• What does the solar system look like?
  
  Our solar system consists of the Sun, nine planets and their moons, and vast numbers of asteroids and comets. Each world has its own unique character, but there are many clear patterns among the worlds.

• What features of our solar system provide clues to how it formed?
  Four major features provide clues:
  (1) The Sun, planets, and large moons generally rotate and orbit in a very organized way.
  (2) With the exception of Pluto, the planets divide clearly into two groups: terrestrial and jovian.
  (3) The solar system contains huge numbers of asteroids and comets.
  (4) There are some notable exceptions to these general patterns.
• What theory best explains the features of our solar system?
  The nebular theory, which holds that the solar system formed from the gravitational collapse of a great cloud of gas.

• Where did the solar system come from?
  
  The cloud of gas that gave birth to our solar system was the product of recycling of gas through many generation of stars within our galaxy. This gas consisted of 98% hydrogen and helium and 2% everything else combined.
• What caused the orderly patterns of motion in our solar system?
  
  A collapsing gas cloud naturally tends to heat up, spin faster, and flatten out as it shrinks in size. Thus, our solar system began as a spinning disk of gas. The orderly motions we observe today all came from the orderly motion of this spinning disk of gas.

• Why are there two types of planets?
  
  Planets formed around solid "seeds" that condensed from gas and then grew through accretion. In the inner solar system, temperatures were so high that only metal and rock could condense, which explains why terrestrial worlds are made of metal and rock. In the outer solar system, cold temperatures allowed more abundant ices to condense along with metal and rock. Icy planetesimals grew large enough for their gravity to draw in hydrogen and helium gas, building massive jovian planets.
• Where did asteroids and comets come from?
  Asteroids are the rocky leftover planetesimals of the inner solar system, and comets are the icy leftover planetesimals of the outer solar system.
• How do we explain the existence of our Moon and other "exceptions to the rules"?
  
  Most of the exceptions probably arose from collisions or close encounters with leftover planetesimals, especially during the heavy bombardment that occurred early in the solar system's history. Our Moon is probably the result of a giant impact between a Mars-size planetesimal and the young Earth.
• When did the planets form?
  The planets began to accrete in the solar nebula about 4.6 billion years ago, a fact we determine from radiometric dating of the oldest meteorites.

• How do we detect planets around other stars?
  
  So far, we are only able to detect extrasolar planets indirectly by observing the planet's effects on the star it orbits. Most discoveries to date have been made with the Doppler technique, in which Doppler shifts reveal the gravitational tug of a planet (or more than one planet) on a star.
• What have other planetary systems taught us about our own?
  Planetary systems exhibit a surprising range of layouts, suggesting that jovian planets sometimes migrate inward from where they are born. This lesson has taught us that despite the successes of the nebular theory, it remains incomplete.

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