The sky is the ultimate art gallery just above us.
— Ralph Waldo Emerson
The Art of the Universe
The sun, with all those planets revolving around it and dependent on it, can still ripen a bunch of grapes as if it had nothing else in the universe to do.
— Galileo Galilei
There are 8 planets and 5 recognized dwarf planets in our Solar System. (If you're a millennial and are about to yell about Pluto, it's time to accept that it's a dwarf planet. Sorry.) Some planets have many moons, some planets have no moons. There are terrestrial planets and jovian planets. Some are large, some are small. We know the most about Earth, but gradually we are getting to know more about them. One day we might be able to visit all of them! But not today.
A planet is defined by the International Astronomical Union as a body that meets three criteria: first, that it orbits the Sun. Second, that it has enough mass to have a spherical shape. Finally, it must have "cleared the neighborhood" (its orbit) of celestial debris. It is for this reason that Pluto is not considered a planet – while it is spherical and orbits the Sun, there is a lot of debris in the path of its orbit. Instead, it was put on the same level as Ceres and Eris; a dwarf planet.
There are two types of planets: terrestrial and jovian. Terrestrial planets have solid surfaces and are made of heavy elements (heavy here means anything on the Table of Elemtents that is not a gas). They are Earth-like in that manner. Jovian planets are not Earth-like at all. They are made of lighter elements and hydrogen compounds. They have no solid surface, except a solid core at their center. These planets are also known as gas giants.
Once we start discussing the gas giants, the information becomes more sparse the further from the sun you get. This is because we have very few ways with which to accurately study the planets, and only a few automated spacecraft have flown out to study them (Voyager 2, for example). Because of this fact, this page has much more information on Jupiter than it does on Neptune or the dwarf planets.
The Planets
Mercury
Distance from Sun: 57,910,000 km (.39 AU)
Orbital period: .2309 Earth Years
Axis tilt: 0Ëš
Radius: 2440 km
Mass: 3.3x10^23 kg (.055 Earth units)
Density: 5.43 g/cm^3
Moons: 0
Mercury is the first planet from the Sun. Its orbit takes 87.9 Earth days, and one rotation – one Mercury day – takes 58.6 Earth days. This means that if you were living on the planet, you would experience three days every two years.
Because it is so close to the Sun, the surface temperature during the day is 700K (that's 426.85ËšC, or 800.33ËšF), hot enough to melt some metals. At night the temperature drops to 100K (-173.15ËšC, or -279.67ËšF). Such extreme temperatures prevent any liquid water from coalescing, and therefore rule out the planet as a possible place for having carbon-based life.
The surface shows impact craters and lava flows, indicating that Mercury experienced some tectonic activity earlier in its existence. An iron core allowed the planet to shrink in some areas without counter expansion, creating massive cliffs along some areas of the crust. However, for all purposes today the planet is geologically dead, although intense heat during the day vaporizes materials from rocks in the crater floors.
There is not currently an atmosphere on the smallest planet, and if there was one, it has been stripped away by solar wind particles and high-energy solar photons knocking away individual atoms and molecules (this is called surface ejection).
Source: solarsystem.nasa.gov
Venus
Distance from Sun: 108,200,000 km (.723 AU)
Orbital period: .6152 Earth years
Axis tilt: 177.3Ëš
Radius: 6051 km
Mass: 4.87x10^24 kg (.815 Earth units)
Density: 5.25 g/cm^3
Moons: 0
Venus is the second planet from the Sun. One Venus year is equivalent to 225 Earth days, and one Venus day is the same length of 243 Earth days. So if you were to spend a year on Venus, you wouldn't even experience a full day!
Although Venus is farther from the Sun than Mercury, Venus actually has a hotter surface temperature due to its thick atmosphere creating an extreme greenhouse effect. The temperature also doesn't vary much between day and night, staying a toasty 740K (466ËšC/872ËšF) throughout the year.
