Theory Observation |
Solar System Planets
IN SHORT - The planets of our solar system were born 4.5 billion years ago from a protoplanetary disc surrounding the Sun. Mercury is Moon-looking with a crust cracked by faults. Venus has a dense atmosphere with sulfuric rains. |
Mars is a desert-like planet. Jupiter, Saturn -with its famed ring, Uranus and Neptune are gas giants with their worlds of moons, as Pluto might be an icy body captured at the ultimate borders of the solar system |
The solar sytem had been born 4.5 billion years ago, when the Sun formed from a gas and dust nebula. A disk of material settled along our star's equatorial plane, forming a proto-planetary disk. This disk split and gravitational instabilities formed smaller bodies. Through further accretion and collisions, such bodies eventually formed the nine planets. As the Sun continued to form, its radiation and heat blew the most volatile elements off the inner regions of the solar system with the planets forming near the Sun mainly formed upon heavy metallic and silicate elements, as further planets forming upon more volatile elements, like ice and gas. Solid materials and gravitation further produced a dynamical interior at the inner, rocky planets. They heated, melted, and layered, a process called differentiation. Gas giants, as far as they are concerned, accreted gas layers around a solid, proto-planet core. Once they had been born, the nine planets endured a period of heavy bombardment, about 4 billion years ago, as numerous leftovers had been left from the planets' formation. They were swept away by the newly formed planets. A flurry of comets and asteroids just punched all of them and their systems. Such a bombardment, on the other hand, brought to the planets the buildings blocks of life, like water and fundamental molecules. Once the leftovers evacuated, the solar system, as we know it, emerged definitively, with the planets, from Mercury to Pluto, the asteroid belt between Mars and Jupiter, and two faraway belts -the Kuiper Belt and the Oort Cloud- of primordial, remaining, materials and bodies
How Did the Solar System Evolve? A recent study, in 2007, modelized a possible evolution of the solar system, with the protoplanetery gas and dust cloud having lasted for about 10 million years while the gas giants, like Neptune and Uranus were close together and close to the Sun and Jupiter and Saturn amazingly featuring a sole mass. Then, at 10 million years after the birth of the solar system, the gas and dust cloud dissipated as the Kuiper Belt forms, being less large than today as it contained 100 times comets more than now! Between 10 and 700 million years, the KB comets come and hit the gas giants, which those, Jupiter excepted, migrating outwards -albeit Neptune keeping coming before Uranus. Then, at 700 million years, it's the telluric planets which are in turn bombarded due to that the gas giants have orbits with larger eccentricites than today and sending the comets into the inner solar system. From 730 to 800 million years after the solar system was born, it eventually reaches its current shape, with Neptune becoming the last planet of the system, the eccentricities of the gas giant decreasing to their values of today. A question posed by the study, at the light of the discovery of solar systems around the other stars, is why our system did not end like most of those found until now, that is a star, and, mostly, "hot Jupiters". Is the solar system a singularity? Or is that appearance due to that the tools of today don't allow to see such systems, akin to ours, around other stars?
The International Astronomical Union (IAU) brought some important change into the solar system nomenclature. Taking act that 'contemporary observations are changing our understanding of planetary systems', the international body established three distinct categories for the planets and other bodies in the solar system: 'planets', which are celestial bodies orbiting the Sun and having a sufficient mass for self-gravity to have shaped them in an about spherical form, as, on the other hand, they have cleared their neighbourhood on their orbit; 'dwarf planets' are bodies orbiting the Sun, having a sufficient mass for self-gravity to have shaped them in an about spherical form too but which have not cleared their neighbourhood on their orbit, as, at last, they are not a satellite; 'small solar-system bodies", which are all other objects in the solar system (like comets, asteroids and the objects of the Kuiper Belt), orbiting the Sun, at the exception of the satellites. The IAU, further, dismissed Pluto from its status of planet. Pluto is now a 'dwarf planet' as, moreover, it's the prototype of a new category of 'trans-Neptunian', 'Pluto-class' objects. This leaves us with 8 'real' planets in the solar system only: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune
Mercury was named from the Roman fleet-footed messenger of Gods, due to that the planet was seen moving swiftly along either the eastern or western horizon, before sunrise or after sunset. Hence Mercury remained an ill-known, elusive planet until the space age. It was always low on the horizon, and badly observable. Mercury, on the other hand, is rotating one and a half time for one orbit around the Sun. The day, at Mercury, is lasting 176 Earth days! Mercury is very similar to our Earth's Moon and has a relatively similar history. Once the heavy bombardment period over, Mercury was flowed by lava which covered the old silicate crust. Then the inner layers of Mercury cooled down to the core. The crust cracked with large faults creating. Some more lava floods occurred at some parts of Mercury, as the bombardment of micrometeorites eventually produced a dusty surface, made of a material called regolith. Some more large impactors created some more craters. Mercury has no atmosphere, as it has a magnetic field and a magnetosphere. The magnetic field might be due to a magneto molten core, like at Earth, or it might be the remnant of a former strong magnetic activity. Mercury's magnetosphere is tilted 7° relative to the planet's axis. Like on the Moon, one's guessing that there might be some water ice at Mercury, inside the polar-located craters. Mercury was explored from orbit by the Mariner 10 spacecraft in 1974-1975 as NASA's mission Messenger is now en route for a renewed study of the planet beginning in 2009. Mercury is trailing behind it a sodium-rich exospheric tail extending more than 25,000 miles (40,000km) from the planet as a first passage of the MESSENGER mission to the planet in 2008 revealed a hydrogen tail of similar dimensions
Mercury Really Seems to Have a Molten Core!
