Planet is a celestial body that is in orbit around a star and that has enough mass for its gravity to overcome the forces of the rigid body.
In the cover image you can see Jupiter rings seen from Voyager 2 probe. Artist’s conception of Cassini orbit insertion. Credit: web “https://airandspace.si.edu”
Also the planets assumes a balanced spherical shape and has cleared of material the vicinity of its orbit. This is the case for the eight planets of the solar system.
There are 8 planets that revolve around the Sun
The Solar System consists of eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
Pluto, which was considered a planet, has been classified as a dwarf planet, by majority vote in the General Assembly of the International Astronomical Union, in August 2006.
Etymologically, the word planet comes from Latin and means “wanderer, wanderer”.
In ancient times they were so called because of the apparent movement of the planets with respect to the fixed background of the stars.
Beyond Neptune the bodies with characteristics similar to Pluto could be several thousand.
The solar system looks like the atomic model
The solar system, made up of the Sun and the planets that surround it, bears a great resemblance to the model of the atom, made up of a central nucleus and a cloud of electrons that revolve around it.
This similarity should not be too surprising, since the law of gravity that affects the Sun and the planets also bears an enormous resemblance to the law of electromagnetism that governs at the atomic level.
The eight planets of the solar system
The size of any planet must not exceed 13 times the mass of Jupiter, which is the mass threshold that prevents nuclear fusion of deuterium.
- Mercury: messenger of the gods.
- Venus: goddess of love and beauty.
- Earth: mother of all gods.
- Mars: god of war.
- Jupiter: supreme god and creator of the universe.
- Saturn: god Titan, father of Jupiter.
- Uranus: god of the sky.
- Neptune: god of the sea.
Some days of the week have been named after Planets. Sunday for the “Sun’s Day”, in German, Sontag. Monday is named after the Moon.
The inner planets are those that are closer to the Sun than the Earth: Mercury and Venus.
The higher planets are all the others.
Planets are also classified by their physical composition
Terrestrial or telluric or rocky planets are those with high density and rocky and solid surface.
They are those that are closest to the Sun: Mercury, Venus, Earth and Mars.
Jovian planets (similar to Jupiter) are those essentially gaseous (hydrogen and helium) and with low density.
The four giant planets of the Solar System are gaseous: Jupiter, Saturn, Uranus and Neptune.
The first asteroids discovered, for example Ceres, were originally also called planets, until it became clear that they were part of a large family of objects: the asteroid belt.
The work of the Kepler mission, created by NASA in 2009, consisted of locating extrasolar planets with characteristics similar to ours around the stars of the Milky Way in “habitable zones”, that is, with temperatures neither very cold nor very hot and with water in its surface.
These Planets are called exoplanets. The Kepler probe first discovered 461 new planets outside the solar system.
Four of them are less than twice the size of Earth and orbit a star at the right distance to host liquid water on its surface.
In the year 2013, NASA estimated, the Milky Way is home to some 17 billion planets similar in size to Earth.
The fact that there are 17,000 million exoplanets similar in size to Earth in our galaxia does not imply that all of them are habitable.
But it increases the probability that in the future, worlds with the capacity to support life will be discovered in the Milky Way.
The Goddard Space Flight Center has a website called “Light in the Dark” where it can see beautiful images of the Earth, the Moon, the Sun, the Planets even of some inhabitants of this our little blue planet.