- The central star of our solar system, the Sun is a main-sequence star, which make up roughly 90% of all stars in the universe. It's roughly 110 times as wide as the Earth is, and contains 99.8% of all the system's mass - which is why everything revolves around it!
- Deep in the sun's core is the beating heart of the entire solar system. It's hard to believe that so much is powered by a reaction on the atomic scale. Under pressure from the gravity of the sun being so massive, hydrogen atoms fuse into helium, providing the energy that lights up the entire solar system. It produces 1.8 billion times the energy of the largest nuclear bomb detonated on Earth... every single second!
- When you look up into the sky (but hopefully not directly!) the bright ball of the photosphere is the part of the sun you're looking at. Though often depicted as being yellow, the light from the sun is white. When it hits the Earth's atmosphere, a phenomenon called Rayleigh scattering causes it to look yellow, as well as the sky blue. The same phenomenon is responsible for the brilliant colors of both sunrises and sunsets.
- Just like the Earth, the sun also has an atmosphere. The largest part of it is known as its corona. Though it is not usually visible thanks to the brightness of the photosphere, during eclipses - when the moon's orbit puts it in the right place to block out the majority of the sun's light - it becomes readily apparent. Particles stream out of the corona to create solar wind, which is responsible for such phenomena as the auroras, and comets having tails, among others.
- Sun spots happen when fluctuations in the sun cause areas of the surface to be not as hot as their surroundings, causing that area to look darker than the rest of the sun. Despite only appearing as spots, they can grow to a size several times our own planet's! They can last anywhere from a few days to a few months, and tend to increase and decrease in frequency based off of eleven-year cycles.
- Solar flares are bright flashes caused by increased activity from the sun, in conjunction with a coronal mass ejection - an intense wave of energized particles that erupt from the sun and fly out into the solar system. While they are relatively common, they can cause electrical problems should the Earth be in the path of a flare, thanks to the disturbances they can cause in the atmosphere.
- Named for the speedy Roman messenger god, Mercury is the closest planet to the sun as well as the smallest. Only about a sixth the width and a twentieth of the size of Earth, it lacks an atmosphere, and temperatures can vary wildly based on whether you're on the bright or dark side of the planet - anywhere from 800 to -300 degrees.
- This crater, located within the Pantheon Fossae, is remarkable because of the long troughs that radiate out from the center. When first discovered, it was nicknamed the Spider because of the web like shape. It is still uncertain if the impact caused them, or whether the asteroid that caused the crater merely landed in the center of a pre-existing formation.
- The Caloris basin is one of the largest impact craters in the Solar System, measuring at over 900 miles wide. The asteroid that created the impact was likely at least 60 miles wide, larger than the one theorized to have caused the dinosaurs' extinction. The impact was so violent that it caused deformations in the terrain on the exact opposite side of the planet (the antipode).
- Despite being the planet closest to the sun, Mercury can get quite cold. At the poles of the planet, hidden perpetually from the sun in the shadows of craters, there's ample evidence that there is large amounts of ice lying frozen. Though a fraction of what exists on Earth's polar regions, it is still enough to be at least a couple miles deep.
- Named after the French composer, this crater, along with a similar one named Hakusai, are prominent enough to be detected from Earth using radio telescopes. It has a very noticeable ray pattern stretching out from the impact center, which indicates that it's relatively new. It was one of the first things photographed by the MESSENGER probe, sent to orbit the planet from 2011 to 2015.
- The Tolstoj crater is notable for its well-preserved ejecta blanket and the reflective material that has settled into the crater plain, leaving it an easy to spot bright patch on the planet's surface. It is one of the oldest craters on Mercury, being estimated to be nearly 4 billion years old.
- The second largest of the terrestrial planets, behind only Earth, which it is very close in size to, Venus is named after the Roman god of love, and is one of the brightest objects in the night sky. Despite the name, however, it is a far from lovely place, featuring a number of hostile elements that make sending even unmanned probes a difficult endeavor.
- The thick atmosphere of Venus means despite being further away from the sun than Mercury, it's actually the hottest planet in the system with a surface temperature of 870 degrees Fahrenheit. It's caused by a runaway greenhouse effect - since it's made of nearly 96% carbon dioxide. The atmosphere is also so dense that the pressure is 90 times that of Earth. Oh, and the clouds and rain are made of sulfuric acid.
- While there are no oceans on Venus - water is incompatible with such harsh conditions - areas of highland terrain that rise up above the volcanic plains make up the rough landmasses of Venus. These are known as terra, and there are three - the two major ones, Aphrodite Terra (around the size of South America) and Ishtar Terra (somewhere between the continental US and Australia), as well as a smaller one, Lada Terra, at Venus's south pole.
