MERCURY's orbit around the Sun takes only 88 days, the shortest of all the planets in the Solar System. It is tidally locked with the Sun in a 3:2 spin-orbit resonance... [see below]
VENUS has the densest atmosphere of the four terrestrial planets, consisting of more than 96% carbon dioxide; the atmospheric pressure... [see below]
[MERCURY] ...meaning that relative to the fixed stars, it rotates on its axis exactly three times for every two revolutions it makes around the Sun. Having almost no atmosphere to retain heat, Mercury has surface temperatures that vary diurnally more than on any other planet in the Solar System, ranging from -173 °C at night to 427 °C during the day (across the equatorial regions).
[VENUS] ...at the planet's surface is 92 times that of Earth. The planet is shrouded by an opaque layer of highly reflective clouds of sulfuric acid, preventing its surface from being seen from space in visible light. By far the hottest planet in the Solar System (with a mean surface temperature of 462 °C), Venus is a dry desertscape interspersed with slab-like rocks and is periodically resurfaced by volcanism. In August 2019, astronomers reported that newly discovered long-term pattern of absorbance and albedo changes high up in the atmosphere of the planet Venus are caused by chemicals or even large colonies of microorganisms.
MESSENGER's path involved a complex series of flybys – the spacecraft flew by Earth once, Venus twice, and Mercury itself three times – allowing to decelerate relative to Mercury using minimal fuel. It entered orbit around Mercury on March 18, 2011, becoming the first spacecraft to do so. Within its primary mission and two mission extensions, MESSENGER achieved 100% mapping of Mercury, a characterization of Mercury's magnetic field, and the discovery of water ice and organic compounds at the planet's north pole. On April 30, 2015, the spacecraft used the last of its maneuvering propellant to deorbit, impacting the surface of Mercury.
The MARINER 10 probe was the first spacecraft to use a gravity assist maneuver to reach another planet – passing by Venus on February 5, 1974 on its way to explore Mercury. During its flyby of Venus, Mariner 10 discovered evidence of rotating clouds and a very weak magnetic field. Planning for MESSENGER relied extensively on data and information collected by Mariner 10. The probe is presumably still orbiting the Sun, although its electronics have probably been damaged by the Sun's radiation.
The MOON is a relatively large, planet-like natural satellite... [see below]
Four billion years ago, chemical reactions led to the first self-replicating molecules on EARTH... [see below]
[EARTH] ...and a half billion years later, the last common ancestor of all current life arose. Over 99% of all species that ever lived on Earth are now extinct. Over 7.6 billion humans live on Earth, and depend on its biosphere and natural resources for their survival.
The habitable zone is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure. The potential habitability depends also on temperature ranges, presence of nutrients and an energy source, and protection from solar wind and cosmic radiation (an effective magnetosphere).
About 1 in 5 Sun-like stars have an "Earth-sized" planet in the habitable zone. Assuming there are 200 billion stars in the Milky Way, it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in our galaxy – rising to 40 billion if planets orbiting the numerous red dwarfs are included.
[MOON] ...with a diameter about one-quarter of Earth's. The natural satellites of other planets are also referred to as "moons", after Earth's. The gravitational attraction between Earth and the Moon causes tides on Earth. The same effect on the Moon has led to its tidal locking: its rotation period is the same as the time it takes to orbit Earth.
Earth's largest artificial satellite is the INTERNATIONAL SPACE STATION.
MARS is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon... [see below]
CERES is the largest object in the main asteroid belt... [see below]
[MARS] ...and the valleys, deserts, and polar ice caps of Earth. The days and seasons are likewise comparable to those of Earth, because the rotational period as well as the tilt of the rotational axis relative to the ecliptic plane are very similar. Mars is the site of Olympus Mons, the largest volcano and highest known mountain in the Solar System, and of Valles Marineris, one of the largest canyons in the Solar System.
