The Four Inner Planets
The four inner planets are Mercury, Venus, Earth, and Mars. Because they are all small, rocky planets close Sol, they are often referred to as the “Terrestrial planets.” (“Terrestrial” means similar to Earth.) Their sizes, in relation to one another, can be seen here:
(This picture does not accurately reﬂect their distances from one another.)
The closest planet to Sol is Mercury. It is one of the four small, rocky, inner planets. It’s between 46,001,200 and 69,816,900 kilometres from Sol. It’s also the smallest of the eight planets and has the volume of 0.054 Earths. Because Mercury is so small, it has very little gravity—0.38 g-force—too little for it to retain any signiﬁcant atmosphere over long periods of time. (Click here for an explanation.)
Because Mercury lacks an atmosphere, it has no weather patterns to wash away evidence of being impacted by comets and asteroids. This is why Mercury is covered in craters, and looks similar to our moon, Luna. Because it is so close to Sol, it’s way too hot to sustain human life. If you were to stand on Mercury, Sol would look very huge. However, you would quickly die from the heat.
The Mercurian year is only eighty-eight (88) Terrestrial days long, as it only takes eighty-eight Terrestrial days (Earth days) for Mercury to orbit Sol. This is ¼ of a Terrestrial year (Earth year). However, Mercury rotates around its axis every one-hundred seventy-six (176) Terrestrial days. That means a Mercurian day is longer than a Mercurian year! The Mercurian day is two (2) Mercurian years long!
The astronomical symbol for Mercury is ☿. It has no moons and no rings.
The second closest planet to Sol is Venus. It’s between 107,476,259 and 108,942,109 kilometres from Sol. It is much larger than Mercury, having the volume of 0.857 Earths and a gravitational pull of 0.904 g-force.
The Venusian year is only 224¾ Terrestrial days long. This is 0.615 Terrestrial years. Venus rotates around its axis every 116¾ Terrestrial days. That means a Venusian year is 1.925 Venusian days long.
The astronomical symbol for Venus is ♀. It has no moons and no rings.
The most interesting thing about Venus is its dense atmosphere. Venus is often called Earth’s “sister planet” because it shares many similarities with Earth, such as a similar size and composition. However, Venus has something our planet does not: a run-away greenhouse effect that keeps Venus extremely hot! The average temperature on Venus is 464º C. (100º C is the temperature at which water boils.) The clouds around Venus are so thick that satellites cannot see through them without entering the atmosphere, and any satellite that does enter the atmosphere quickly melts. Sulphuric acid rains down from these clouds.
The third planet from Sol is Earth. It’s between 147,098,074 and 152,097,701 kilometres from Sol. It is the largest of the four terrestrial planets, having the volume of 1 Earth and a gravitational pull of 1 g-force.
The Terrestrial year is only 365¼ Terrestrial days long, as it only takes 365¼ days for Earth to orbit Sol. This is 1 Terrestrial years. Earth rotates around its axis every 24 hours.
The astronomical symbol for Earth is ♁. It has no rings, but does have a moon named Luna. It is the largest moon in the solar system relative to its planet.
The most interesting thing about Earth is its that it is the only planet thus-far known to man to possess life. The Earth also is the only planet known to possess large amounts of liquid water—and it’s deﬁnitely got a lot of it! In fact, 71% of its surface is covered in water.
The fourth planet from Sol is Mars. It’s between 206,669,000 and 249,209,300 kilometres from Sol. It’s larger than Mercury, but not nearly as large as Venus; it has the volume of 0.15 Earths and a gravitational pull of 0.376 g-force.
The Martian year is only six-hundred eighty-seven (687) Terrestrial days long. This is 1.88 Terrestrial years. Mars rotates around its axis every 24⅔ hours. That means a Martian year is 668.6 Martian days long.
The astronomical symbol for Mars is ♂. It has no rings, but two tiny moons, Phobos and Deimos! Whereas Deimos orbits Mars every 30 hours, it takes Phobos 64.8 hours to orbit Mars. Before becoming Mars’s moons, they were both very likely asteroids.
Because Mars is small and has little gravity, it has little atmosphere. Venus is sometimes called Earth’s “little brother.” Although no known life is on Mars, humans could potentially walk there. Plans have been made to send astronauts to Mars by 2037!
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