Solar System

• 1. Introduction

  ›

  The Sun holds 99.86% of all the mass in the Solar System. Everything else, the eight planets, the dwarf planets, the moons, the asteroids and comets, together adds up to less than 0.002% of the total. That single number reframes how we picture our home in space. The Sun is not the center of a balanced family of worlds. It is the overwhelming presence, and the rest is leftover crumbs.

  The system formed about 4.6 billion years ago, when a dense region of a molecular cloud collapsed. Out of that collapse came a star and a spinning disc of gas and dust. From the disc, the planets assembled. Earth is one of them. So how did a cloud of hydrogen become worlds with oceans, rings, and erupting geysers of nitrogen?

  This documentary follows the Solar System outward. It starts at the blazing core of the Sun and travels past the rocky inner planets, the gas and ice giants, and the cold belts of debris beyond Neptune. It asks where the system ends, how astronomers slowly mapped it, and what will happen when the Sun finally runs out of fuel. The answers stretch from a transit of Mercury in 1631 to a sphere of ice that may reach 200,000 astronomical units from the Sun.

• 2. Birth From A Cloud

  ›

  At least 4.568 billion years ago, a region within a large molecular cloud gave way to gravity. That cloud was likely several light-years across and probably birthed several stars at once. It was made mostly of hydrogen, with some helium and small amounts of heavier elements forged by earlier generations of stars.

  Conservation of angular momentum took over as the pre-solar nebula collapsed. The cloud spun faster and faster, flattening into a protoplanetary disc with a hot, dense protostar at its center. Dust and gas in the disc gravitationally attracted one another, coalescing into ever larger bodies through accretion. Hundreds of protoplanets may have existed in the early system. They merged, or were destroyed, or were ejected.

  Heat sorted the disc into two kinds of worlds. Close to the Sun, temperatures exceeded the boiling point of hydrocarbons for the first million years, so only refractory materials like metals and silicates stayed solid. Those scarce materials built the small rocky planets: Mercury, Venus, Earth, and Mars. Beyond the frost line, where volatile icy compounds could remain solid, ices were far more plentiful. The giant planets grew massive enough to capture huge atmospheres of hydrogen and helium.

  Within 50 million years, the pressure and density at the protostar's center became great enough to ignite thermonuclear fusion. Solar wind then swept the remaining gas and dust into interstellar space. Later, the Nice model proposes, gravitational encounters pushed the gas giants into new orbits. The grand tack hypothesis suggests a final inward swing of Jupiter scattered much of the asteroid belt, triggering the Late Heavy Bombardment of the inner planets.

• 3. The Star Itself

  ›

  The Sun carries the mass of 332,900 Earths. That mass produces the temperatures and densities needed to fuse hydrogen into helium in its core, releasing energy that radiates into space, peaking in visible light. It is a G2-type main-sequence star, a classification tied to its effective temperature, intermediate between the hottest and coolest stars.

  Red dwarfs, far dimmer and cooler than the Sun, make up about 75% of the fusor stars in the Milky Way. The Sun stands apart in another way too. It is a population I star, formed in the galaxy's spiral arms, and rich in elements heavier than hydrogen and helium. Astronomers call those heavier elements metals. That higher metallicity is thought to have been crucial, because planets form from the accretion of metals.

  The Sun radiates more than light. A continuous stream of charged particles called the solar wind pours outward at speeds from 900,000 km/h to 2,880,000 km/h. This wind fills the vacuum between the bodies of the system and shapes the heliosphere, the region of space dominated by the solar magnetosphere. The largest stable structure inside it is the heliospheric current sheet, a spiral form created by the Sun's rotating magnetic field.

  Activity on the surface disrupts this calm. Solar flares and coronal mass ejections blow magnetic fields and material off the Sun, creating space weather and geomagnetic storms. When that material reaches Earth, it funnels charged particles into the upper atmosphere, where they create the aurorae seen near the magnetic poles.