There are ongoing investigations assessing the past habitability potential of Mars, as well as the possibility of extant life. Liquid water cannot exist on the surface of Mars due to low atmospheric pressure, which is less than 1% of the Earth's, except at the lowest elevations for short periods. The two polar ice caps appear to be made largely of water; the volume of water ice in the south polar ice cap, if melted, would be sufficient to cover the entire planetary surface to a depth of 11 meters. Mars has two moons, PHOBOS and DEIMOS, which are small and irregularly shaped – these may be captured asteroids.
ROSETTA's primary mission was to perform a detailed study of comet 67P/Churyumov-Gerasimenko. The comet's orbit was previously known to an accuracy of approximately 100 km; information gathered by onboard cameras beginning at a distance of 24 million kilometers were processed to refine the position of the comet to a few kilometers. In 2007, Rosetta made a Mars gravity assist – the flyby was successful, also returning detailed images of the surface and atmosphere of the planet. On September 2014 the probe entered orbit around 67P, and two years later ended its mission by hard-landing on the comet in its Ma'at region.
Mars Reconnaissance Orbiter (MRO) is a multipurpose spacecraft designed to conduct exploration of Mars from orbit. During orbital insertion, all six of MRO's main engines burned for 27 minutes (counting for 70% of its total propellant) to slow down the probe from 2,900 to 1,900 m/s. In November 2006, after five months of aerobraking, it entered its final science orbit. MRO's HiRISE camera has a resolution of 0.3 m from an altitude of 300 km (satellite images of Earth on Google Maps are available to 1 m). MRO is paving the way for future spacecraft by monitoring Mars' daily weather, studying potential landing sites, and hosting a highly capable telecommunications system; it has enough propellant to keep functioning into the 2030s.
[CERES] ...constituting about 25% of its total mass. It's neither an asteroid (because rounded by its own gravity) nor a planet (it does not dominate its orbit, sharing it with thousands of asteroids) – but a dwarf planet. The surface of Ceres is a mixture of water ice and various hydrated minerals such as carbonates and clay; there is evidence that Ceres' icy mantle was once a watery subterranean ocean.
The robotic spacecraft DAWN entered orbit around Ceres on March 2015, and remained active to November 2018. Dawn is still close to Ceres, in an uncontrolled orbit.
JUPITER is a gas giant with a mass one-thousandth that of the Sun and two-and-a-half that of all the other planets... [see below]
[JUPITER] ...in the Solar System combined. Jupiter is so big that it does not technically orbit the Sun: both orbit a combined center of gravity (barycenter), some 30,000 miles away from the Sun's surface. The Great Red Spot is a giant storm that is known to have existed since at least the 17th century when it was first seen by telescope. Surrounding Jupiter is a powerful magnetosphere, a faint planetary ring and 79 known moons, including the four large Galilean moons: IO, EUROPA, GANYMEDE and CALLISTO (all four orbiting within the magnetosphere, hence protected from the solar wind).
The three inner moons—Io, Europa, and Ganymede—are in a 4:2:1 orbital resonance with each other, meaning that for every four orbits that Io makes around Jupiter, Europa makes exactly two and Ganymede makes exactly one. So each moon receives an extra tug from its neighbors at the same point in every orbit it makes, and the induced eccentricity of their orbits causes regular flexing of the moons' shapes, with Jupiter's gravity physically stretching them out as they approach it (perijove), and allowing them to spring back to more spherical shapes as they swing away (apojove). This tidal flexing heats the moons' interiors by friction, and is seen most dramatically in the extraordinary volcanic activity of innermost Io, and to a lesser degree in the geological youth of Europa's surface.
Io's volcanic plumes and lava flows paint its surface in various subtle shades of yellow, red, white, black, and green. Volcanic plumes can climb as high as 500 km above the surface, and some of this ionized material escapes Io's gravitational pull and goes into orbit around the planet, giving Jupiter a magnetic field inflated to more than twice the size it would otherwise have.
