First it is necessary to understand the new story of the solar system. The planets, comets, and other bodies orbiting the sun were born, some 4.5 billion years ago, from a spinning disk of dust and gas called the solar nebula. It was long assumed that things formed more or less where they orbit now – nine planets orbiting in well-determined orbits like clockwork, forever in the past, and forever in the future.
But when dust flakes from a comet were captured, a new theory was needed to explain the metals found in the dust. That comet had theoretically resided outside of Neptune’s orbit for its entire life, but exotic minerals, like tungsten and titanium nitride, that could only have been formed near the newborn sun in temperatures of more than 3000 F were in the dust. Some violent process must have thrown them into the outer solar system. The solar system had literally turned itself inside out at one time.
Since Newton, it has been recognized that the planets must interact with one another. Their gravitational tuggings are far weaker than those of the sun, but over time, they affect the paths of neighbors. As a result, there is no such thing as a circular orbit.
Pluto provided the first evidence of interplanetary interaction. The oddball of the solar system, it dips far above and below the pancake-like plane in which the eight planets travel; it swoops on an elongated orbit that takes it from 30-50 times the Earth’s distance from the sun. But the most curious thing about Pluto is its bond with Neptune. It’s called resonance: For every three times that Neptune orbits the sun, Pluto orbits twice, and in such a way that the bodies never approach each other.
A theory has been proven to explain how that synchrony could have evolved. When the solar system was young and full of asteroids and comets, Neptune was closer to the sun. If one of those bodies approached Neptune, the planet’s powerful gravity would either fling the comet or meteor closer to the sun or out of the solar system entirely, in a cosmic version of crack the whip. Because action begets reaction, Neptune’s orbit would shift a tiny bit too. The effect of trillions of such interactions compelled Neptune to migrate away from the sun. That led it to “capture” Pluto, which was already farther out and sweep it into gravitational lockstep.
Further proof was provided by the Kuiper belt, a dark region extending far beyond Neptune. Telescopes have unveiled bunches of Plutinos – icy dwarf worlds that have the same two-to-three resonance with Neptune. That could only have happened if Neptune had advanced toward the Kuiper belt like a gravitational snowplow, piling up dwarf planets into new orbits.
The planets had not condensed gently from the solar nebula; instead they had grown to full size by absorbing planetesimals – rocky asteroids, icy comets, and larger objects – that smashed into them at high speed. One theory suggests that the moon coalesced from the spray of molten rock that was blasted into orbit when a body the size of Mars collided with Earth. All this happened in the first 100 million years of formation of the solar system.
Another enigma was revealed in the Kuiper Belt. Besides Plutinos, it was littered with bodies on wildly different orbits. Some were grouped in a flat disk, some in a puffy doughnut-shaped cloud; others in orbits even more crazily eccentric and elongated. The smooth outward migration of Neptune should have not strewn debris so widely. Hundred of planets around other stars have now been detected. Some are in tightly bunched orbits, much closer together than the planets in our solar system. Some are Jupiter- or Neptune-size worlds that race on insanely hot orbits close to their suns. Others loop deep into space on weird trajectories – on average the orbits of extrasolar planets are more eccentric than those in our solar system. Some planets even float freely in interstellar space. That is not what would be expected from planets born in a spinning disk around a star and staying quietly in their birthplace. That process should produce well-spaced near-circular orbits.
It is predicted that the solar systems history was anything but smooth – that it had somehow endured a “global gravitational instability.” It was proposed that the solar system’s four giant planets –Jupiter, Saturn, Uranus, and Neptune – had started out closely packed together, in nearly circular orbits, next to the sun. 4.4 billion years ago, Jupiter was closest, then Saturn, and Uranus may have been outside Neptune. Early on they were embedded in the disk-shaped solar nebula, which was still full of icy and rocky debris. As the planets absorbed those planetesimals or flung them away after close encounters, they cleared gaps in the disk. Their orbits slowly shifted as they cleared away this debris of comets and asteroids. A dense comet belt lingered beyond Uranus.
Because the planets were also tugging on one another, the whole system was fragile – almost infinitely chaotic. The most powerful gravitational link was between Jupiter and Saturn, the two biggest planets and a yank on them could jolt the whole system. When the solar system was about 500-700 million years old, 3.8 billion years ago, as the planets interacted with planetesimals, their own orbits shifted. Jupiter moved slightly inward; Saturn moved slightly outward, as did Uranus and Neptune. Everything happened slowly – until at a certain point, Saturn was completing exactly one orbit for every two of Jupiter’s. That was the tipping point. That one-to-two resonance wasn’t stable like the one between Neptune and Pluto; it was a brief vigorous yank. As Jupiter and Saturn approached each other repeatedly at the same point in their orbits, those near circular orbits were stretched into the ellipses we see today. That soon ended the precise resonance, and Jupiter’s gravity pumped Saturn closer to Uranus and Neptune. This drove them into the comet belt, flinging comets all over – including at Earth. Those two planets hurtled violently outward and may have even swapped places.