• 4. Worlds Of Rock

  ›

  Mercury, at 0.31 to 0.59 AU from the Sun, is the smallest planet in the system. Its equatorial regions swing from minus 170 degrees Celsius at night to 420 degrees Celsius in sunlight. The surface carries an expansive system of cliffs, called rupes, generated by thrust faults, and bright ray systems left by impacts. Mercury has no natural satellites and only a very tenuous atmosphere of solar-wind particles and ejected atoms.

  Venus hides beneath a reflective, whitish atmosphere of carbon dioxide so thick that surface pressure is ninety times that of Earth at sea level. Surface temperatures climb over 400 degrees Celsius, driven by greenhouse gases. With no protective magnetic field, Venus loses material to the solar wind, and its atmosphere appears to be sustained by volcanic activity. It too has no natural satellites.

  Earth, between 0.98 and 1.02 AU, is the only place in the universe where life and surface liquid water are known to exist. Its atmosphere is 78% nitrogen and 21% oxygen, a balance that is itself the result of life. Plate tectonics shaped its continents, and its magnetosphere shields the surface from radiation. Its only natural satellite, the Moon, has a diameter one-quarter that of Earth and a surface of fine regolith dominated by impact craters and dark maria.

  Mars, with a radius about half of Earth's, glows red from iron oxide in its soil. Its thin carbon dioxide atmosphere holds just 0.6% of Earth's surface pressure, yet still supports some weather. Mars lost its magnetosphere 4 billion years ago. It keeps two tiny moons. Phobos, the inner one, carries a Stickney crater roughly 4.5 km in radius. Deimos, with a mean radius of 6 km, wears a smoother face because regolith partly buries its craters.

• 5. The Belt Between

  ›

  Between 2.3 and 3.3 AU from the Sun, in the gap between Mars and Jupiter, lies the asteroid belt. It holds tens of thousands, possibly millions, of objects over one kilometer across. Yet its total mass is unlikely to exceed a thousandth that of Earth. The belt is so sparsely populated that spacecraft routinely pass through without incident. It is thought to be material that never coalesced, blocked by the gravitational interference of Jupiter.

  Ceres is the largest object in the belt, with a diameter of 940 km, and the only dwarf planet within it. Its surface mixes carbon, frozen water, and hydrated minerals, and shows signs of past cryovolcanic activity in its bright spots. Vesta, the second largest, left fragments that survive as the Vesta asteroid family and as HED meteorites found on Earth. Its basaltic surface bears two giant craters, Rheasilvia and Veneneia. Pallas, the third largest, has never been visited by a spacecraft.

  Asteroids are sorted by where their orbits take them. At least 362 Mercury-crossing asteroids are known, and one object, 594913 Aylochaxnim, orbits entirely within Venus's orbit. Over 37,000 near-Earth asteroids are known, some of them potentially hazardous. NASA lists 26,182 confirmed Mars-crossing asteroids. It is now widely accepted that past collisions have shaped the geological and biological history of Earth.

  Some bodies travel in step with the planets. Hilda asteroids sit in a 3:2 resonance with Jupiter, circling the Sun three times for every two Jovian orbits. Trojans cluster at gravitationally stable Lagrange points, 60 degrees ahead of or behind a planet. Every planet except Mercury has at least one trojan, and after Jupiter, Neptune holds the most confirmed, at 28.

• 6. The Giant Planets

  ›

  Jupiter, between 4.95 and 5.46 AU, is the biggest and most massive planet in the system. Orange-brown and white cloud bands wrap its surface, where giant storms swirl, including the Great Red Spot and white ovals. Its magnetosphere is strong enough to redirect ionizing radiation and light auroras at its poles. Jupiter has 115 confirmed satellites, including the four large Galilean moons: Ganymede, Callisto, Io, and Europa.

  Saturn, from 9.08 to 10.12 AU, wears a visible ring system of small ice and rock particles circling its equator. At its poles sit peculiar hexagon-shaped storms larger than the diameter of Earth. Saturn has 292 confirmed satellites. Among its outer large moons is Titan, the only satellite in the Solar System with a substantial atmosphere. Tiny trojan moons like Calypso and Telesto share orbits with the moon Tethys.