- Regio are large plateaus that rise above Venus's surface. They are composed of some of the most intriguing tectonic features of Venus, such as the highly deformed tesserae terrain in the Alpha Regio, or the large Devana Chasma rift zone that cuts through the Beta Regio. These were some of the first regions of the surface to be detected by radar, as the cloud cover made telescopic observation impossible.
- Maxwell Montes is the highest mountain on Venus, standing at about 35,000 feet tall. Rising from the Ishtar Terra on the northern half of the planet, the mountain is one of just three features not named after women or ancient goddesses on the entire surface of the planet, having been grandfathered in before the International Astronomical Union made the decision on nomenclature for further discoveries.
- Named for the Egyptian goddess of truth, Maat Mons is the largest volcano on Venus and the second highest on the planet. Venus is the most volcanically active planet in the entire solar system, with over 1600 major volcanoes across the surface. Indeed, there's evidence that the entire planet is periodically resurfaced in floods of lava.
- Our closest celestial companion, the moon orbits around our planet Earth roughly every 28 days, and rotates around its axis in the same time frame, meaning that we only ever see the familiar side of the surface. It is the fifth-largest moon in the Solar System, behind Saturn's moon Titan and three of the four Galilean moons of Jupiter (Ganymede, Callisto and Io).
- Mare, or seas, are dark features most common on the near side of the moon - where volcanic eruptions have covered these regions in basalt, which reflects less of the sun's light. Generally wide and flat, the most famous of these is the Sea of Tranquility, where on July 20, 1969, Apollo 11 became the first manned lunar landing.
- Perhaps the most notable crater on the near side of the moon thanks to its distinctive rays that spread out across the southern side. It is a relatively young crater by the standards of the moon, having impacted roughly 108 million years ago, which means unlike other prominent craters, it has yet to be malformed by further impacts.
- The largest impact crater on the moon, and one of the largest in the entire solar system is located on the South Pole, creating the South Pole-Aitken Basin. Mostly visible from the far side of the moon, the crater is 1600 miles wide, and 4 to 5 miles deep. On the near side visible from Earth, only the outer ring of the basin, known as the Leibnitz mountains, can be seen.
- Of all the dark spots that splotch the near side of the moon, only Procellarum, the largest by some stretch, was given the title Oceanus - or ocean, in comparison to the mare, or seas, that are defined elsewhere. It covers 10% of the entire lunar surface, and has been visited by a series of probes - the Soviet Luna 9 and 13, and the American Surveyor 1 and 3. Apollo 12 would land close to the Surveyor 3 site, returning parts of the probe home with them, the only time this was ever done.
- Though the name means Eastern Sea, it is so called because it appears on the east from Earth - it is actually on the west side of the moon. Though difficult to see because it lies on the boundary of what is visible, satellite imagery in the 1960s revealed it to have a striking set of concentric circles, making almost a target bullseye on the moon. The inner ring of mountains is known as the Montes Rook, while the outer ring are known as the Montes Cordillera.
- The furthest out of the terrestrial planets, Mars stands out for both its brilliant red hue as well as its likelihood as the next target of human space travel. Named for the Roman god of war, it is the second smallest of all the planets, larger than only Mercury. It's roughly half the width of Earth and about a sixth of the size. Temperatures in the thin atmosphere can range from -225 to 100 degrees Fahrenheit.
- The largest mountain on any planet in the solar system, the volcano Olympus Mons towers over the Martian surface, rising 85,000 feet from the surrounding terrain. It is, in fact, so tall that it almost rises above the Martian atmosphere, close to sticking out all the way into space. At two and a half times the size of Mount Everest, were it to be placed in the deepest part of the Pacific Ocean, it would still rise higher than most commercial jetliners fly.
- The Valles Marineris is a noticeable scar on the Martian surface, a canyon that would dwarf the Earth's Grand Canyon, at four times the depth, a dozen times the length and over ten times the Grand Canyon's maximum width. The most likely reason for its formation is a tectonic crack in the planet's surface, widened over time via erosion and collapses of the rift walls.
- Most of the north of Mars lies in a very wide basin that covers 40% of the entire planet. The basin is considerably smoother than the rest of the planet's surface and is mostly flat, though the volcanic Tharsis Bulge rises above it in the planet's western hemisphere. Speculation abounds as to why the north and south of the planet are so different - whether it be a large asteroid impact (if so, the basin would be the largest crater in the Solar System by a fourfold margin) or perhaps a vast ocean in Mars' distant past.
