Four Major Characteristics of the Solar System
- Large bodies in the solar system have orderly motions
- All planets and most satellites have nearly circular orbits in
nearly the same plane and revolve in the
same sense (counterclockwise when
looking down on the North Pole).
- 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.)
- Planets fall into two main categories:
- Terrestrial--small, solid, near the Sun.
- Jovian--large, gaseous, farther out.
- 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 and
- Several notable exceptions to the general trends:
- Planets with unusual tilt (Venus, Uranus, Pluto).
- Surprisingly large moons and moons with unusual orbits.
Terrestrial vs Jovian
|Mercury, Venus, Earth, Mars
||Jupiter, Saturn, Uranus, Neptune
|smaller size and mass
||larger size and mass
||no solid surface
|higher density (rocks, metals)
||lower density (light gases, hydrogen compounds)
| metallic core
and a rocky outer layer
|| rocky core which
resembles the terrestrial planets
|orbits closely spaced
||orbits spaced far apart
|few (if any) moons, no rings
||many moons, rings
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
- 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
- According to the definition, there are currently eight planets and three
dwarf planets known in the solar system. The definition distinguishes
planets from smaller bodies, and is not used outside the solar system, where
smaller bodies cannot be found yet. Exoplanets are covered separately under
a complementary 2003 draft guideline for the definition of planets, which
distinguishes them from dwarf stars, which are larger.
- 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.
Mass and Density
- 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
astrophysical calculator that will display basic astronomical constants and
solar system data at the touch of a button, and also allow calculations using
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
- Meteorites around 4.5 b.y.
- Computer simulations of the Sun ~ 5 b.y.
- 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
- 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
- As of March 2014, at least 1692 extrasolar planets (exoplanets) have been
confirmed, with thousands more under investigation. For the latest on exoplanet research, visit
- None of the extra-solar systems discovered so far look much like our
- Most of the currently discovered exoplanets are massive giant planets
likely to resemble Jupiter.
- 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 actual imaging.
- Indirect observation typically involves the detection of a parent
star's wobble as the planet orbits it.
- Wobble is most easily observed when the planet is large, so this
technique favors the discovery of large, massive planets.
- Direct observation of exoplanets is challenging because of the
blinding glare of the parent star. This is analogous to the challenge of
seeing stars in our daytime sky.
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