Europa has a water-ice crust and erupting cryogeysers. The apparent youth and smoothness of the surface have led to the hypothesis that a water ocean exists beneath it, which could conceivably harbor life. The predominant model suggests that heat from tidal flexing causes the ocean to remain liquid and drives ice movement similar to plate tectonics.
In December 1995, GALILEO became the first spacecraft to orbit Jupiter; it also launched the first probe into Jupiter, directly measuring its atmosphere. Besides recording Io's volcanism and plasma interactions with Jupiter's magnetosphere, Galileo collected data supporting the theory of the liquid ocean under the icy surface of Europa, and there were indications of similar liquid-saltwater layers under the surfaces of Ganymede and Callisto. After 14 years in space and 8 years in the Jovian system, Galileo's mission was terminated by sending it into Jupiter's atmosphere, eliminating the possibility of contaminating local moons with terrestrial bacteria.
JUNO is a space probe orbiting Jupiter from July, 2016; its primary mission is to measure Jupiter's composition, gravity and magnetic field, and its deep winds – which can reach speeds up to 600 km/h. Juno is the first spacecraft sent to the outer planets, powered by solar arrays – the largest ever deployed on a planetary probe (away from the Sun, spacecraft are commonly powered by radioactive decay within RTGs).
Like Jupiter, SATURN's interior is probably composed of a core of iron-nickel and rock, surrounded by a deep layer of metallic hydrogen, an intermediate layer of liquid hydrogen and helium, and finally... [see below]
[SATURN] ...a gaseous outer layer. If placed on water (!) Saturn will (be the only planet in the Solar System that will) float, being overall 30% less dense. Also, rainfalls of diamonds have been suggested to occur within Saturn, as well as in Jupiter and ice giants Uranus and Neptune. However, the planet's most famous feature remains its prominent ring system that is composed mostly of ice particles, with a smaller amount of rocky debris and dust.
At least 82 moons are known to orbit Saturn; among them, TITAN is the largest and the only one in the Solar System to have a substantial atmosphere (largely nitrogen) and stable bodies of surface liquid (methane and ethane). Titan is more likely composed of a rocky core surrounded by various layers of ice, including a crust of ice and a subsurface layer of ammonia-rich liquid water. The climate – including wind and rain – creates surface features similar to those of Earth, such as dunes, rivers, lakes, seas and deltas. Titan's methane cycle is analogous to Earth's water cycle at quite the same pressure (1.45 atm) but much lower temperature (-179.2 °C). Titan spends 95% of its time within Saturn's magnetosphere, which shields it from the solar wind.
Titan is thought to be a prebiotic environment rich in complex organic compounds. It has been speculated that life could exist in the lakes of liquid methane on Titan, just as organisms on Earth live in water. Conditions on Titan could become far more habitable in the far future: five billion years from now, as the Sun becomes a red giant, its surface temperature could rise enough for Titan to support liquid water on its surface, making it habitable for several hundred million years.
RHEA, the second-largest moon of Saturn, is made of ~25% rock and ~75% water ice. IAPETUS is often characterized as walnut-shaped, because its unique equatorial ridge is so high that it visibly distorts Iapetus's contour even when viewed from a distance.
ENCELADUS is mostly covered by fresh, clean ice, with water-rich plumes venting from the south polar region. These cryovolcanoes, alimented probably by a large south polar subsurface ocean with a thickness of around 10 km, shoot (to a maximum height of 500 km) also molecular hydrogen, sodium chloride crystals and even complex organic compounds. Like occurring with Io, Enceladus' orbital eccentricity damped by Saturn's tidal forces are responsible for this geological activity.
The CASSINI-HUYGENS Flagship-class spacecraft comprised both NASA's Cassini probe and ESA's Huygens lander. With 13 years spent around Saturn (following orbital insertion on July 1, 2004), the mission is widely perceived as one that has revolutionized our understanding of where life might be found in the Solar System. The Huygens probe entered the atmosphere of Titan on January 14, 2005, and after a two-and-a-half-hour descent landed by parachute on solid ground – this was the first landing ever accomplished in the outer Solar System and the first landing on a moon other than Earth's Moon.
