itself, Jupiter is made up mostly of hydrogen and helium, though it probably has a small rocky core, somewhere deep inside. The atmosphere is an exceptionally violent place, with bands of storms perpetually circling the planet. One of these bands, in the south, is home to an enormous storm known as the Great Red Spot, the size of several Earths put together. Despite its size, but because it is mostly made of gas, a day on Jupiter lasts just ten hours. Several probes have flown by Jupiter, including both Voyager spacecraft, but more recently it has been studied in detail by the Galileo probe, and soon NASA will send another probe, Juno, towards the giant planet.
Although less massive and diverse than Saturn’s moon system (up next), Jupiter currently hosts 63 known satellites, three-quarters of which are very small and have been discovered only since space probes and new telescopes came online during the 1970s. The four largest and best known moons were discovered by Galileo centuries ago: Io, Europa, Ganymede, and Callisto. Each are distinctly different, and all are larger than our Moon. Io is exceptionally volcanic, spouting off molten gas off its surface and into orbit; Callisto is an exceptionally old ice formation; and Ganymede is the only moon to possess its own magnetic field. However, Europa has become most interesting to scientists because it appears to be covered in a deep ocean of liquid water below its frozen surface.
SATURN
Out another billion miles (almost as far again as Jupiter is from the Sun) lies the planet Saturn. The second of the gas giants, Saturn also consists mostly of hydrogen and helium, with a tiny rocky core. However, it is best known for its bland atmosphere (wind speeds are higher than Jupiter, but it lacks the distinctive bands of storms on the larger planet) but also for its enormous and distinctive ring system. The beautiful rings are surprisingly thin – perhaps 70 feet thick in places, and mostly composed of ice – and were probably formed either out of the same early nebular material that also formed the planet below, or from the debris of destroyed moons. The Cassini spacecraft reached Saturn in 2004 and has studied it in detail for the past six years.
Saturn has 62 known moons, of which perhaps a dozen are large enough to be of astronomical significance. The largest of these is Titan, which is larger than Mercury, nearly the size of the planet Mars, and the only moon to possess its own thick atmosphere and liquid pools on its surface. Far larger than our own Moon, Titan has a strikingly Earth-like surface and an atmosphere made up of methane and nitrogen. This has led to speculation that Titan could one day support life, although the very cold temperature and its great distance from the Sun mean that this will probably never happen, at least until the Sun heats up and expands in the next several billion years. The Cassini probe landed a probe on Titan which garnered immense new information about this important moon.
URANUS
Uranus is the third gas giant, located twice as far from the Sun as its closest neighbour, Saturn. (As readers may be concluding, distances in the outer solar system tend to be considerably vaster than those in the comparatively cramped inner solar system.) Like its other neighbour, Neptune, Uranus has a much higher ice content due to its greater distance from the Sun, which has led to these two planets being colloquially referred to as the “ice giants,” rather than gas giants per se. Uranus is also the coldest planet in the solar system, with its coldest regions just 50 degrees above absolute zero. This unusually low temperature is not well understood: the three other gas giants are noticeably hot; in fact, so hot that they actually produce more heat than they receive from the Sun.
The answer may lie in the planet’s strange past. Unlike all other planets, Uranus orbits virtually tilted over on its axis (in other words, the north pole points nearly directly at the Sun). This presumably resulted from an enormous impact in its distant past, which may have accounted for most of the loss of heat as well. It is also possible that the temperature difference is due to some sort of atmospheric condition. Much less is known about the ice giants than about Saturn or Jupiter, but Uranus is believed to be made up of three layers: a rocky inner core about the size of Earth, covered by a thick mantle of ice, and finally the hydrogen-helium atmosphere characteristic of all gas giants, all of which amounts to about 14 times the mass of Earth. We will know more if a probe ever visits Uranus again; unfortunately, we have not got a close look at the planet since the Voyager probes sped by nearly thirty years ago.
Uranus has a ring system, though it is much thinner and darker than Saturn’s. It also has 27 known moons, including Ariel, Titania, and Oberon. None of these moons is known to have any particular astronomical significance, although like Uranus itself, none have yet been the subject of close-up study.
NEPTUNE
Neptune is even farther out, as far from Uranus as Uranus is from Neptune – and 30 times as far from the Sun as we are, on Earth. Discovered in the 1840s, Neptune was first confirmed to exist by mathematicians studying perturbations in planetary orbits rather than by astronomers, who only later found the planet itself. The Neptunian day is difficult to calculate because of the dynamics of its atmosphere: the core (with its magnetic field) experiences a 16-hour day, but farther out from the core, within the atmosphere, the day ranges from 12 hours at the pole to 18 hours at the equator.
