The Solar System: Planets, Dwarf Planets and Small Bodies
An Ordered Family
The Solar System contains the Sun, eight planets, five recognised dwarf planets, and an enormous number of Small Solar System Objects (SSSOs). Understanding the distinctions between these categories — and where each type of body sits in the Solar System — is fundamental to understanding how the planetary system formed and evolved.
The Eight Planets
The eight planets divide naturally into two groups. The four terrestrial planets (Mercury, Venus, Earth, Mars) are small, rocky and dense, with few or no moons and no ring systems. They formed inside the frost line, where temperatures were too high for ices to survive, so they accreted mostly from rock and metal. The four gas giants (Jupiter, Saturn, Uranus, Neptune) are vastly larger, primarily composed of hydrogen and helium (Jupiter and Saturn) or ice-dominated mixtures (Uranus and Neptune, sometimes called ice giants). They formed beyond the frost line where icy solid cores could grow large enough to gravitationally capture large envelopes of gas.
Key comparative properties to remember:
- Jupiter is the largest planet — over 300 times Earth's mass and 11 times its diameter.
- Saturn has the most prominent ring system and the lowest average density of any planet — less than water.
- Venus has the highest surface temperature (~460 °C), due to an extreme greenhouse effect, not its distance from the Sun.
- Mars has the largest volcano in the Solar System (Olympus Mons) and two small moons (Phobos and Deimos).
- Planets orbit the Sun in the same direction (anticlockwise viewed from above the north pole) and their orbits are nearly coplanar, lying close to the ecliptic plane.
Terrestrial planets (Mercury, Venus, Earth, Mars) are small, rocky and formed inside the frost line. Gas/ice giants (Jupiter, Saturn, Uranus, Neptune) are large, gas-rich and formed beyond the frost line where icy cores could grow large enough to capture hydrogen and helium.
Dwarf Planets
The International Astronomical Union (IAU) defined dwarf planets in 2006 as bodies that orbit the Sun, are massive enough for gravity to make them roughly spherical, but have not cleared their orbital neighbourhood. The five currently recognised dwarf planets are Pluto, Ceres, Eris, Haumea, and Makemake. Pluto was reclassified from planet to dwarf planet in 2006 because it shares the Kuiper Belt with thousands of similar Kuiper Belt Objects. Ceres is the largest object in the asteroid belt. Eris is more massive than Pluto and was a key trigger for the reclassification debate.
Asteroids and Meteoroids
Most asteroids orbit in the asteroid belt between Mars and Jupiter. They are remnant planetesimals — rocky material that never accreted into a planet because Jupiter's strong gravitational influence constantly stirred the region. Meteoroids are smaller fragments in space. When a meteoroid enters Earth's atmosphere and ablates (burns up) due to friction, we see a bright streak called a meteor (shooting star). If any material survives and reaches the ground, it is a meteorite. Meteorites are classified into stony, iron and stony-iron types, and some preserve pristine material from the early Solar System.
Three distance units are used in astronomy: the astronomical unit (AU), defined as the average Earth-Sun distance (approximately 1.5 × 1011 m); the light year (ly), the distance light travels in one year (approximately 9.46 × 1015 m); and the parsec (pc), defined by stellar parallax and equal to approximately 3.09 × 1016 m (or 3.26 light years). Within the Solar System, AU is most convenient. Stellar distances use pc. Cosmological distances use Mpc (megaparsecs). Converting fluently between these units is a key maths skill for this course.
Comets: Structure and Origins
Comets are icy bodies that develop spectacular extended features when they approach the Sun. The solid central body is the nucleus, typically only a few kilometres across. As a comet warms near the Sun, surface ices sublime (convert directly from solid to gas) and release dust, forming a bright cloud called the coma. The coma can extend hundreds of thousands of kilometres.
Comets develop two distinct tails pointing away from the Sun:
- Dust tail: dust particles pushed by radiation pressure; curves gently along the orbit path and appears whitish-yellow.
- Ion tail (plasma tail): ionised gas blown directly away from the Sun by the solar wind; always points straight away from the Sun regardless of the comet's direction of travel, appearing bluer.
A comet's ion tail always points directly away from the Sun (blown by the solar wind), while the dust tail curves along the orbital path. Both always point away from the Sun — but in slightly different directions.
Short-period and Long-period Comets
Short-period comets have orbital periods of less than 200 years and are believed to originate from the Kuiper Belt — a disc of icy bodies lying about 30 AU to 50 AU from the Sun, beyond Neptune's orbit. Interactions with the giant planets occasionally redirect Kuiper Belt Objects into the inner Solar System. The most famous short-period comet is Halley's Comet, with a period of about 75–76 years; it last appeared in 1986 and will next return around 2061.
Long-period comets have periods of thousands to millions of years and come from the Oort Cloud — a hypothetical vast spherical shell of icy bodies surrounding the Solar System at distances of 2,000 to 100,000 AU. Long-period comets arrive from all directions (not confined to the ecliptic plane), consistent with an origin in a spherical cloud rather than a flat disc.
The transit of Venus method, championed by Edmond Halley in 1716, exploited the transit (Venus passing across the Sun's face) to measure the absolute value of the AU. Observers at different latitudes on Earth would see Venus trace slightly different paths across the Sun (the parallax effect). By precisely timing the duration of each transit from multiple locations and applying trigonometry, astronomers could calculate the AU in kilometres. Transits of Venus are rare, occurring in pairs separated by 8 years then gaps of over a century. The last pair was in 2004 and 2012; the next will not occur until 2117.