We don't actually know a lot about the surface of this planet because it's difficult to study. The atmosphere is so thick that we can't see through it to the surface. However, thanks to the use of radar technology we have been able to determine that there are few impact craters on the surface, meaning that there has been geologic activity since the Late Heavy Bombardment period. There is also much evidence for lava flows that probably "repaved" the planet about 750 million years ago. While we don't see any indications of plate tectonics, the planet is almost certainly geologically active today, due to its similar size to Earth and and the presence of sulfur dioxide in the atmosphere.
Unlike Mercury, which has no atmosphere, Venus has the thickest atmosphere of the terrestrial planets. There is so much atmosphere, in fact, that the pressure at Venus' surface is 90 times the atmospheric pressure on the surface of Earth. Venus' atmosphere is made up of mostly carbon dioxide, with some sulfur dioxide and virtually no oxygen to speak of. The density of this atmosphere is 10% of the density of water, significantly thicker than the density of the atmosphere on our own planet. Due to the chemical makeup of the atmosphere, the sky would appear reddish-orange to our eyes. However, it's not likely anyone you know will ever get the opportunity to see it for themselves, because conditions are so harsh that both of the successful automated missions to the surface – Vega 1 and Vega 2 – stopped transmitting soon after landing on the surface, although they did provide valuable data about the atmosphere and soil of the planet.
Source: solarsystem.nasa.gov
Earth
Mars
Distance from Sun: 227,900,000 km (1.524 AU)
Orbital period: 1.881 Earth years
Axis tilt: 25.2Ëš (can vary between 0Ëš and 60Ëš)
Radius: 3397 km
Mass: 6.42x10^23 kg (.107 Earth units)
Density: 3.93 g/cm^3
Moons: 2
The fourth planet from the Sun, Mars has a rotation of just over 24 hours – its days are only slightly longer than one day on Earth. However, a Mars year is much longer than an Earth year; it's orbit takes 687 Earth days.
Mars, being the farthest terrestrial planet from the Sun, predictably has the coldest surface temperature. The temperature is fairly steady, keeping at around 220K (-53.15ËšC/-63.67ËšF).
Mars' surface has been extensively shaped by impact cratering, lava flows, and volcanoes. The southern hemisphere of the planet has fewer craters and is, on average, a lower altitude than the northern hemisphere. Historically, there was much more volcanic activity in the northern hemisphere, and many volcanoes can be found there, including Olympus Mons, the largest volcano in the Solar System. There are also massive valley systems that are four times deeper than the Grand Canyon and longer than the width of the United States. However, today volcanic activity is mostly dormant. The surface also shows evidence of the past existence of liquid water on the surface in the form of delta deposits, erosion of crater rims, rocks similar to ones on Earth that are only formed in or by water, and gullies on crater and channel walls. There are also polar ice caps that are at least partially made up of water ice, leading us to conclude that liquid water must have once existed on Mars.
Today there is a very thin atmosphere around the red planet. The atmosphere is mostly carbon dioxide, nitrogen, and argon, but there is no ozone and not enough gas to create a greenhouse effect, leaving the planet frigid. There are, however strong winds that sweep across the surface of the planet and can create massive dust storms that cover the entire planet. In fact, there is so much dust suspended in the atmosphere at all times that the sky is a red-brown color.
Mars has two moons: Phobos and Deimos. However, unlike our moon they are not large enough to be spherical, and are not much larger than asteroids. Phobos is the larger of the two, and it orbits a mere 6,000 km above the surface of Mars – the closest of any satellite in the Solar System. Its orbit is so fast that it passes around the planet three times per day. Slowly, it is being pulled closer and closer to Mars, and will one day (in 50 million years or so) either crash into the surface or break up and form a ring around the planet. Deimos, on the other hand, orbits farther from the planet, and passes once around every 30 hours.
Source: solarsystem.nasa.gov
Jupiter
Distance from Sun: 778,300,000 km
Orbital period: 11.86 Earth years
Axis tilt: 3.08Ëš
Radius: 71,492 km
Mass: 1.9x10^27
Density: 1.33 g/cm^3
Moons: 67
The first of the Jovian planets, Jupiter rotates once every 10 hours, while one Jupiter year takes almost 12 Earth years.