Operations ended by sending Cassini into Saturn's upper atmosphere, where it burned up. The mission, involving the participation of 27 nations, called for 2.5 million commands executed (of which 360 engine burns), 635 GB science data collected (including 453,048 images), 4.9 billion miles traveled, 162 targeted flybys of Saturn's moons, and 3,948 science papers published.
URANUS' atmosphere is similar to Jupiter's and Saturn's... [see below]
NEPTUNE is the only planet in the Solar System found by... [see below]
[URANUS] ...in its primary composition of hydrogen and helium, but it contains more "ices" such as water, ammonia, and methane, along with traces of other hydrocarbons. It has the coldest planetary atmosphere in the Solar System, with a minimum temperature of -224 °C; wind speeds can reach 900 km/h. In 2033, the planet will have made its third complete orbit around the Sun since being discovered in 1781. Uranus has 27 known natural satellites, of which TITANIA is the largest. OBERON is differentiated into a rocky core and an icy mantle; a layer of liquid water may be present at the boundary between them.
[NEPTUNE] ...mathematical prediction rather than by empirical observation. It features the strongest sustained winds of any planet in the Solar System, with recorded wind speeds as high as 2,200 km/h – nearly reaching supersonic flow. Neptune's orbit has a profound impact on the region directly beyond it, known as the Kuiper belt . The most heavily populated resonance in the Kuiper belt, with over 200 known objects, is the 2:3 resonance; objects in this resonance complete 2 orbits for every 3 of Neptune, and are known as plutinos because the largest of the known Kuiper belt objects, Pluto, is among them. Although Pluto crosses Neptune's orbit regularly, the 2:3 resonance ensures they can never collide. Neptune has 14 known moons; TRITON is the largest, comprising more than 99.5% of the mass in orbit around Neptune, and is the only one massive enough to be spheroidal. Triton is one of the few moons in the Solar System known to be geologically active (the others being Io, Europa, Enceladus and Titan).
VOYAGER 2 is a space probe launched on August 20, 1977, to study the outer planets. An particular planet alignment that occurs once every 175 years enabled Voyager 2 to visit Jupiter (July 1979), Saturn (August 1981), Uranus (January 1986) and Neptune (August 1989) by using successive gravity assist maneuvers. The probe left the heliosphere for interstellar space on November 5, 2018, becoming the second artificial object to do so (subsequently of Voyager 1). After 42 years, the Voyagers are still (partially) active.
PLUTO is a dwarf planet in the Kuiper belt; it is the largest known... [see below]
Pluto and CHARON are considered a binary system because the barycenter of their orbits... [see below]
[PLUTO] ...trans-Neptunian object by volume but is less massive than Eris. Pluto is primarily made of ice and rock and is about one-sixth the mass of the Moon and one-third its volume. Pluto has five known moons, of which CHARON is the largest, with a diameter just over half that of Pluto.
[CHARON] ...does not lie within either body. The reddish-brown cap of the north pole of Charon is composed of tholins, organic macromolecules that may be ingredients for the emergence of life, and produced from methane, nitrogen and other gases released from the atmosphere of Pluto and transferred about 19,000 km to the orbiting moon.
The NEW HORIZONS probe performed a flyby of Pluto on July 14, 2015, becoming the first ever spacecraft to do so. Jupiter provided a gravity assist to the probe, and most of the post-Jupiter voyage was spent in hibernation mode to preserve on-board systems. Having completed its flyby of Pluto, New Horizons then maneuvered for a flyby of Kuiper belt object "Ultima Thule", which occurred on January 1, 2019.
Currently, Pioneer 10 and 11, Voyager 1 and 2, and New Horizons are (the only spacecraft) leaving the Solar System because their velocity and direction are taking them away from the Sun, and at their distance from the Sun, its gravitation is not sufficient to pull them back or into orbit.
