Composition of the Solar System

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Composition of the Solar System
Humans have been interested in the Universe and our place in it since pre-recorded history, indeed probably since before we could be considered ‘humans’ in the modern sense. The Solar System represents only a small fraction of the Universe as a whole. However, because it is our “neighborhood”, its members can be considered Earth’s immediate family, Over the centuries our understanding of the Solar System has evolved as our technologies have developed. In the 21st Century, with a battery of sophisticated telescopes, orbiting detectors at many wavelengths across the electromagnetic spectrum, and robot probes to other planets, satellites, asteroids, and comets, humankind has a deep but certainly not complete knowledge of the solar family.

The dominant figure in this cast of characters is the Sun. Consisting almost entirely of hydrogen (74%) and helium (24%), our local star contains some 99.8% of the mass of the entire system, its volume big enough to comfortably swallow a million Earths. 

The Sun is located in one of the spiral arms of the Milky Way Galaxy, one house in an immense ‘city of stars’ of some 400 billion members. The Sun revolves about the center of the Milky Way every 200 million years or so, and the galaxy itself is in motion relative to other galaxies in the local universe. But wherever the Sun goes, the planets move with it like the occupants of a moving vehicle, so such motion can be discounted when considering just the Solar System itself. It’s perhaps easiest to consider the Sun to be fixed in the middle, and everything else to be in motion relative to it. 

This wasn’t always thought to be so. Although Aristarchus proposed a heliocentric (Sun-centered) system in the 3rd Century BCE, the idea lay dormant for many centuries until once again suggested by Nicholas Copernicus in 1543. Although the idea solved many of the problems in observational astronomy, it was considered outlandish to most scientists and religious leaders. The idea of Earth hurtling around the Sun in a billion-kilometer annual round trip, or rotating on its own axis once a day at over 1,000 km/hr, seemed unbelievable. Eventually through the work of such acclaimed scientists as Johannes Kepler, Galileo Galilei, and Isaac Newton, the evidence supporting the heliocentric system became overwhelming. 

The entire solar system condensed out of an immense cloud of gas – the ‘solar nebula’ – some 4.6 billion years ago. All of the parts of the solar system are considered to be of similar age. Because this nebula was spinning as it collapsed, all of its major components co-exist on a relatively flat plane (the ecliptic). The Moon and planets can always be found in a narrow band of the sky very close to the ecliptic.

The immediate family members are the major planets, which account for some 0.1% of the Solar System’s matter. There are generally thought to be eight of these – in order, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto was demoted from planetary status because it was too small and rocky, more like a moon. There are two distinct groups of planets: the four inner, rocky ‘terrestrial planets’ (each some 5,000 to 13,000 km in diameter) and then the four outer, ‘gas giant planets’ (50-143,000 km). They were formed out of leftover material from the collapse of the solar nebula. The denser elements stayed closer to the sun and lighter elements moved further away.

The planets were not formed whole, but were formed as rings of material around the Sun started to clump together into planetesimals through gravitational attraction. This process, known as ‘accretion’, featured a large number of violent collisions, especially in the earliest years of the Solar System. One such collision between early Earth and a Mars-sized object is believed to have resulted in our Moon. After about a billion and a half years, or one-third the Solar System’s current age, this process was largely complete. However, by no means is it fully so, and cosmic collisions remain an occasional fact of life in this relatively crowded neighborhood. 

Most of the planets have their own satellites, or moons, in orbit around them. This is especially true of the gas giants, which have large families of dozens of rocky, icy moons, each such system is much like a mini-solar system. 


  1. Fill in the pie chart and make a key for the composition of the solar system:

Mass of sun:

Mass of all other planets:

  1. Draw a diagram of the solar system that shows the location of rocky planets and gas giants.

  1. Draw a picture of the “ecliptic” and label where you think our solar system is, based on the description in the reading:

  1. Use the “neighborhood” analogy to draw a solar system “city”. Do not just randomly choose a building; make a thoughtful choice. Make sure you include the sun, inner planets, outer planets, nearest star and nearest galaxy.

5. How did you choose the buildings for your neighborhood?


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