A planet is a celestial body that traces its orbit around the Sun. Those "planets" that orbit around other stars are called exoplanets. In the solar system there are eight planets: Neptune, which is the furthest from the Sun and is made up of gas and a solid core; Uranus, formed by an atmosphere of hydrogen, helium and a nucleus of ice and rocks; Saturn, characterized by its rings and composed mainly of gas; Jupiter, also gaseous and the largest; Mars, which is the closest to Earth; Earth, the only planet on which life is known to exist; Venus, already known in prehistoric times; and finally Mercury, which is the closest to the sun.
Pluto, formerly considered a planet by astronomers, is now considered a dwarf planet; This change was largely motivated by the discovery of a body called Eris, which is less small than Pluto. Basically, the difference between dwarf planets like Pluto and the other planets is that the latter have cleared their orbit, opening up the possibility that they have a different origin.
For hundreds of years, the cosmos has been a great object of study for physicists, mathematicians, and astronomers. Each of these eight planets that make up our galaxy, called the Milky Way, have been progressively “discovered”. Man's curiosity, supported by his intelligence, has allowed him to develop measurement and observation instruments to deepen the knowledge about the cosmos, and the planetary study.
In the past, with geocentric theory in vogue, planets were classified according to the angle they made with the sun from Earth's perspective; thus, they received the name of inferior planets and superior planets. This behavior observed in ancient times is explained in the heliocentric theory from the interiority or exteriority with respect to the orbit of the earth.
Planets are also classified according to their diameter and density. Thus, we have the terrestrial planets, of small diameter and high density, and the Jovian planets, of large diameter and low density. In the first group we can find the Earth, Venus, Mercury and Mars, while in the second group there are Jupiter, Uranus, Saturn and Neptune.
As we said before, the planets that make up the solar system have been the subject of countless scientific experiments, from (predominantly) the Middle Ages to the present day. If in the times of Galileo Galilei the telescope allowed great advances in the establishment of astronomical theories, today the expeditions of organisms such as NASA have developed important instruments for the observation "in situ" of planets, that is, programmed satellites are sent to collect certain types of data, which are transmitted to NASA's monitoring centers on Earth, more precisely, in the United States.
In this sense, Mars has been one of the most explored planets, and where the greatest number of elements have been found that present some type of similarity with terrestrial elements, such as rocks or certain types of minerals. It is the planet that, after Earth, many say that life could be possible.