A radar study in 2007 might have that Mercury really has a molten core due to that the radar echoes showed tiny twists in Mercury's orbit around the Sun, hinting to such a molten core, or, at least, an outer molten core. Such a molten state might well be the explanation to the current, slight magnetic field observed at Mercury. To maintain such a molten core, Mercury would have had to contain a lighter element, such as sulfur in its center. At last, to get sulfur in its composition, Mercury would have had to combine elements from close to the Sun and from farther away -an idea known under 'radial mixing'
Recentest Findings About Mercury Recentest data returned by NASA's MESSENGER mission during its first steps towards Mercury are the following: Caloris basin, one of the largest impact basins in the solar system had its interior filled with lava according to a process similar to what occurred at the Moon; the magnetic field of Mercury likely is due to a core magneto; it's the cooling of the oversized core which triggered a remarkable contraction of the planet, with scarps and faults, with the contraction of Mercury one third greater than previously thought; the magnetosphere of Mercury is filled with many charged particles forming a 'plasma nebula' akin to Io plasma torus; the thin Mercury atmosphere -its 'exosphere' is replenished with particles from the surface processes and provides for a long tail of atoms flowing behind the planet
Venus was ill-known until space exploration began, as its surface was hidden to observation by a dense cloud cover and as the planet, like Mercury, was an evening or morning star. Its size, mass, density, and volume are similar to the ones of Earth. Its atmosphere is mainly composed of carbon dioxide however, as sulfuric acid rains are falling from its clouds. Recent studies by an European mission to Venus have shown that lightnings in the atmosphere may trigger the formation of molecules. Venus, along with Jupiter, Saturn and the Earth thus is a planet where lightnings occur. The surface atmospheric pressure is 92 times Earth's and the temperature is about 900° F (482° C). The atmosphere of Venus endured a runaway greenhouse effect as the Sun's radiation was trapped by the cloud cover in a cumulative way. Venus' day is 243 days long and the planet is orbiting the Sun in 225 days. It's likely that the planet's axis of rotation is nearly upside down, giving Venus an apparent retrograde rotation, that is that Venus is rotating clockwise. The planet's cloud cover is "super-rotating" that is that the clouds are rotating faster than the surface below them. Venus has a volcanic activity, which is is shaping, and resurfacing the landscape. Venusian relief features are believed to have been last resurfaced 300 to 500 million years ago. Venus was mainly explored by the Magellan spacecraft at the beginning of the 1990s. Craft from the former USSR managed to land on the surface between 1975 and 1981
Mars is the famous Red Planet. Its red color brought to its name taken from the Roman god of war. It was thoroughly observed by diverse crafts and probes, from orbit or from the surface, like Mariner 4 in 1965, the Viking landers or more recent missions. Mars is a rocky world with craters, ancient volcanoes, large plains, and a famous canyons' system, Vallis Marineris. Mars' famous polar caps are alternately freezing and vaporizing along the seasons. The average temperature at Mars is - 81° F (- 63° C), as the pressure is low. Mars has a thin atmosphere which is composed of carbon dioxide. Mars was always considered the best match for a life-hospitable place in the solar system. Percival Lowell, at the end of the 19th century, thought Mars had a channels' net linked to living creatures. The life question was scientifically investigated by the Viking landers about 1976, and the answer was mostly negative. The question of life at Mars is always pending and still agressively searched, as it's now related to the presence of water. Two small moons are orbiting around Mars: Phobos and Deimos
Jupiter is the solar system's largest planet, and the first of the gas giants. Such large, gazeous planets are located beyond Mars and the asteroid belt, and they are mainly made of gas which accreted around a very small solid core. The gas is organized in layers, the nearer the core, the denser. Swirls, bands and spots are the marks of weather events in the layers' upper part. One of these spots, the Great Red Spot, is an hurricane which had been born three centuries ago. Jupiter has a large satellite system, with four, large, well-known satellites. These are called the "Galilean satellites" because they were discovered by Galileo Galilei, the first astronomer to use a telescope ever in the 17th century. Jupiter has the most powerful magnetosphere in the solar system. It's acting like a powerful radio beacon, as its tail probably extends until the orbit of Saturn! Jupiter was explored by the Pioneer and Voyager missions, and, lately, by the Galileo spacecraft which spent some years in orbit around Jupiter, providing a thorougful study of the Jovian system. First missions found that Jupiter had a faint ring system. The shadow of Jupiter is a factor about the rings of the planet
