PH 205

We live on the third planet from the Sun, in a system that contains a smallish star, a few small rocky planets, gas giant planets complete with their own systems of moons, and various smaller objects like minor planets, rocky asteroids, icy comets, meteors and even smaller bits of dust and gas.

We will begin this section of our study by taking stock of the general makeup and composition of our solar system. We will need to make sense of the stark  differences between terrestrial planets and gas giants, and their placement in our system.

We will use our observations of the solar system to build a model of how it must have evolved. Our model will eventually need to explain not only the characteristics of our own solar system, but also the characteristics of other systems of planets we have observed, orbiting distant stars. Some of these extrasolar systems exhibit traits very unlike our own, so our model will need to be very robust in explaining how these different types of systems could have formed.

This artist's conception from NASA compares what a planetary system orbiting a brown dwarf might look like in comparison to our own solar system. Since a brown dwarf star is smaller than our Sun, the planets would orbit much closer to the star.

The image also shows the main layout of our solar system, with small terrestrial planets close to the Sun and gas giants farther out, in more widely-spaced orbits. The distances are not to scale.

The above image compares the relative sizes of the planets. Note the stark contrast of the gas giant planets, compared to the much smaller terrestrial planets.

This image shows the relative size of the Sun, compared to the planets. The sun contains some 99% of the mass of the solar system, even counting the asteroids and comets as well as the planets. Jupiter is the second largest body in our solar system, and it is tiny compared to the Sun.

This video gives a nice comparison of the sizes of the planets, and continues on to show how the Sun compares in size to some other stars.

Terrestrial planets are small, rocky planets. In our solar system, the terrestrial planets are Mercury, Venus, Earth and Mars. Gas giant planets are often called "Jovian" planets, after the largest gas giant planet in our solar system, Jupiter. The Jovian planets in our solar system are Jupiter, Uranus, Saturn and Neptune. Pluto was considered a planet until Aug. 4, 2006 when it was reclassified as a "dwarf planet."

Planet tilts and spins

All the planets rotate on axes, some faster than others. The rotation axes of the planets are tilted from being perpendicular to the plane of their orbits, to varying degrees.

This NASA video illustrates the rotations and axial tilts of the planets. Notice that Uranus is extremely tilted compared to the others and that Venus rotates very slowly and in the reverse direction. Our theory of the formation of the Solar system will need to address similarities and differences like these.

The planets lie close to the ecliptic, or the plane of Earth's orbit about the Sun. The planes of the other planets' orbits do not lie exactly in the same plane, but are within a few degrees, so the planets do not typically line up exactly.

Since Mercury and Venus are closer to the Sun than Earth is, they are always close to the horizon. Planets that are located farther away from the Sun than Earth can be found higher in the sky.

Once in awhile, the planets appear close together in the sky. In the above photo, five planets appear relatively close to each other. Of course, Saturn is really much farther away than Mars but here they lie in the same direction. An alignment of all eight planets and Pluto is very rare, happening only once in about 500 years.

Asteroid belt

The Sun and planets are not the only objects in our Solar system. Asteroids are smaller rocky and metallic objects that orbit the sun. Many of them can be found in the asteroid belt between Mars and Jupiter.

Kuiper belt and Oort cloud

Farther out, beyond the orbit of Neptune, lies the Kuiper belt. This region of the Solar system is where the comets spend most of their time. Pluto's elliptical and tilted orbit is shown here in yellow. It spends part of its time in the Kuiper belt but sometimes can be found inside Neptune's orbit.

Even though the Kuiper belt is very large, it is dwarfed by the Oort cloud. This is the far region of the Solar system where the long-period comets reside. Notice that it is not flattened like the rest of the Solar system.