• Video overview of the solar system

1. Large bodies in the solar system have orderly motions
   • All planets and most satellites have elliptical, often nearly circular, orbits in nearly the same plane and revolve in the same sense (counterclockwise when looking down on the North Pole).
   • The predictable nature of the orbits is traditionally described by Kepler's laws.
   • The Sun and most of the planets rotate in this same sense as well. (In other words, the rotation axis is roughly perpendicular to the orbital plane. The deviation of the spin axis from the perpendicular is called tilt or, more formally, obliquity.)
   • In this applet, you can create your own solar system and experiment with the size of the orbiting bodies.
2. Planets fall into two main categories:
   • Terrestrial--small, solid, near the Sun.
   • Jovian--large, gaseous, farther out.
   • Note: Pluto is no longer officially classified as a planet (see more below) but is still informally referred to as a planet
3. Swarms of asteroids and comets populate the solar system.
   • Asteroids are concentrated in the asteroid belt and to a much lesser extent in Jupiter's orbit (Trojans).
   • Comets populate the Kuiper belt (or trans-Neptunian object, TNO) and Oort cloud.
   • Pluto is now considered a member of the Kuiper belt rather than a planet
4. Several notable exceptions to the general trends:
   • Planets with unusual tilt (Venus, Uranus, Pluto).
   • Surprisingly large moons and moons with unusual orbits.

• There are over 1 million known asteroids in our solar system. Most of this ancient space rubble can be found orbiting our Sun between Mars and Jupiter within the main asteroid belt.
• Asteroids range in size from Vesta – the largest at about 329 miles (530 kilometers) in diameter – to bodies that are less than 33 feet (10 meters) across.
• The total mass of all the asteroids combined is less than that of Earth's Moon.

Definition of planet

• On August 24, 2006 the International Astronomical Union (IAU) defined the term "planet" for the first time. The definition was a controversial one and has drawn both support and criticism from different astronomers.
• The 2006 definition of "planet" by the IAU states that in the solar system a planet is a celestial body that:
  • is in orbit around the Sun
  • has sufficient mass so that it achieves gravitational rounding (round shape)
  • has "cleared the neighborhood" around its orbit.
• A non-satellite body fulfilling only the first two of these criteria is classified as a "dwarf planet", while a non-satellite body fulfilling only the first criterion is termed a "small solar system body" (SSSB).
• This definition excluded Pluto because its orbit is quite elliptical and crosses that of Neptune. The IAU therefore reclassified Pluto as a member of the new category of dwarf planets, along with Eris (previously known as 2003 UB313 or 'Xena') and Ceres (the largest asteroid).
• Because of its orbital location, Pluto is now considered not only a dwarf planet but also a Kuiper Belt object (though, for historical reasons, will still occasionally be referred to as a planet). Click here for a video empathizing with Pluto's demotion. There is even hate mail from third graders.
• According to the definition, there are currently eight planets, three dwarf planets known in our solar system, along with a myriad of smaller bodies (comets, asteroids, meteoroids, moons).

• The Sun has roughly 10 times the diameter of Jupiter
• Jupiter has roughly 10 times the diameter of the Earth
• The Earth has 5-6 times the diameter of Pluto.
• Ceres, the largest asteroid, has a diameter roughly half that of Pluto.

• Most of the mass of our Solar System is concentrated in our Sun.
• The masses of the planets are concentrated in the Gas Giant planets Jupiter, Saturn, Uranus, and Neptune.
• Jupiter is more than twice as massive as all the other planets combined.
• However, the large mass of these planets comes from their absolute sizes, not their densities. The inner planets are by far the most dense.
• This distribution of masses and densities in the Solar System is a key observation that a theory of the origin of the Solar System must explain.
• Here is an interactive tour of the solar system where you can explore the various aspects of the solar system.

Age of the Solar System

The age of our solar system has been established to be about 4.6 billions years, based on the following pieces of evidence:

• Oldest fossil around 3.5 b.y.
• Earth rocks at least 3.9 b.y.
• Moon rocks radiometrically dated to around 4.5 b.y.
• Meteorites around 4.5 b.y.
• Computer simulations of the Sun ~ 5 b.y.

Moons

• A precise number of moons for the Jovian planets cannot be given, as there is no objective boundary between the countless small anonymous objects that form the ring system and the larger objects that have been named as moons.
• Jupiter's moon Ganymede is larger than Mercury and is the largest moon in the solar system; Callisto is roughly the size of Mercury.
• Saturn's moon Titan, the solar system's second largest moon (slightly smaller than Ganymede and slightly larger than Mercury), is the only moon in the solar system to possess a significant atmosphere, which is, in fact, denser than ours.

Extrasolar systems

• As of January 2023, more than 5000 extrasolar planets (exoplanets) have been confirmed, with thousands more under investigation. For the latest on exoplanet research, visit Nasa's Exoplanet Exploration.
• Most of the extrasolar systems discovered so far do not look much like our solar system, though more and more terrestrial exoplanets have discovered lately.
• Most of the currently discovered exoplanets are massive giant planets likely to resemble Jupiter, but this might change rapidly as our detection technology improves.
• The first confirmed exoplanet detection occurred in 1992.
• In September of 2009, one exoplanet, COROT-7b, was confirmed to have Earthlike density.
• Many of the extrasolar Jupiter-like giants orbit their parent star within a fraction of 1 AU, hence the label "hot Jupiters".
• The vast majority were detected through various indirect methods rather than direct observation.
  • Most exoplanets are discovered via the transit method, which involves seeing a dip in the light from the parent star as the planet orbits around it and blocks a little bit of the light.
  • Some exoplanets are discovered via the wobble method, which involves the detection of a parent star's wobble as the planet orbits and tugs on it gravitationally.
    • Wobble is most easily observed when the planet is large, so this technique favors the discovery of massive planets.
    • The wobble is detected with the Doppler effect.
  • A few exoplanets are discovered through direct observation (imaging), in which the blinding glare of the parent star is removed through some clever techniques. This is a relatively new method and is analogous to the challenge of seeing stars in our daytime sky.
  • A few exoplanets are discovered via the microlensing method, in which light from background stars is distorted by the presence of the exoplanet.

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