The planets grew to full size by absorbing rival planet embryos in a series of titanic collisions – one of which probably gave Earth its moon. A Mars-size protoplanet struck Earth, vaporizing itself and part of Earth’s rocky mantle. Rocky debris blasted into orbit coalesced into a moon – or maybe two – in less than a century. Most of the incoming protoplanet’s iron sank into Earth’s core, so the moon is less dense than Earth. Lunar gravity raised a tidal bulge on Earth; its spinning in turn accelerates the moon, causing it to spiral slowly outward. A sister moon, about a third as wide, orbited at a distance. Within tens of millions of years, the moon reeled in its sister. Splatting onto the moon’s far side, it creates a highlands there – a striking contrast to the low plains, called maria, on the side we see.
As Uranus and Neptune plowed through zones of the solar system that were still full of icy planetesimals, they triggered a devastating cascade. Ice balls were catapulted in all directions. The giant planets captured a few as oddly orbiting moons. Many objects were scattered into the Kuiper Belt. An untold number – perhaps a trillion – were banished even farther to the Oort cloud, a vast cocoon of comets reaching halfway to the next star. A lot of comets were also hurled into the inner solar system, where they crashed into planets or fell apart in the heat of the sun.
The Kuiper belt is a region of the Solar System beyond the planets, extending from the orbit of Neptune (at 30 AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but it is far larger—20 times as wide and 20 to 200 times as massive. Like the asteroid belt, it consists mainly of small bodies, or remnants from the Solar System’s formation. Although some asteroids are composed primarily of rock and metal, most Kuiper belt objects are composed largely of frozen volatiles (termed “ices”), such as methane, ammonia and water. The classical belt is home to at least three dwarf planets: Pluto, Haumea, and Makemake. Some of the Solar System’s moons, such as Neptune’s Triton and Saturn’s Phoebe, are also believed to have originated in the region.
Since the belt was discovered in 1992, the number of known Kuiper belt objects (KBOs) has increased to over a thousand, and more than 100,000 KBOs over 100 km (62 mi) in diameter are believed to exist. The Kuiper belt was initially thought to be the main repository for periodic comets, those with orbits lasting less than 200 years. However, studies since the mid-1990s have shown that the classical belt is dynamically stable, and that comets’ true place of origin is the scattered disc, a dynamically active zone created by the outward motion of Neptune 4.5 billion years ago; scattered disc objects such as Eris have extremely eccentric orbits that take them as far as 100 AU from the Sun.
Pluto is the largest known member of the Kuiper belt, and one of the two largest known TNOs, together with scattered disc object Eris. Originally considered a planet, Pluto’s status as part of the Kuiper belt caused it to be reclassified as a “dwarf planet” in 2006. It is compositionally similar to many other objects of the Kuiper belt, and its orbital period is characteristic of a class of KBOs, known as “plutinos”, that share the same 2:3 resonance with Neptune.
Meanwhile the giant-planet migrations also disrupted the rocky asteroid belt between Jupiter and Mars. Scattering asteroids joined comets from farther out to create the Late Heavy Bombardment. Our moon suffered badly then and earlier in its history: Its entire crust was deeply fractured. Earth would have caught even more flack, but shifting tectonic plates have erased the craters. Any early life could only have survived deep underground. The Late Heavy Bombardment lasted less than 100 million years.
Once Uranus and Neptune swept up most of the comets from their new orbits (and also switch places), the Late Heavy Bombardment ends. The four giant planets settle into their current, slightly elliptical orbits.
When an asteroid slams into Earth, tiny droplets of molten rock are lofted high into the atmosphere, and they later rain out as solid, glassy beads called spherules Deposits of spherules from the six-mile-wide asteroid that hit the Yucatan some 65 million years ago, wiping out the dinosaurs, have been discovered all over the world. At least 12 comparable spherule beds have been found dating from 1.8-3.7 billion years ago. A now vanished inner rim of the asteroid belt, which shed asteroids for two billion years after Jupiter disturbed it. As many as 70 may have struck Earth, each comparable to the one that extinguished the dinosaurs. Vesta, at 300 miles across, is the third largest asteroid in the belt between Mars and Jupiter. It endured eons of impacts, and it is believed that chips off Vesta make up 6% of the meteorites that fall on Earth.
Solar system evolution is dynamic and violent. Ours is probably on the mild side compared to others. That mildness was needed in order to have a habitable planet.
New telescopes will expose far more objects in the Kuiper belt and maybe even in the Oort cloud. It could be littered with lots of planets the size of Earth or Mars.
It is likely that the four giant planets have finished wandering and will be on the same orbits five billion years from now, when the aging sun is expected to balloon outward and engulf the inner planets: Mercury, Venus, Earth and Mars. There is a 1% chance the solar system will go dramatically unstable in the next 5 billion years. The problem is a weird long-distance connection between Jupiter and Mercury. When Jupiter’s closest approach to the sun lines up with Mercury’s noticeably squashed orbit in just the right way, Jupiter exerts a slight but steady tug. Over billions of years this gives Mercury a 1-in-a-100 chance of crossing the orbit of Venus. There is a further 1-in-a-500 chance that if Mercury goes nuts, it will also perturb the orbit of Venus or Mars enough for one of them to hit Earth – or miss it by several thousand miles, which would be almost as bad. The entire Earth would get stretched and melted like taffy.
