A Guide to the Solar System

According to NASA our solar system has 8 major planets, approximately 150 moons, a handful of “dwarf planets” (Pluto and Eris, for instance), and millions of observable objects like comets and asteroids.  Fathoming how expansive this solar system is can be a tough feat — Neptune is 3 billion miles away from the sun, and as the farthest planet its orbit is used as a conservative estimate for the breadth of the entire solar system. The gravitational influence of the sun’s mass extends much further than Neptune, however.  If you take into account the cloud of debris that populates the outer reaches of this influence, the size of the solar system can then be estimated at 100,000 astronomical units (AU). To put that in perspective, one AU is 93 million miles!

Although the distances between objects in space are great, there are plenty of other strange phenomena for scientists to explore in our solar system. This resource provides a brief introduction to history of the solar system’s exploration, discusses its extent, and highlights the eight official planets that scientists know very well today. Throughout the text are links to highly credible and comprehensive resources that delve into subtopics with great detail. This is a primer to the large body of research that is collectively conducted by NASA, the Russian Federal Space Agency, the European Space Agency, and other respected organizations.

Discovery and Exploration

Some notion of the solar system has existed for as long as human beings have been observing the sun and planets, the latter of which appeared as stars that moved across the sky. The Babylonian and Sumerian civilizations have the earliest recorded observations of the four terrestrial planets (see below), plus Jupiter and Saturn. Galileo’s telescopic observations in the 1600s provided the first definitive proof that the planets orbited the Sun, implying strongly that the Earth itself was a planet and was doing the same. A telescope was also responsible for the discovery of Uranus by Sir William Herschel in 1781. Neptune, on the other hand, was discovered by pure extrapolation by Alexis Bouvard in 1846 by observing its gravitational effect on Uranus. The first close-up images of all the planets were provided by the Voyager, Pioneer, Galileo, Venera probes launched from the 1960s to the 1990s. There have been several unmanned expeditions to Mars in recent years.

The Structure of the Solar System

Although nearly all scientists agree that solar system is at least as large as the outermost orbits of Neptune, the place where the solar system ends and interstellar space (the area between star systems) begins is more ambiguous. Objects similar in size to Pluto, such as Eris and Makemake, occupy a large ribbon of debris beyond the orbits of the farthest planets called the Kuiper Belt.

Solar winds extend beyond the Kuiper Belt and terminate at the Heliopause, the border between the interstellar medium (gas and tiny particles) and the sun’s influence. The hypothesized Oort Cloud has billions of icy objects similar to comets that are held in place by the sun’s feeble gravity; though unobserved, it is thought to exist beyond the explored solar system for over a light year. The shape of either the Oort Cloud or the Heliopause are often said to define the overall structure of the solar system.

The Planets

The following terrestrial planets are the smallest and rockiest since Pluto, previously the most distant planet, was downgraded in status to a dwarf planet. They are given in order of their distance from the Sun:

• Mercury is the smallest planet in the solar system and is known for its wild temperature swings. Without a substantial atmosphere to retain heat, the temperature can range from 800 degrees Fahrenheit on the side facing the sun to 280 degrees below freezing on the side facing away.
• Venus, closest to the Earth in size and somewhat closer than Mars, has a thick atmosphere of mostly carbon dioxide that causes an out-of-control greenhouse effect, trapping so much heat from the sun’s rays that the temperature of Venus is high enough to melt lead.
• Earth, besides the obvious distinction of housing all life as we know it, is also notable for its oxygen-rich atmosphere and massive amounts of liquid water, two constituents of life as scientists currently understand it.
• Mars, the Red Planet, has received the most attention from space agencies due to both its proximity and similarity to Earth. Nevertheless, Mars is still quite hostile to life since temperatures can drop to 200 degrees below zero Fahrenheit, the atmosphere is a thin layer of carbon dioxide, and frequent dust storms can last for months and cover the entire planet. Olympus Mons, the tallest mountain in the solar system at three times the height of Mt. Everest, is located on Mars.

The remaining worlds are the gas giants, or Jovian planets, also listed in order of their distance from the sun. Between Mars and Jupiter is an asteroid belt that is home to some other large objects in the solar system that are of interest to scientists.

• Jupiter is the largest planet in the solar system, possessing a thick, crushing atmosphere that blocks any view of the supposedly solid core. The Great Red Spot that marks the outer surface of Jupiter is a massive storm large enough to contain Earth within it and has lasted for centuries. Jupiter’s massive gravity serves somewhat as a magnet for comets and asteroids, protecting the Earth. One of the planet’s larger moons, Europa, exhibits evidence that it may contain liquid water beneath a frozen surface and thus also contain life.
• Saturn’s rings are comprised of dust, rocks, and ice particles and are the most striking in the solar system (the other gas giants’ rings are much thinner). Like Jupiter, Saturn has a large number of moons, one of which, Titan, has a thick atmosphere.
• Uranus is an almost featureless blue sphere comprised of hydrogen and helium with traces of ice throughout its atmosphere. John Flamsteed discovered it in the late 1600s, although Hershel recognized it as a planet. Uranus has numerous moons, many of which exhibit highly irregular orbits.
• Neptune, since Pluto was redefined, is the farthest planet from the sun and is marked by two characteristics: the strongest winds in the solar system (which nearly break the sound barrier) and the staggering amount of meteorological activity, the largest storm being the Great Dark Spot identified by both Voyagers 1 and 2. At times, Neptune’s eccentric orbit takes it farther away from the Sun than even Pluto.