Planets of the Solar System

Definition

The word "planet" comes from a Greek word meaning "wanderer", but classing anything in the sky that moves as a planet is no longer adequate.

The definition of a planet (as of September 2006) states that, in the Solar System, a planet is a celestial body which:

Is in orbit around the Sun,
Has sufficient mass to assume a nearly round shape, and
Has "cleared the neighbourhood" around its orbit.

There is another slightly older qualification to be a planet (from 2001), that the body is too small to be able to fuse hydrogen. In other words, a star. This does not apply to bodies known to be orbiting the Sun.

Terrestrial planets

Mercury

Mercury is the closest to the Sun and the smallest planet in our Solar System. Visible from Earth just before sunrise or just after sunset depending on its position.

Mercury weirdness:

A day on Mercury lasts two of its years.
Mercury has a magnetic field.
Mercury is not the hottest planet. That distinction goes to Venus.

Venus

Venus is the second planet from the Sun. Second brightest object in the night sky, but only ever gets 47.8° from the Sun as viewed from Earth.

Venus weirdness:

Venus' surface cannot be seen because of thick clouds of sulphuric acid.
The clouds reflect 90% of the light falling ot them, making Venus a bright object.
There is an oval in the clouds at Venus' south pole.
Acid rain from the clouds evaporates 25 km above the surface.
Venus is the hottest planet.
Venus' atmosphere is 92 times as dense as Earth's.
Venus' atmosphere is made of 96.5% carbon dioxide.

Earth

Earth is the third planet from the Sun.

Earth is very unique.

The only planet anywhere known to hold life.
The only planet anywhere known to have liquid water on its surface.
The largest moon:planet mass ratio in the Solar System (roughly 1:81) other than the Pluto/Charon binary system.

Mars

Mars in the fourth planet from the Sun. Easily visible from Earth, has a orangey colour, which means it's sometimes confused with the star Antares.

Mars weirdness:

Mars' atmosphere is about 1% the density of Earth's, the equivalent to the pressure at an altitude of about 30 km on Earth.
Mars' atmosphere is 96% carbon dioxide.
During each pole's winter up to 30% of the atmosphere is deposited on the polar ice cap.
There is enough water in Mars' south polar ice cap to cover the entire surface to a depth of 11 metres.

Mars has two small moons, both discovered in 1877 by Asaph Hall. Neither moon is big enough to be spherical.

Gas Giants

Jupiter

Jupiter is the fifth planet from the Sun. Largest planet in the Solar System, normally the third brightest object in the night sky (Mars can at times get brighter than Jupiter).

Jupiter weirdness:

Its mass (about 2 thousand trillion trillion kg) is about 2.5 times that of all the other planets combined.
That means the barycentre of for it and the Sun is a little above the Sun's surface.
Jupiter has a high rotation rate. A Jovian day is just under 10 hours long.
Its high mass combined with its high rotation rate means it contains almost all the angular momentum in the Solar System.
It also means Jupiter has quite a pronounced equatorial bulge. In other words, if you view Jupiter through a good telescope and you think it doesn't look like a perfect circle, that's because it's not. This was first observed by Giovanni Cassini in the 1660s.
Jupiter emits about 1.9 times the amount of energy it gets from the Sun.
Jupiter is quite a strong radio emitter (that is, it emits radio waves).

Since Jupiter emits so much energy, how far is Jupiter from becoming a star (and fusing hydrogen into helium)?

A old brown dwarf star might look very much like Jupiter does now, and the same size except it would be more massive – the smallest brown dwarf star found so far is just 8 times Jupiter's mass. If Jupiter was a young brown dwarf it would look like a small star, and to us on Earth it would seem like a very bright star in Jupiter's place.
The diameter of Proxima Centauri is only 1.5 times Jupiter's.

Saturn

Saturn is the sixth planet from the Sun. Saturn is easily visible from Earth. The rings can be seen with a low power telescope.

Saturn weirdness:

Saturn has features not observed on other planets. While all the gas giants (and some other bodies) have tenuous rings, Saturn is the only planet with prominent rings.
There is a hexagon in the clouds at Saturn's north pole, with sides longer than the diameter of Earth. It is apparently caused by different rotation speeds of the material inside and outside the hexagon, causing turbulence. The effect can be recreated in a lab, as shown in this video. With different conditions (such as rotation speeds), different polygons can be made.
Jupiter and Saturn together make up 90% of the mass of the planets. When they are on the same side of the Sun, the barycentre of the whole solar system is outside the Sun.

Ice Giants

Uranus

Uranus is the seventh planet from the Sun. Barely visible with the naked eye, but it was not recognised as a planet until discovery by William Herschel in 1781.

Uranus weirdness:

Relative to its orbit, Uranus is tipped over on its side at a 98° angle.

Neptune

Neptune is the eighth planet from the Sun. Not visible with the naked eye, observed by Galileo Galilei in 1612 but not recognised as a planet, and was eventually discovered in 1846, just 1° from where Urbain Le Verrier had predicted it would be.

In 2011 Neptune completed its first orbit since its discovery 165 years earlier.

Other Planets

Planet Nine

As far back as 2004, there were hints that another sizeable planet still remains to be found. Specifically, Sedna's highly elongated orbit; something must have caused it to have that shape.

In 2008, more tenuous hints emerged in the form of the inclined orbits and high eccentricities of many more trans-Neptunian objects.

In 2014 better (yet still indirect) evidence surfaced for the existence of a ninth planet, with more evidence presented in 2016. Basically, all six TNOs that have perihelia far enough out to not be affected by Neptune have similarities in their orbits that could be explained by a ninth planet. They were all discovered with different telescopes and in different surveys, reducing the chance of observer bias.

The planet the evidence implies exists would have a mass of about 10 times that of Earth, with an elliptical orbit (eccentricity roughly 0.6), inclined at about 30° and taking on the order of 15,000 years for an orbit. Its perihelion would be about 200 AU, which is twice as far out as the furthest known object is presently located, but it could be around 1,000 AU at present.

For comparison, Uranus and Neptune have masses of about 14.5 and 17 times that of Earth, respectively.

If found, Planet Nine would satisfy the three present requirements for qualifying as a planet. However, I note the significant differences in inclination and eccentricity its orbit would have with the 8 presently known planets.

No longer planets

There are quite a number of objects in the Solar System that were once considered planets, but are not now.

The Sun and Moon were once considered to be planets because they move across the sky. The word "planet" comes from a Greek word meaning "wanderer".
The four Galilean moons of Jupiter and the five largest moons of Saturn were "secondary planets".
The first 15 asteroids discovered, from 1801 to 1851. Because so many objects were being discovered between Mars and Jupiter they were reclassified as asteroids in 1854.
Pluto was officially a planet for 76 years after its discovery in 1930. Once again, too many objects were being discovered in the same region for it to hold its planet status. Its orbit is also quite different to those of the 8 planets, being inclined to the ecliptic, elliptical, and crossing the orbit of a planet. Pluto has a 2:3 orbital resonance with Neptune, and there is now a whole category of objects with the same resonance – the plutinos.

Other Solar System bodies

See the Solar System page for details of other objects in the Solar System, or the Minor Planets page for details of some of the smaller bodies that orbit the Sun.