The Planets
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.
