The Sun, Mars, Jupiter, and Saturn are masculine planets; Mercury and Uranus are neutral; while the Moon, Venus, Neptune, and Pluto are female planets (though Pluto is related to Mars despite its Dark Mother feminine archetype). Mercury is the closest planet to the Sun, at about 58 million km. It travels around the Sun in almost exactly 88 days.
These ideas have their origins in the ancient world where each planet was associated with various powers over human life. Today we know that our solar system is full of planets, but back then people knew only the Earth and the Moon were relevant.
Mercury is the smallest and nearest planet to the Sun, and so it experiences the highest temperatures of any planet except for close to the Sun. Its average temperature is 507°F (259°C), but it can get as high as 890°F (470°C) on occasion. Most of the time however, mercury is cold enough for ice to exist on its surface.
Its orbit is eccentric, which means that it takes 87.9 days to go around the Sun. So every two years it spends about 11 months outside the Sun's influence (called an "eclipse" by astronomers). During these periods it is dark out and cannot be seen with the naked eye, although telescopes can reveal details about its surface.
The planets Mercury, Venus, Earth, and Mars are known as terrestrials because their surfaces are compact and rocky, similar to Earth's terra firma. The terrestrial planets are the solar system's four innermost planets. They are always located within 450 million km of the Sun, except for when Earth passes between the Sun and Mars at about 878 million km out from the Sun.
Mercury is the smallest planet and also the closest to the Sun. It orbits so close that its surface is constantly heated by its proximity to the star. As a result, Mercury's atmosphere is made up of gases such as hydrogen and helium with only a few atoms per cubic meter.
Venus is the second planet from the Sun and the third-closest planet to Earth (after the Moon). It has one of the most unusual atmospheres in the Solar System. 95% of Venus is made up of oxygen and nitrogen with only small amounts of other gases such as carbon dioxide and traces of methane.
Earth is the largest planet and also the only one that isn't completely covered in water. It orbits around the Sun once every 24 hours, but due to its eccentric orbit, this distance can vary between 393,926 km and 63,000 km.
Mercury is the solar system's smallest planet. (Pluto used to possess the title, but it was demoted to minor planet status.) Although its surface resembles that of our moon, the small planet has a density comparable to that of Earth. Because of this, much of mercury's surface is made up of iron ore and rock salt.
Due to high temperatures and lack of air, most parts of mercury are uninhabitable for humans or any other species. However, several regions have large quantities of ice that have preserved their water content from the last time mercury had an atmosphere. These frozen oceans cover more than half of the planet.
The remaining half of mercury's surface is dominated by two huge lobes caused by powerful impacts with other planets. The largest of these, named Marius Hills because they were first seen in images taken by the Soviet spacecraft Venera 9, covers nearly 400,000 square miles and is composed of ancient lava flows from when mercury was still hot enough for liquid rock to flow.
Although mars appears to be devoid of life, there is evidence that it once supported extensive microbial communities. Data from NASA's Mars Rover Curiosity suggests that some of these may have been capable of producing organic chemicals that could have helped them adapt to environmental changes and survive for a few hundred thousand years before being destroyed when the climate changed.
Pluto is one of a class of objects known as Kuiper Belt Objects. It, like Ceres, does not match the standard description of a planet. Pluto's status as a dwarf planet, incidentally, does not make it any less intriguing or significant. True, Mercury is far bigger and heavier than Pluto. But that's because it orbits much closer to the sun—about 60 million km compared to 48.5 million km for Pluto—and so it experiences much higher temperatures inside its atmosphere. The reason we don't call Mercury a dwarf planet is that it was never considered to be part of the planetary system.
Pluto was originally designated a planet by the International Astronomical Union (IAU) in 2006. However, in 2015 it was determined that there were certain criteria that had to be met for an object to be classified as a dwarf planet. These requirements included that an object have enough mass to be rounded by its own gravity, that its surface be made of rock or ice, and that it not be associated with a larger body such as another planet or moon. By all accounts, Pluto meets these criteria. As such, in July 2016 the IAU announced that it was removing Pluto from its list of planets.
However, although Pluto no longer has any official recognition as a planet, it still features prominently in astronomy books and articles. And so scientists are still studying it to learn more about other objects in space beyond our own planet.
Mercury is the smallest planet in our solar system, while Jupiter is the biggest. They both have a metallic hue to them when seen from Earth and are covered by a thin veneer of atmosphere. However they differ in many other ways as well.
Jupiter has four major moons: Jupiter, Europa, Ganymede, and Io. All up, it's considered the most massive planet outside our solar system. It has been estimated that Jupiter weighs about 300 million km^3 (150 million mi^3). That makes it more than 99.9% water, with only a small fraction of rock.
Venus is the second-closest planet to the Sun and the third-largest planet in the Solar System. It has one of the most opaque atmospheres in the Solar System due to its high cloud cover. If you could see underneath the clouds, you would find that 95% of Venus is made up of water.
Mars is the Red Planet that we commonly know it as. It is the fourth planet from the Sun and the closest planet to Earth once you exclude the Moon. Mars is smaller than Earth but much more massive, making it a "rocky" planet.