- This crater impact site is not quite the largest on Mars, but is noticeable for being incredibly deep - over thirty thousand feet lower than the surrounding terrain. The crater is so deep that it causes changes to the Martian atmosphere, and the air pressure at the bottom of the crater is double what it is elsewhere on the Martian surface. In fact, it's one of the few places on Mars that has sufficient pressure to sustain liquid water, were the temperature to stay warm enough.
- The two moons of Mars are tiny - Phobos is less than 15 miles long, with Deimos being less than 10. They are still visible from the Martian surface, however, thanks to orbiting much closer to the planet. They are named after twin sons of the Greek god Ares, who is the analogue to the Roman god Mars. Though it is not known for certain, the most likely scenario is that Mars captured these moons from the asteroid belt.
- Jupiter, named after the king of the Roman gods, is by a considerable margin the largest of all the planets, and the closest of the gas giants in the entire solar system. It has 70% of all the planetary mass in the solar system - even though that still only contributes to about one thousandth of the sun's mass - and is roughly 11 times as wide as the Earth and a thousand times as large. It has no surface, but the temperature in the atmosphere is roughly -150 degrees Fahrenheit.
- The clouds of Jupiter swirl constantly through its atmosphere, easily reaching speeds of 225 miles per hour. Most prominent among the clouds however, is the Great Red Spot, a large and persistent storm south of the planet's equator. It has been observed in the Jovian atmosphere since at least the 17th century, and is larger than the size of the Earth. That said, the storm has been observed getting smaller in recent years.
- Ganymede is the largest moon in the entire Solar System, and is, indeed, even larger than the planet Mercury. The four largest moons of Jupiter are known as the Galilean moons, because they were discovered by Gailileo Galilei, and were the first celestial objects that were seen to be rotating around another planet. Ganymede is the only moon to have a magnetic field, though its magnetosphere is subsumed within Jupiter's much larger one.
- Callisto is the most distant of the four Galilean moons, and the second largest. Like the others, it is named after a mythological lover of Zeus, Jupiter's Greek equivalent. These four moons consist of 99.997% of all the mass in the Jovian system. Callisto is notable for having perhaps the oldest and most pockmarked surface in the entire Solar System - lacking volcanism or plate tectonics, the features of its surface are entirely created by craters.
- Io is the closest of the four Galilean moons to Jupiter, though it is not the closest to the planet - there are four much smaller ones that orbit even closer, known as the Amalthean group. The proximity to Jupiter, however, has made the moon intensely volcanic - it is, in fact, the most volcanically active object within the solar system, even moreso than Venus.
- Made famous by Stanley Kubrick's 2001: A Space Odyssey, Europa is notable for evidence that underneath its icy exterior there may lay a fully liquid water ocean, heated by the tides caused from the orbital resonance between it, Jupiter, Io and Ganymede. This makes it one of the most sought after places to further analyze in the search for biological life forms outside of Earth.
- Named after the Roman god of wealth, agriculture and time, who also served as the father of Jupiter, Saturn is the other gas giant of the Solar System. It's a very light planet, as despite being nine times as wide and about 750 times as big as the Earth, it's only 100 times as heavy - meaning that it could actually float, were there a body of water long enough. Like the other giant planets, it has no true surface, but the temperature in the atmosphere is about -200 degrees Fahrenheit.
- Easily the most defining attribute of Saturn is its extensive ring system. While all of the giant planets of the system have some sort of ring structure, Saturn's is by far the most prominent. Going from roughly 4000 miles above the equator to fifty thousand miles out, the rings are remarkably thin - less than a mile thick
- and made almost completely out of ice.
- Both the north and south pole of Saturn are the center of some long lasting storms with some curious features. A large hexagon shaped storm, eighteen thousand miles wide spins around the North Pole of Saturn at roughly 200 miles per hour. At the South Pole, a hurricane like vortex - with an eyewall formation never seen outside our planet - is roughly the same size of Earth and has winds up to 340 miles per hour.
- The largest moon around Saturn and the second largest moon in the entire Solar System behind only Ganymede, Titan is unique among all satellites in that it has a full atmosphere covering it. Indeed, until the Cassini-Huygens probe began investigating it in the early 2000s, much underneath the atmosphere remained a mystery. However, underneath the clouds lies the only other known location of liquid seas - though they're of hydrocarbons instead of water.
- Mimas is the closest and smallest of Saturn's major moons - though there are over 80 of them, most of the mass is contained in 7 - and of those, 96% is in Titan. Mimas has a large crater called Herschel, with a diameter a third of the entire moon, and which gives it a more than passing resemblance to the Death Star of Star Wars fame.