Because it is a gas giant, there is no solid surface on Jupiter, although it is believed that there is a solid core at its center that is approximately the size of Earth. What it lacks in surface, however, it makes up for with sheer size. Jupiter has a thin ring that is only visible from visiting spacecraft or really good telescopes.
Jupiter's atmospheric structure is similar to Earth's; at least in terms of temperature. There is a troposphere, a stratosphere, and a thermosphere. The thermosphere is way beyond the cloudtops and has very hot – about 1000K (726.85˚C/1340.33˚F) – low-density gas. The stratosphere is in between the clouds and the thermosphere and has a much cooler temperature of approximately 200K (-73.13˚C/-99.67˚F) due to the absorption of ultriviolet rays by minor ingredients in the gases that make up this layer. The lowest layer, the troposphere, is where the clouds are located. Warmer temperatures in the lower regions of this layer fuels convection, which forms clouds in the higher regions. However, the clouds in Jupiter's atomosphere are not like clouds on Earth. On this jovian planet there are three layers of clouds: the uppermost layer is made of condensed ammonia, the middle layer is composed of condensed ammonium hydrosulfide, and the bottom layer of clouds is condensed water. There are more types of clouds in Jupiter's atmosphere because the temperature is much colder than it is on Earth, so more gases can condense to form clouds.
Below the atmosphere lies the interior of the planet. Here the composition of the planet is mostly hydrogen, and as you get closer to the center it gets hotter and denser and the hydrogen begins acting less like a gas and more like a liquid or a metal. The first of the four layers is gaseous hydrogen, which also acts as part of the atmosphere. The pressure is approximately 1 bar, the same pressure as being at sea level on Earth, and the temperature is a chilly 125K (-148.15˚C/-234.67˚F). Next down, at 7000 kilometers into the interior, is a layer of liquid hydrogen – hydrogen that acts more like a liquid than a gas – at 2000K (1726.85˚C/1340.33˚F) and a pressure of 500,000 bars. Beneath the liquid hydrogen at a depth of 14,000 kilometers is metallic hydrogen. We don't really know what it is like, as we don't have any conditions under which it exists here on Earth, but we know that metallic hydrogen acts more like a metal than a gas and that it is highly conducive. Pressure here is 2,000,000 bars and the temperature is 5000K (4726.85˚C/8540.33˚F). The final layer is the core, composed of rock, metals, and hydrogen compounds. At a depth of 10,000 kilometers, a pressure of 100 million bars, and a temperature of 20,000K (19726.85˚C/35540.33˚F), the density is 25 g/cm^3; siginificantly denser than anything on Earth, and about 10 times more massive than our own planet.
Jupiter has 67 moons; 50 that have been recognized and 17 that are still waiting for recognition as official moons. All of the moons are named after the mythological lovers of Jupiter/Zeus, king of the ancient Greek gods. There are four main moons known as the Galilean moons, having been discovered by Galileo in the 1600s. They are Io, Europa, Ganymede, and Callisto. Io is the third largest moon of Jupiter, and is still volcanically active today. It is composed of either sulfur or silicate rock and has a small atmosphere of sulfur dioxide. Europa has a surface of solid water ice and a thin atmosphere of oxygen. It is believed that under the surface there is a liquid water ocean covering the entire planet, and because of this fact it is a prime location to look for extraterrestrial life. Ganymede is the next Galilean moon, and its surface is also made of ice (although it's not all water ice). The ice is about 800 kilometers thick, and makes up the outermost of 3 layers that make up the satellite – the other two are an iron core and a rocky mantle. The final Galilean moon is Callisto. It has the lowest density of the moons and the oldest landscape in the solar system; by scientists' reckoning it hasn't seen any resurfacing since it was formed around 4 billion years ago. It has a similar composition to Ganymede.