Neptune is characterized by its bright blue atmosphere, resulting from high amounts of methane in the upper atmosphere; other than this, the planet is fairly similar to Uranus in terms of its chemical makeup. Unlike Uranus, however, the atmosphere is highly active, more like Jupiter’s – to the point that a massive southern storm, the Great Dark Spot, was spotted by Voyager 2 in 1989. With the recent demotion of Pluto to minor planet status, Neptune is the outermost planet in the solar system.
Also like Uranus, Neptune possesses a ring system, but one much less noticeable than Saturn’s. Unlike Uranus’s dark system, Neptune’s rings are mostly reds, due to the presence of ice and silicates. They are also extremely unstable, and will likely show significant degradation over as little as the next century. It also has 13 known moons, the largest of which is the massive Triton, probably originally a dwarf planet in its own right (like Pluto) which was captured by Neptune’s gravity and pulled into orbit. For this reason, Triton’s orbit is actually spiralling gradually inward, until in several billion years it will be torn apart.
KUIPER BELT and KUIPER BELT OBJECTS (KBOs)
Until recently, this overview would have included a section for the ninth planet, Pluto. However, Pluto has always been an outlier among planets: discovered only in the 20th century, it was far smaller than the gas giants – smaller than Earth’s moon, in fact – and followed a most unplanetlike elliptical orbit, dipping within Neptune’s orbit at its closest to the sun but then swinging far out beyond any of the other planets on a more than 200-year trip around the sun. The discovery of a more distant object that was larger than Pluto, Eris, required astronomers to do so some linguistic shifting around. Today, Pluto and several other large bodies are classified as “dwarf planets.” Pluto and Eris in particular are also classified by the region in which they are found – the Kuiper belt. Eris is a member of an even more distant group of objects – the scattered disk.
The Kuiper Belt consists of small bodies similar to the closer-in Asteroid Belt, except that they are spread from roughly Neptune’s orbit out to about 55 times Earth’s distance from the sun. Perhaps a hundred thousand objects of substantial size can be found in this region, although only a small percentage have ever been sighted conclusively. Occasionally these objects are caught and spun inwards by the gravity of the gas giants, forming asteroid groups like the centaurs, or even becoming comets.
Pluto is about one-third the size of Earth’s moon and has three moons, Charon, Nix and Hydra. It is believed to possess an atmosphere for part of its orbit – vapour rising from the evaporating surface, heated by the Sun during the inward leg of its highly elliptical orbit, which freezes and falls back onto the surface during the outward portion of Plutos’ orbit. Until NASA’s New Horizons space probe speeds by Pluto in 2015, the best images of Pluto are a small number of very low-resolution images from the Hubble Space Telescope, showing a surface of mixed orange, white, and black. Its orbit is 250 years; one day on Pluto lasts roughly six days on Earth.
Two other dwarf planets are known to exist in the Kuiper Belt, Haumea (discovered in 2004) and Makemake (discovered in 2005). Both are smaller and colder than Pluto, and neither has a moon.
SCATTERED DISK
The scattered disk lies beyond the Kuiper belt, between about 30 and 100 times as far from the Sun as the Earth is. Objects in this region are dark and cold by nature, and therefore difficult to detect from Earth. However, a hundred objects have been located here, including Eris, a dwarf planet larger than Pluto. Eris is probably uncharacteristic of scattered disk objects, given its size; by the same token, given its size, it was the easiest to detect, and its existence was announced in 2005. It was the discovery of Eris which provoked the debate over Pluto’s own planetary status. It also has its own small moon, Dysnomia.
OORT CLOUD
The most distant reaches of the solar system are the clouds of comets believed to exist in the Oort cloud, stretching as much as a light-year from the Sun (50,000 times as far as we are on Earth). The Oort cloud is believed to be the origins of comets such as Halley’s comet as well as several asteroids and comets pulled into the solar system by Jupiter’s gravity well. Because of its vast distance and the relatively small size of any objects here, the Oort cloud currently exists more in theory than in fact. One recently discovered dwarf planet, Sedna, is the farthest known object from the Sun (at 76 times the distance that Earth is from the sun), and may in fact be a member of the Oort cloud, though it may simply be a particularly distant member of the scattered disk population.
Beyond the Oort cloud, no orbiting objects are theorized to exist. This far out, the Sun’s gravity would likely be too weak to capture objects for any consistent period of time. This would truly be the end of the solar system.
