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Terrestrial, rocky or interior planets

The inner planets are the four closest planets to the Sun. They are small, dense (3 to 5 gr/cm3) and rocky. They have light atmospheres .

The inner planets are:

Beyond the orbit of Mars is the asteroid belt, a region of the Solar System in which there are abundant asteroids that never formed a planet.


Mercury is the planet closest to the Sun and the smallest . Its diameter is approximately one third of the Earth's diameter.

Mercury and Venus are the only two planets in the Solar System that do not have satellites.

Mercury has no atmosphere (almost nonexistent) for two reasons:

  • Since it is a very small planet, there is not enough force of gravity to hold gases close to its surface.
  • The solar wind carries almost its entire atmosphere.

As it has almost no atmosphere, there are no atmospheric agents that modify the landscape, having an appearance very similar to that of the Moon , with many craters from meteorite impacts.

The absence of atmosphere also causes a great difference between the daytime temperature (350 ºC, part illuminated by the Sun) and the night temperature (-170 ºC, part not illuminated). It is the planet with the greatest temperature differences between its maximum and minimum temperatures.

Mercury's mass is 0.055 times that of Earth. There, we would weigh 0.37 times what we weigh on Earth.

Aspecto craterizado de Mercurio.


Venus is the closest planet to Earth. It is a rocky and terrestrial planet, which is also said to be Earth's sister planet because they are very similar in size, mass, and composition.

Venus is a very bright planet, which is why it is also known as "Evening Star", for being the one that appears first, and "Morning Star", for being the last to be hidden.

Like Mercury, it does not have any satellites orbiting it.

The orbit of Venus is the most circular of all the planets in the Solar System, an ellipse with an eccentricity of less than 1%.

Although it is further from the Sun than Mercury, its average temperature is higher. This is because Venus's dense atmosphere is made up of greenhouse gases, such as carbon dioxide. In addition there are layers of sulfuric acid clouds many kilometers thick. All this means that they allow solar radiation to pass to the surface but the heat cannot escape to the outside, so the surface of Venus reaches an average temperature of 463.85 ºC.

The mass of Venus is 0.815 that of Earth, and we would weigh 0.88 times our weight on Earth there.

The Earth

Although we will talk about the characteristics of the Earth later, now we will only cite its four main characteristics:

  • The Earth has a large satellite, the Moon, compared to the size of the Earth. The Moon produces the tides, stabilizes the tilt of the Earth's axis and gradually reduces the rotation speed of the planet.
  • magnetic field of appreciable intensity. This magnetic field, in addition to preventing the atmosphere from being swept away by the solar wind, protects us from harmful radiation from the Sun.
  • Almost three-quarters (70.8%) of its land surface is covered by water (hydrosphere).
  • The distance from the Earth to the Sun, the presence of water, etc., has made possible the presence of life, almost as old as the Earth, which has been the cause of the current composition of the atmosphere.

La Tierra vista desde el espacio

By NASA (Earth Viewed by Apollo 8) [Public domain], via Wikimedia Commons


Mars is also known as the red planet by the reddish appearance that gives the abundance of iron oxide minerals from its surface .

Liquid water has recently been found on its surface, and it is the only planet on which microscopic life could exist, although there is no evidence to indicate that.

It is, behind Mercury, the second smallest planet in the Solar System.

It has a thin atmosphere of carbon dioxide.

Two small and irregularly shaped satellites, Phobos and Deimos (children of the Greek god), which could be asteroids captured by gravity, orbit around Mars.

The Martian surface is very spectacular, with large geological formations. The Valle Marineris stands out, a huge canyon 4000 km long and 2 to 7 km deep. There are also large volcanoes, such as Mount Olympus, about 22 km long that make it the highest mountain in the Solar System. There are also large meteorite impact craters, gullies, ravines, dunes, etc.


By USGS [Public domain], via Wikimedia Commons