What Do We Really Know About the Solar System?

In a very real sense, then, we do have—in meteorites and moon rocks—“witnesses” to the creation of the solar system. These geological remnants are relatively unchanged from the time the solar system was born. But how do we make up for an absence of precedents from which to draw potentially illuminating analogies? Why can’t we just go find another planetary system forming (around a star younger than the sun) and draw our analogies from it? Well, that has been one of the main goals of astronomers in the past decade or so. In fact, NASA has defined one of its primary missions in terms of this search, called the Origins program. We are just now starting to see the results of these searches. The Hubble Space Telescope, in particular, has given us tantalizing clues about the formation of planetary systems. Around the star Beta Pictoris, astonomers have imaged a disk of material larger than the orbit of the most distant planet in our solar system, Pluto. Are the inner reaches of this disk even now taking shape as planets around that star?
The truth is, even with the best instruments that we have today, we can still learn a lot more about how our solar system formed by looking closer to home. There are a number of fundamental things that we know about our solar system, and any explanation that we come up with must, at the very least, account for what we observe. Here are some undeniable facts that the last 300 years of planetary exploration have given us:

• Most of the planets in the solar system rotate on their axis in the same direction as they orbit the sun (counterclockwise as seen from the North Pole of Earth), and their moons orbit around them in the same direction.
• The planets in the inner reaches of the solar system are rocky and bunched together, and those in the outer part are gaseous and widely spaced.
• Most of the planets (with the exception of Pluto) orbit the sun in elliptical paths that are very nearly circles.
• Except for the innermost planet (Mercury) and the outermost (Pluto), the planets orbit in approximately the same plane (near the ecliptic), and they all orbit in the same direction.
• Asteroids and comets are very old, and are located in particular places in the solar system. Comets are found in the Kuiper Belt and Oort Cloud, and asteroids in the asteroid belt between Mars and Jupiter.

In addition, it is clear that the asteroids we have examined are some of the oldest unchanged objects in the solar system, and that comets travel in highly elliptical orbits, originating in the far reaches of the solar system. The most important conclusion we can draw from these observations is that the solar system appears to be fundamentally orderly rather than random. It doesn’t appear that the sun formed first, and then gradually captured its nine planets from surrounding space.
Although there are important exceptions, the counterclockwise”
(as viewed from above the North Pole of the Earth) aspect of so many properties in the solar system suggests that the planets fragmented and formed from a large rotating cloud of material. That the orbits of the remaining planets are very nearly circular suggests that the solar system has settled down, as it were. Any planets or planetesimals that were on highly elliptical orbits have been cleared out in the last 4.6 billion years. The inclined, markedly elliptical orbit of Pluto is one of the arguments for its being an escaped moon from an outer planet. The physical differences in planets that are related to their distances from the sun suggests that the sun influenced the formation of the planets; that is, the sun must have formed first.