Source: solarsystem.nasa.gov
Saturn
Distance from Sun: 1,427,000,000 km
Orbital period: 29.5 Earth years
Axis tilt: 26.73Ëš
Radius: 60,268 km
Mass: 5.69x10^26
Density: .70 g/cm^3
Moons: 53
Saturn is the second jovian planet. Its day is approximately 10.7 hours long and its year is about 29 Earth years in length.
Just like Jupiter, there is no solid surface on this planet and it is mostly hydrogen and helium. Unlike Jupiter, however, Saturn has seven broad rings. They are clearly visible as one large ring even through a cheap telescope. The rings are made up of dust, ice, and large rocks.
Its atmosphere is structured similarly to Jupiter's, with the three temperature layers: the thermosphere, stratosphere, and thermosphere. In Saturn's stratosphere, there are three layers of clouds. The highest layer, visible to us, is made up of condensed ammonia, which gives the ringed planet its yellow appearance. The middle layer is comprised of ammonium hydrosulfide. Like Jupiter, the lowest clouds are condensed water.
The interior of Saturn is similar to the interior of Jupiter, with four layers of gaseous hydrogen, liquid hydrogen, metallic hydrogen, and the core. Together the gaseous and liquid hydrogen layers extend 30,000 kilometers into the planet, with the density and temperature both increasing as you get closer to the core. From about 30,000 to around 44,000 kilometers into the the interior, the hydrogen chanes from liquid to metallic. Beyond the metallic hydrogen is the core, where the temperature is 12,000K (11726.85ËšC/21140.33ËšF) and the pressure is about 18 million bars. There is no possiblity for life as we know it on this planet.
Along with its impressive rings, Saturn boasts 53 moons. The most prominent moons are Titan, Iapetus, Rhea, Dione, Tethys, Mimas, and Enceladus. Titan, the largest moon, has a thick, complex atmosphere, which scientists believe may be similar to Earth's before life existed. The surface of the moon has lakes, rivers, and seas of liquid methane and ethane. Under the surface, there is evidence of liquid water, which may or may not be hospitable to some life. Iapetus is two sided; its leading hemisphere (the side that leads the orbit) is dark, and has an albedo (reflectivity) similar to coal, and its trailing hemisphere is very bright and icy. Rhea is the second-largest moon of Saturn, but is still only about one-third the size of Titan. It is thought to be a homogenous mixture of 75% ice and 25% rock. Dione is probably made up of a dense rock core surrounded by ice, and is constantly bombarded by fine dust from Saturn's E-ring. Tethys is very reflective, suggesting that it's made of water ice. Like Dione, it is bombarded by dust from the E-ring, which gets its water-ice particles from geysers on Enceladus. Mimas is the smallest and innermost of Saturn's main moons. Its orbit takes 23 hours and has a completely solid surface. Enceladus, the last of the major moons of Saturn, reflects almost all of the sunlight that reaches it. Water vapor geysers create a halo of ice dust around the moon, which, as stated before, are pulled into orbit around Saturn as part of its E-ring.
Source: solarsystem.nasa.gov
Uranus
Distance from Sun: 2,870,000,000 km
Orbital period: 84.01 Earth years
Axis tilt: 97.92Ëš
Radius: 25,559 km
Mass: 8.66x10^25
Density: 1.32 g/cm^3
Moons: 27
One day on Uranus is about 17 hours long, and it orbits the Sun once every 84 Earth years. Like Venus, it has a retrograde rotation, meaning that the sun rises in the west and sets in the east. The planet has the most extreme axis tilt of any in our Solar System: over 90Ëš. This means that the planet is essentially on its side, and only one of its poles gets sunlight..
As with the previous two planets, Uranus has no solid surface. It is also mostly made of hydrogen and helium, however, it also contains methane. Uranus has thicker rings than Jupiter, but its rings are no match for Saturn's.