Saturn is the solar system's second-largest planet, after Jupiter. It's very similar to the latter. Saturn is a gas giant, with gas in layers around a small, solid inner core. The most distinctive trait of Saturn is obviously its famed ring which is a fine show to any amateur astronomer. Saturn's ring is thought to have originated from collisions between the planet's moons or from a foreign body which was disrupted by the planet's gravitational field. The ring, in no place, is thicker than a 2-story building, as it's composed of billions of individual, icy or rocky particles, each one orbiting the planet on its own path. The weather activity is not as pronounced at Saturn than the one which is seen at Jupiter. Winds are reaching much stronger speeds however. Saturn was explored by the Voyager 1 and 2 missions, as the most recent Cassini mission will spend four years in orbit studying the Saturnian system and Titan, Saturn's main moon. Saturn, like Jupiter, has a vast system of satellites. The ringed planet takes 29 years to complete its orbit. Saturn's atmosphere is affected by a wave oscillation on a pattern of 15 years (each Saturn's half year) either side of the equator, with the changes in temperature layers bringing the winds keeping changing direction and generating that oscillation. Such phenomenons exist too at Jupiter and the Earth
Uranus was fully revealed, in 1986, by the Voyager 2 mission which showed it like a fine, light blue-green world. As seen from Earth, Uranus is at the limit of the visual magnitude. It was discovered in 1781 only by William Herschel, an English astronomer. Uranus is another gas planet and it's orbiting the Sun in 84 years. Its axis of rotation is strongly tilted by 98 °, hence the planet is seen lying on its side! Voyager 2 found that Uranus had a ring system. This faraway world has about 20 large and smaller satellites. Uranus has a magnetosphere which is tilted 60° relative to the weird planet's axis
Neptune is the last of the gas giants. Its size is similar to Uranus'. It is structured into gas layers as it has a strongest weather activity than the previous planet. Some large spots, similar to those of Jupiter, may be observed. Neptune has the strongest winds in the solar system, with speeds above 1,240 mph (2,000 km/h). Neptune has an axis tilt of about 28° as its magnetosphere is tilted by 47° and shifted by half a radius off the planet's center. Such an odd tilt and shift are thought to originate at the planet's internal gas flows, like at Uranus. Neptune was discovered in 1846 only, from orbital predictions made by Le Verrier. Neptune has a ring system and about 10 moons. The planet was first visited by Voyager 2 in 1989. Neptune takes a long 165 years to orbit the Sun! The length of summer, hence the uninterrupted daylight, during 40 years to one of the planet's poles, brings to that the methane in the stratosphere is warming and escaping into space there, as the pole is warmer than the rest of the planet by 18° F (10° C) as this might be a factor too to the strong winds of Neptune
Pluto is the solar system's ninth and last planet. It was discovered in 1930, from a systematic search along the ecliptic. Pluto is located at the far reaches of the Sun where this faraway planet is completing its orbit in 249 years -two centuries and a half!. Pluto's orbit is highly eccentric, and highly inclined by 17° above the ecliptic, the average plane of the solar system. The eccentricity of the orbit takes Pluto sometimes inside the orbit of Neptune. Pluto is not a gas giant. It's a rocky planet instead, like Earth or Mercury. It has an atmosphere. Pluto is strongly tilted on its axis by 122°, which means that the planet is lying below the horizontal, with its south pole pointing upwards. Pluto has a thin atmosphere. NASA's New Horizons, scheduled to launch in 2006 for an arrival in 2015, will be the first space mission ever to reach there! Some more science about Pluto came about 1985-1900 when Charon, the planet's moon, was seen orbiting edge-on from ground observatories. Hubble provided some pictures too. Some are questioning the planetary nature of Pluto, as they think the planet might be better related to the Kuiper Belt, this zone of primordial objects dating back to the formation of the solar system, and found beyond the orbit of Pluto
Our solar system has been our backyard since mankind appeared. It now seems not to be so unique in the Universe however. Recentest science thinks that solar systems are inherent to stars' formation. A lot of them, Earth-like planets included, are expected to be found around numerous stars
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