what is a planet
This seemingly simple question doesn't have a simple answer. Everyone knows that Earth, Mars and Jupiter are planets. At least, they are for now. Both Pluto and Ceres were once considered planets until new discoveries triggered scientific debate about how to best describe them—a vigorous debate that continues to this day. The most recent definition of a planet was adopted by the International Astronomical Union in 2006. It says a planet must do three things:
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Mercury is the planet in the solar system closest to the Sun and the smallest. It is part of the so-called inner or terrestrial planets and lacks natural satellites as well as Venus. Little was known about its surface until the Mariner 10 planetary probe was sent and observations were made with radar and radio telescopes.
Formerly it was thought that Mercury always presented the same face to the Sun, a situation similar to the case of the Moon with the Earth; that is, that his period of rotation was equal to his period of translation, both of 88 days. However, in 1965, radar impulses were sent to Mercury, with which it was definitively demonstrated that its rotation period was 58.7 days, which is 2/3 of its period of translation. This is not a coincidence, and it is a situation called orbital resonance.
Although Venus is not the planet closest to the Sun, it is the hottest. It has a dense atmosphere, full of carbon dioxide that causes the greenhouse effect and clouds composed of sulfuric acid. The gases trap the heat and keep Venus very warm. In fact, it's so hot on Venus that metals like lead would be puddles of molten metal.
Venus looks like a very active planet. It has mountains and volcanoes. Venus is similar to Earth in size. The Earth is just a little bigger.
Venus is rare because it turns in the opposite direction to that of Earth and most other planets. And its rotation is very slow. It takes around 243 Earth days to turn only once. Because it is so close to the Sun, a year goes by very fast. Venus takes 225 Earth days to go around the Sun. This means that on Venus, one day is a little longer than a year.
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