The atmosphere on this planet is similar structurally to Saturn's and Jupiters, but at cooler temperatures due to its distance from the Sun. Clouds in the troposphere are made of ammonia and methane, although there are more methane clouds. The methane clouds are also at higher altitudes than the ammonium clouds, making the planet appear a light blue-green color.
Uranus' interior is comparable to Jupiter's and Saturn's, but because Uranus isn't quite as massive as the first two jovian planets, there are no metallic or liquid hydrogen layers. Instead, there is a gaseous hydrogen layer around a two-layer core. The inner core is comprised of rock and metals, while the outer core is (possibly) liquid water, methane, and ammonia.
The 27 known moons of Uranus are named after characters from the plays of William Shakespeare and Alexander Pope, not characters from Greek mythology. The two largest moons are named Oberon and Titania. Oberon is the second largest satellite of this planet. It is half rock and half ice and is covered in craters. Titania is the largest of the Uranian moons. It shows signs of having once been geologically active, and is a neutral grey color that most other moons orbiting Uranus share.
Source: solarsystem.nasa.gov
Neptune
Distance from Sun: 4,497,000,000 km
Orbital period: 164.8 Earth years
Axis tilt: 29.6Ëš
Radius: 24,764 km
Mass: 1.03x10^26
Density: 1.64 g/cm^3
Moons: 13
A Neptunian day is 16 hours long, which is no time at all compared to its year. Neptune takes over 160 Earth years to orbit the Sun, much longer than the average lifespan of a human.
Like all the other gas giants, there is no solid surface on the outermost planet. Although it has a similar atmospheric structure to the other jovians, there are only methane clouds in the atmosphere, leading to its unique bright blue color. There are six faint rings around the planet, the outermost of which has three prominent arcs named for the three main ideas of the French Revolution/Constitution: Liberty, Equality, and Fraternity.
Neptune's interior is most similar to Uranus'. There is an outer layer of gaseous hydrogen, an outer core layer of (probably) liquid water, methane, and ammonia, and an interior core of rock and metals. Despite Neptune being the smaller of the two outer planets, it actually has more mass.
Of Neptune's 13 moons, only two were discovered before the Voyager 2 spacecraft visited the planet. The three largest moons are Triton, Proteus, and Nereid. Triton is the largest of the moons and orbits in the opposite direction of all the other moons. That suggests that it was once an independent object that became trapped in Neptune's orbit. If this is the case, it could explain why Nereid, the third-largest moon, has such an eccentric orbit, with one end being nearly seven times farther than its close end. Proteus, the second largest moon, is irregular and icy. It is also not very reflective, and orbits very close to the planet.
Source: solarsystem.nasa.gov
Dwarf Planets
Pluto
Ah, Pluto, our beloved planet-that-will-always-be-a-planet-in-our-hearts. Even though it's been demoted to a dwarf planet, it doesn't cease to be interesting. It orbits the sun at a distance of 5,906,000,000 kilometers, although its orbit is elliptical so that part of its orbit it is closer to the Sun than Neptune. It takes a lengthy 248 Earth years to complete an orbit and has five moons. While it ordinarily has no atmosphere, when it is closest to the Sun its surface ices melt and create a thin atmosphere. Pluto consists of a rocky core surrounded by water ice.
Eris
Eris is another dwarf planet beyond the orbit of Pluto. It takes 557 Earth years to complete a singular orbit. Its orbit is even more elliptical than Pluto's, crossing inside Pluto's orbit and coming nearly tangent to Neptune's. Its average distance from the Sun is 10,120,000,000 kilometers. Eris has one moon, Dysnomia. Like Pluto, this dwarf is smaller than our moon.
Ceres
Ceres is the closest dwarf planet to us. It is located in the asteroid belt. This little dwarf completes its orbit in 4.2 Earth years. It is believed that beneath its surface are large quantities of pure water ice. There is evidence that plumes of water are shot up from the surface when the surface is warmed by the sun, indicating that the surface is made up of ice and that there is an atmosphere.
Source: solarsystem.nasa.gov
Source: solarsystem.nasa.gov
Source: solarsystem.nasa.gov