“Since nothing prevents the Earth from moving, I suggest that we should now consider also whether several motions suit it, so that it can be regarded as one of the planets. For it is not the center of all the revolutions.” ~ Nicolaus Copernicus
The Sun captured 99.8% of local system mass. (About 1 million Earths could fit inside the Sun.) Jupiter grabbed most of the leftover matter, leaving Saturn as an also-ran. Jupiter’s early formation is the reason that the inner solar system has such puny planets.
The early formation of the solar system was a carnival of debris collisions. The basic idea that tiny grains stick together into a gathering ball and swoop up gas conceals many levels of intricacy.
A chaotic interplay among various mechanisms occurs during star system development. A divergent diversity of outcomes is possible. The formation of the solar system is physics applied to happenstance.
Though shifts in the solar system were affected by the sweeping motions of both gas giants, Jupiter’s movements were decisive in the solar system’s early evolution, owing to its girth: 2.5 times the mass of all other planets combined.
4 rocky bits stabilized into the inner planets, all terrestrial: Mercury, Venus, Earth, and Mars.
In geological time, planets form fast. Jupiter gained its great girth within a very few million years of the solar system starting up. Transforming nebular dust into a rocky planet takes only tens of millions of years.
Planetary core formation begins within a million years of the first solids beginning to condense. Once a few hundred kilometers in radius, a planet on the make is large enough to retain heat.
Thermodynamics spur transformation. Chemical components differentiate by weight. Molten metals sink to form a core. Lighter liquefied silicate rises to form a crust.
The inner 4 have rocky outer shells and metallic cores. The family resemblance ends there.
Earth and Venus are roughly the same mass, size, and composition, but Earth is swathed in a life-sustaining atmosphere, while the Venusian atmosphere is acid-laced, crushingly dense, and hot enough to melt lead. Venus sports a single, immobile shell of rock. Earth is encased in tectonic plates topped by cruising continental crusts. Earth is oceanic. There is no sign that Venus ever hosted an ocean.
Earth’s churning iron core generates a magnetic field. Earth has a large moon that sways its tides, and it rotates 365 times per orbit. In stark contrast, Venus is moonless and bereft of a magnetic field. Venus rotates, but backward, and less than once per Venusian year, which is 224.7 Earth days.
The more diminutive pairing of Mercury and Mars is another study in contrast. Mars, with 11% of the mass of Earth, has twice the mass of Mercury, but its core-generated magnetic field sputtered out early on.
While smaller, Mercury is denser than Mars. Minuscule Mercury is still spinning a magnetic field, albeit a weak one, centered far from the center of the planet.
Of the first 4 planets in the solar system, only Mercury lacks an atmosphere. Mercury is so close to the Sun that it is constantly scoured by the solar wind.
Mercury’s huge metallic core is cooling. It has shrunk 11 km since its formation. Part of this owes to Mercury’s thermal character. With no atmosphere to retain heat, Mercury’s surface ranges from 100 K (–173 °C) during the night to 700 K (427 °C) during the day at the sunny equator; the greatest variation among solar planets.
A migrant that made its way in from further out in the solar system, Mercury is the most deeply cratered of the solar planets.
With a rotational axis perpendicular to its orbital plane, Mercury’s poles never tip to the Sun. Hence, many of its polar craters never see the Sun. Deep within they preserve a trillion tonnes of water ice.
Closer to the rim of craters, where the ice warms to a watery sheen, organic matter has been discovered. While “organic” should not be confused with “biological,” the possibility of mercurial microorganisms exists.
Mars formed rather quickly, within 2 to 4 million years. This hasty accretion created a planetary structure with a less intense convection dynamic, making for modest magnetic mojo.
Mars should be 1.5 to 2 times the mass of Earth. Mars is instead only 10%. The gas left over from Jupiter’s formation meddled with the rocks forming Mars, making them fall apart rather than clump together.
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Jupiter began forming from an ice asteroid 4.5 BYA, 4 times further from the Sun than it is now. Over 700,000 years, Jupiter carved a spiraling path to its current orbit.
4.4 BYA, the newborn giants – Jupiter and Saturn – orbited in a tight circle, their orbits influencing each other. Beyond them were Neptune and Uranus, with Uranus on the outside.