  Uranus and Neptune are ice giants, built largely from compounds like water, methane, and ammonia. Uranus, from 18.3 to 20.1 AU, is unique in orbiting on its side, with an axial tilt greater than 90 degrees. That tilt gives it extreme seasons as each pole points alternately toward and away from the Sun. It has 29 confirmed satellites, including Miranda, made primarily of ice.

  Neptune, between 29.9 and 30.5 AU, is the furthest known planet. Its magnetosphere is strongly tilted, at 47 degrees, for reasons still unexplained. Of its 16 confirmed satellites, the largest is Triton. Triton is geologically active, with erupting geysers of nitrogen gas and a thin, cloudy nitrogen atmosphere. Together the four giant planets make up 99% of the mass orbiting the Sun.

• 7. Beyond Neptune

  ›

  Past Neptune's orbit lies a region sometimes called the third zone of the Solar System. The doughnut-shaped Kuiper belt extends between 30 and 50 AU from the Sun. It holds an estimated 100,000 objects with diameters greater than 50 km, yet its total mass may be only a tenth or a hundredth that of Earth. Beyond it stretches the scattered disc, tilted toward the plane of the system and reaching much further out.

  Pluto is the largest known object in the Kuiper belt. It follows an eccentric orbit inclined 17 degrees to the ecliptic and holds a 2:3 resonance with Neptune, orbiting the Sun twice for every three Neptunian orbits. Objects sharing that resonance are called plutinos. Pluto has five moons: Charon, Styx, Nix, Kerberos, and Hydra. Charon is so large that the two bodies orbit a barycenter above their surfaces, appearing to orbit each other.

  Other dwarf planets crowd this region. Haumea, discovered in 2005, rotates once every 3.9 hours, fast enough to stretch into an ellipsoid, and carries a ring system. Makemake, named in 2008, is the brightest Kuiper belt object after Pluto. In the scattered disc, Eris is the most massive known dwarf planet, 25% more massive than Pluto. Its discovery fed the debate about what counts as a planet.

  The centaurs bridge the giant planets, with orbits between 5.5 and 30 AU. They are former Kuiper belt and scattered disc objects, expected to become comets or be ejected. The first one discovered, 2060 Chiron, develops a coma like a comet and is also classified as comet 95P. Farther still are the extreme trans-Neptunian objects, including Sedna, the first such object found, which takes roughly 11,400 years to complete one orbit.

• 8. Mapping The Edge

  ›

  The term Solar System entered the English language by 1704, when John Locke used it for the Sun, planets, and comets. Before that, from Europe to India, astronomers had long held Earth to be stationary at the center of the universe. Aristarchus of Samos had speculated about a heliocentric cosmos, but Nicolaus Copernicus was the first known to build a mathematically predictive heliocentric system.

  Johannes Kepler carried the idea forward by allowing orbits to be elliptical. Using the precise observations of Tycho Brahe, he produced the Rudolphine Tables. Pierre Gassendi used them to predict a transit of Mercury in 1631, and Jeremiah Horrocks did the same for a transit of Venus in 1639. In 1687, Isaac Newton's Principia Mathematica showed that the same laws of motion and gravity govern both Earth and the heavens.

  Discovery kept pushing the boundary outward. In 1705, Edmond Halley realized that repeated sightings of a comet were one object returning every 75 to 76 years. Uranus was recognized as a planet beyond Saturn by 1783. Neptune followed in 1846, found through its gravitational pull on the orbit of Uranus. In 1838, Friedrich Bessel measured a stellar parallax, the first direct experimental proof of heliocentrism.

  The far edges remain unknown. The region beyond 100 AU is virtually unexplored. The heliopause marks the beginning of the interstellar medium, and beyond it, around 230 AU, lies the bow shock left by the Sun's passage through the Milky Way. The Oort cloud, a theorized shell of up to a trillion icy objects, may stretch to around 200,000 AU. No present imaging technology can observe it directly. Yet the Sun's gravity is thought to hold sway out to the edge of its Hill sphere, at 178,000 to 227,000 AU, where its pull finally matches that of the galaxy.