Non-terrestrial planets Not all planets are terrestrial. In our solar system, Jupiter, Saturn, Uranus and Neptune are gas giants, also known as Jovian planets. It’s unclear what the dividing line is between a rocky planet and a terrestrial planet; some super-Earths may have a liquid surface, for example.

Venus, Mars and Earth, three out of the four inner or ‘rocky’ planets of the Solar System, have a lot in common – a solid surface you could walk on, a comparable surface composition, an atmosphere and a weather system.

Solar System Scavenger Hunt

The Terrestrial Planets. From top: Mercury, Venus, Earth and Mars. The planets Mercury, Venus, Earth, and Mars, are called terrestrial because they have a compact, rocky surface like Earth’s terra firma.

Neptune

The Sun is our nearest star. It is, as all stars are, a hot ball of gas made up mostly of Hydrogen. The Sun is so hot that most of the gas is actually plasma, the fourth state of matter. Gas is the third state of matter.

The gas giants of our solar system are Jupiter, Saturn, Uranus and Neptune. These four large planets, also called jovian planets after Jupiter, reside in the outer part of the solar system past the orbits of Mars and the asteroid belt.

Kuiper Belt

As the authority on the naming and classification of celestial objects, the International Astronomical Union officially recognizes five dwarf planets in the solar system:

• Pluto.
• Eris.
• Ceres.
• Makemake.
• Haumea.

The four outer planets have faster orbits and spins, a composition of gases and liquids, numerous moons, and rings. The outer planets are made of hydrogen and helium, so they are called gas giants.

Like the Inner Planets, the Outer Planets share similarities but each also has its own unique characteristics. As well as all being huge and being made up mostly of gases, all four have rings spinning around them, with Saturn having the most famous rings. All four planets also have large numbers of moons orbiting them.

The characteristics that the outer planets have in common include their large size and their large number of moons when compared with the inner, or terrestrial, planets. They also lack solid surfaces, and as far as astronomers know, none are capable of supporting life.

Venus

Mercury

Saturn

Titan is the second largest moon in our solar system. Only Jupiter’s moon Ganymede is larger, by just 2 percent. Titan is bigger than Earth’s moon, and larger than even the planet Mercury.

Saturn has 82 moons. Fifty-three moons are confirmed and named and another 29 moons are awaiting confirmation of discovery and official naming. Saturn’s moons range in size from larger than the planet Mercury — the giant moon Titan — to as small as a sports arena.

Scientists have observed four rings around Jupiter. They are made of small bits of dust, which makes them very faint and difficult to see unless they are backlit by the Sun. In fact, they were first discovered only fairly recently, by the Voyager I spacecraft in 1979.

Saturn has overtaken Jupiter as the planet with the most moons, according to US researchers. A team discovered a haul of 20 new moons orbiting the ringed planet, bringing its total to 82; Jupiter, by contrast, has 79 natural satellites. The moons were discovered using the Subaru telescope on Maunakea, Hawaii.

On Earth, a solar day is around 24 hours. However, Earth’s orbit is elliptical, meaning it’s not a perfect circle. That means some solar days on Earth are a few minutes longer than 24 hours and some are a few minutes shorter. On Earth, a sidereal day is almost exactly 23 hours and 56 minutes.