As Jupiter and Saturn sauntered into place 4.1 BYA, their gravitational tug flung asteroid fragments willy-nilly. This high-speed slam fest lasted hundreds of millions of years, all the while altering the participants’ chemistry via the altercations.
During this time, Saturn swung into an orbital period twice that of Jupiter. This further scattered cosmic debris about, bombarding Earth and the other inner planets.
Saturn’s shift drove Uranus and Neptune outwards, into the comet belt, causing them to fling these cold bits all over, including hurtling more meteorites toward the inner planets as they sweep clear their orbit. In their swirl they switched places, with Neptune now further out.
Once Jupiter and Saturn settled in, a raft of rocks, numbering in the millions, were held between the tug of the Sun and Jupiter, and so formed an asteroid belt between Mars and Jupiter.
The gravitational perturbations from Jupiter kept the asteroids and debris from accreting into a planet. The extra orbital energy from Jupiter’s gravity instead caused collisions that shattered the protoplanets.
From the debris, Saturn spun its stunning signature ring system, including remnants from a vanished moon. Jupiter too has a ring, but quite faint.
During formation, the 2 gas giants gobbled much material that would have otherwise made moons. Only the late starters survived to spin about as satellites.
Jupiter captured 63 sizable moons compared to Saturn’s 62. (In 2018 Jupiter’s moon total was upped to 79. The new outer moons were at most only a few kilometers in diameter.) Jupiter has 4 large satellites, all discovered by Galileo. Saturn has only 1 big moon: Titan.
“Titan is bloody complicated.” ~ American astrophysicist Ralph Lorenz
Titan is an exceptional moon. Spawned from giant asteroid impacts, Titan is the only satellite known to have a dense atmosphere, with a layered temperature profile like Earth, though much colder: 93 K (–180 °C).
Solar irradiation produces polymer aerosols, which give Titan its famed orange glow. Titan’s atmosphere is laden with organic molecules, formed by sunlight striking the atmospheric methane.
Titan has weather and seasons. The southern hemisphere has lingering clouds during the summer.
Methane plays the role that water does on Earth. Titan has a methane cycle like Earth’s water cycle. Hydrocarbon gases condense and fall as methane rain. Titan’s surface sports methane seas, lakes, and networks of rivers.
As Titan revolves around Saturn every 16 days, it has tides that are pulled by proximity to Saturn.
Titan does have water, but underground: with a thick ice layer near the surface, and a salty, ammonia-laden watery ocean underneath, heated from below by the core.
Titan has a rock-iron core. Titan does have mantle dynamics, though in fits and starts, unlike Earth’s continual movements.
Titan lacks Earth’s plate tectonics: the movement of large crustal plates which bump and grind to produce terrestrial effects – a lithosphere in motion. Still, Titan has a geographically active surface. Methane-laden lavas flow from cold volcanoes, replenishing the atmospheric methane that is constantly decimated by solar ultraviolet rays. The volcanic eruptions also carry iced ammonia to the surface, which may mix with the methane and nitrogen to create a prebiotic brew. From that life could emerge. Preliminary evidence hints that methane-munching bacteria may reside on Titan’s surface.
“Methane has the disadvantage that it is nonpolar, and hence a poor solvent for the polar compounds necessary for the complex interactions required for life.” ~ English ecologist Andrew Clarke
Europa is the smallest of the 4 Galilean satellites of Jupiter, the 6th closest around the gas giant. Slightly smaller than the Moon, Europa is the 6th largest moon in the solar system.
Europa is a silicate rock with an iron core. Its tenuous atmosphere is primarily oxygen.
Situated past the planetary snow line, Europa’s surface is H2O ice, 15–25 km thick. Europa’s face is one of the smoothest in the solar system, albeit pockmarked in patterned scratches.
Underneath the ice is a dark, global saltwater ocean, 160 km deep. Turbulence in Europa’s subsurface sea, inspired by Jupiter’s gravitational tugs, causes chaotic cracks on its surface, prompting water plumes that rise 20 times the height of Mt. Everest.
Europa has subduction-driven plate tectonics like Earth, though on Europa, it is an icy shell that submerges into a warmer mantle.
