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1.5.2. The planets

The planets

The planets are stars more or less spherical shape, orbiting a star. Planets do not emit their own light, but rather reflect the light they receive from their star.

In the case of the Solar System, the planets revolve around the Sun following elliptical paths located in the same plane called the eclipticIn addition, the planets also rotate about themselves, on an axis more or less inclined with respect to the plane of the ecliptic.

Two types of planets are distinguished:

Comparison of some data of the planets of the Solar System.

  Sun Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune
Distance to the Sun (million km) --- 57 114 150 237 780 1425 2880 4590
Translation in years --- 87.9 (days) 224.7 (days) one 1.9 11.8 29.4 84 164.8
Rotation 25-36 days 58.6 days 243.1 days 1 day 1.03 days 9.8 hours 10.5 hours 16.8 hours 16.1 hours
Radius (km) 695000 2439.7 6051.8 6378 3397 71492 60268 25559 24746
Average T during the day (ºC)

6000 (on surface)

Day 623 K (350 ° C)
Night 103 K (-170 ° C)
Minimum 90 K (-183 ° C)
Half 440 K (166 ° C)
Maximum 700 K (427 ° C)
480 22 -23 -150 -180 -210 -220
No. of satellites --- 0 0 one two 79 62 27 14

The Solar System to scale

Answer in your notebook

Why do the planets shine?

Why are planets so called?

The names of the planets


Mercury is the planet with the fastest translation speed around the Sun. It takes the least time to go around the Sun.

Its name comes from the Latin Mercurius, (god of merchants), from merx (merchandise). Mercurius was the Roman Roman god of commerce and the messenger of the gods. He had sandals with wings that allowed him to travel very fast, like this planet, the fastest to go around the Sun, that is why it was given this name.


Venus is the planet that reflects the most light, which is why it is considered the most beautiful planet. Its brilliance and beauty caused it to be called Venus, in honor of the Roman goddess of love (or Aphrodite, the Greek goddess).

The earth

The Earth receives its name in honor of the Greek goddess Gea or Gaia, which today could be translated as Mother Earth. Gaea was the goddess who knew the secrets of nature, the one who allowed crops and protected people from earthquakes. Later, the Romans, as they had other names for the same gods, called it Terra. Hence came the name of Earth, our planet.

Some words that you know as Geology, Geography, Pangea, etc., have this same origin.


Mars is known as the " Red Planet ", so his blood red, he was associated with the god of war Mars, by the blood spilled in battle.


Jupiter is the largest planet in the Solar System, which is why it is named after the greatest of the gods in Roman mythology, Jupiter. Jupiter is the father of gods and men, equivalent to Zeus in Greek mythology.


The characteristic rings of Saturn seem to be giving it an aged look, thus reminiscent of the god of time Saturn. Furthermore, like the God Saturn, this planet was the slowest of the known planets (neither Uranus nor Neptune were known until the end of the 18th century), the one that took the longest to go around the Sun.


The sky blue color of the planet Uranus has made it related to Uranus, the Greek god of the sky.


Neptune's name also comes from its characteristic navy blue color, similar to the color of the Earth's sea, which is why it was associated with Neptune, the Roman god of the seas and oceans. Neptune, with his trident, can churn the waves, make springs gush, etc. Its equivalent in Greek mythology is Poseidon.