- Third largest and furthest out of the major moons of Saturn, Iapetus has a couple of striking features of note. First is the equatorial ridge that runs most of the way around the moon, giving it a sort of walnut shape. The other is that it has two distinctive sides - one a bright, reflective white, and the other a dark reddish-brown, which causes it to be more visible from one side of Saturn than the other.
- Unique in several aspects, Uranus is named after the Greek, not Roman, god of the sky, father to Cronus (Saturn's equivalent) and grandfather to Zeus (Jupiter's equivalent). It is the first planet to have been discovered, by William Herschel, in 1781, rather than been known to the ancients. It and Neptune form the ice giants - smaller than the two gas giants, but still considerably larger than the terrestrial planets. Four times as wide and sixty times as large as Earth, the sheer distance from the sun means that the average temperature runs roughly -320 degrees Fahrenheit.
- While most planets spin around themselves vertically - or at the very least only a slight tilt, Uranus is an oddball in the sense that it rotates more or less on its side, having an axial tilt of 97 degrees. This means that the two poles alternate between facing the sun - and given the length of the Uranian year, the day-night cycles for the poles last over 40 Earth years.
- The smallest of Uranus' five major moons, Miranda is perhaps the most intriguing because of its unique topography - rather than a round sphere, it is has jagged and broken terrain. Among these features lies the largest cliff in the Solar System
- a scarp called Verona Rupes, 12 miles from top to bottom. Its low gravity, however, means that it'd take roughly 12 minutes for you to actually fall off of it, and might still be survivable with enough impact cushioning.
- Named for a shadowy fairy from the literature of Alexander Pope, Umbriel is the third largest of Uranus' moons, but also the darkest. There is, however, a crater on the moon, named Wunda, that has left behind a ring of bright material that stands out on the duskier surface. It lies on the moon's equator - though because all of Uranus' moons share the planet's odd axial tilt, it appears in the Voyager 2 images as if it were on the moon's north pole from a casual observation.
- The largest of Uranus' moons, Oberon and Titania were discovered shortly after the planet itself in 1787, by the same William Herschel who had found the planet in the first place. As the first planet not known to antiquity, naming the planet was something of an unexplored avenue. It was originally named after the British King George, before Uranus was proposed a year later. The nod to its British discoverer would be retained in its moons - they're all characters from English literature, Oberon and Titania being the king and queen of fairies from Shakespeare's Midsummer Night's Dream.
- The outermost of the planets since Pluto's demotion after the discovery of the Kuiper Belt, Neptune is named after the Roman god of the sea - an apropos name, given the deep, vibrant blue of the planet. It is the only planet that wasn't found by direct observation - astronomers noted slight disturbances in the orbit of Uranus that implied another planet was out there and was then found within a degree of where calculations expected it. It's only slightly smaller than Uranus
- a fraction under 4 Earth widths and 60 Earth sizes and shares a similar temperature.
- The Great Dark Spot was first discovered in 1989, when Voyager 2 was approaching the planet for a fly-by. Though similar to the Great Red Spot on Jupiter, the storm was smaller - though still roughly Earth sized - and much fiercer, with the highest winds in the Solar System - 1300 miles per hour. In 1994, when observed by Hubble, however, the storm had disappeared. Since then, it's become known that these storms are transient - lasting years, but not permanently. They are however, frequent - an observation in 2016 showed a new one had formed, and has remained in latest observations.
- Triton is the by far the largest of Neptune's moons, accounting for a full 99.7% of all of the mass of all of the moons around the planet, and the seventh largest in the entire Solar System behind the Galileans, Titan and our own. It is also unique in that it was originally an object in the outer Solar System, like Pluto, before being captured by Neptune to become a moon. This event may have destroyed some of the moons already around Neptune, giving the planet its faint rings.
- Because of Neptune's distance, and their relatively small size, there has been extremely limited direct observation on the rest of the moons of Neptune. The only probe to pass through the system was Voyager 2 back in 1989 and did not approach any of the small moons close enough for many detailed images. There's still a lot we can detect from afar, however. Proteus, the second largest, for example, was likely amalgamated from debris caused by Triton's capture. Nereid, the third largest, was thrown into a highly eccentric orbit. They both may have also fragmented at some point, creating the moons Hippocamp and Halimede, respectively.
- As the furthest major planet from the sun, Neptune has a large influence on the orbits of the objects within the Kuiper Belt. Astronomers have begun categorizing them by the type of orbital resonance they have with Neptune. The most famous of these is Pluto, the largest Trans-Neptunian Object (TNO) found so far, and the defining example of what have come to be known as Plutinos, which have a 2:3 resonance with Neptune (that is, they revolve twice for every 3 times Neptune does), but there are countless different types - some of the more common are 3:5, 4:7 and 1:2 (the last of which have been nicknamed twotinos)