Warmed by internal heat from the core, which creates global convection currents, there may be microbial life in the stormy subsurface sea of Europa.
Saturn is 60% the size of Jupiter, but less than a 3rd as massive, making it the least dense planet in the solar system. Saturn is the only planet less dense than water.
Saturn’s famous rings are only 100 million years old or less and are now slowly fading. The rings, which are a composite of rocky bits and ice, are about halfway through their life. The rings are held in place by magnetic field lines.
“We just missed out on seeing giant ring systems of Jupiter, Uranus, and Neptune, which have only thin ringlets today.” ~ English astronomer James O’Donoghue
Gas giants are mostly hydrogen and helium. Jupiter’s mass creates enormous gravitational pressure that squeezes most of its hydrogen into a metallic fluid that conducts electricity. Compression radically alters electron orbitals in atoms. Chemistry is a different beast under high pressure.
Jupiter has a core of iron, rock, and ice which weighs 10 times as much as Earth. Because of the intense pressure (40 million Earth atmospheres), Jupiter’s core temperature is 16,000 K; hotter than the Sun’s surface. The convection at the core boundary may be so extreme as to cause the core to slowly dissolve near its boundary, eroding the core into the fluid hydrogen and helium that surrounds it.
Past Saturn are 2 more planets: Uranus and Neptune. Uranus is the lightest of the outer planets. Uranus is slightly larger than Neptune. Both planets are about 4 times the size of Earth. Both have rocky cores.
Neptune’s density, and the relative ease of aggregating matter in the inner system, indicates that Neptune formed closer to the Sun before migrating outward.
While the other planets orbit the Sun axially standing up, Uranus orbits the Sun on its side (a 97° axial tilt). Uranus has a much colder core than the other gas giants, and so radiates very little heat into space.
All the planetary orbits have small deviations, which cannot be accounted for by Newtonian physics. Einstein’s general relativity theory adequately explains these peculiarities.
Mercury has an especially eccentric orbit: neither circular nor elliptical, but instead rosette-like, its perihelion (closest point to the Sun) precessing (gyral rotation) at more than 43 arc seconds per century. Yet Mercury has the smallest axial tilt (2.11°).
The 4 outermost planets have magnetic fields, though each is at a different tilt to the axis of rotation. Saturn’s magnetic field is perfectly aligned with its rotation axis. Jupiter is slightly tilted. Neptune tilts 47°. Uranus is askew a whopping 60°.
Roaming outside the planets – past Neptune – are smaller chunks, still held in sway by the Sun: the Kuiper belt. A Kuiper belt resident, Pluto, was counted as the 9th planet upon its discovery in 1930. Pluto has at least 3 moons.
In 2006, Pluto’s mass was unchanged, but it diminished in stature: no longer regarded as a planet proper; instead, merely a “dwarf planet.” Pluto was downgraded for untidiness: not having cleared other objects from its orbit. However rotund a rock one may be, having an icy composition does not win friends.
(Historically, determining a planet had been based upon how the celestial body formed. The criterion of orbit-clearing for defining a planet had previously been mentioned in only 1 publication, dated 1802, and its reasoning since disproven: by that criterion, there are no planets. The downgrading of Pluto demonstrated core incompetence in the International Astronomical Union.)
2 planetoids have been found outside the Kuiper belt. There are likely many more, but observation from Earth is problematic, as objects so far out are faint. The outer solar system is dark.
All told, there are 8 major planets: the iron-core inner 4 (Mercury, Venus, Earth, Mars) and 4 outer gas giants (Jupiter, Saturn, Uranus, and Neptune), along with 5 dwarf planets (Pluto, Ceres, Eris, Haumea and Makemake). Riding planetary shotgun in the system are 162 major satellites.
The planets have a roughly circular orbit, but comets and other cosmic bits have much more elliptical orbits. Some travel as far as the Oort cloud.
The Oort cloud is teeming with a trillion icy objects. It is nearly a light-year from the Sun; almost a quarter the distance to Proxima Centauri, the closest star to the Sun. The Oort cloud was formed from gyral scatterings created by the orbital wanderings of the 4 gas giants.