With its profusion of landscapes and climates, animals and people, our Earth certainly seems to be something special and deserving of the name “cradle of humankind.” To planetary astronomers, however, the Earth and the other planets of the Solar System are cosmic flotsam, debris left over from the formation of the Sun, itself just an average star in a commonplace galaxy.
The Earth and the rest of the Solar System formed during a hellish maelstrom around 4.6 billion years ago. Such activity is driven by gravity and continues in other parts of the Universe today, forming new stars and planets, but it is all hidden away inside dusty cocoons that absorb ordinary wavelengths of light. To peer inside these celestial construction sites, astronomers have to use other wavelengths, such as infrared or microwaves. The goal of this research is simple: to see what is happening in these regions of activity and relate it to what must have occurred during the formation of our own Solar System. By doing this, the astronomers hope to solve a number of mysteries.
The first of these unknowns is why the planets of the Solar System all orbit in more or less the same plane. Presumably, there must have been some natural process that prevented the planets from assuming random orientations and corralled them into nested orbits. The second is, why there is such an abrupt change in the composition of the planets from the inner to the outer Solar System. Some process segregated the raw material to produce relatively small rocky worlds in the inner Solar System and giant gaseous planets further out. To answer these questions, astronomers must identify and observe sites of star and planet formation elsewhere in the Galaxy.
Stars and planets account for 85 percent of all the atoms in the Galaxy, leaving around 15 percent available in interstellar regions where new stars and planets form. This material floats through space creating a tenuous mist of gas and dust, of which hydrogen is the most abundant constituent, and the “dust” consists of particles of heavier elements from planetary nebulae and supernovae (see What Are Stars Made From?).
In space there are typically only 100,000 hydrogen atoms in every cubic meter (at sea level on Earth the atmosphere contains around 200 million trillion atoms). While floating through the Galaxy, the hydrogen atoms tend to pair up into molecules, and the molecules gather together into enormous clouds. Astronomers estimate that there are about 4000 of these giant molecular clouds in the Milky Way Galaxy. Each one is typically between 150 and 250 light years in diameter and contains enough gas to make up to ten million stars like the Sun. As the molecular gas falls together it sets the cloud rotating, a fact that will become very important for the subsequent formation of stars and planets.
Within each giant molecular cloud there are a multitude of slightly denser clumps, each measuring between a quarter and a third of a light year across. These clumps are the seeds from which stars grow—or, more accurately, shrink. A clump generates gravity and pulls itself together in a slow process that takes anything from a million to ten million years. Shockwaves from nearby supernovae explosions may speed things up by compressing the gas and also triggering further clumps to form. As a clump shrinks, so the gas and dust inside becomes denser. Dust accounts for only one to ten percent of a clump’s mass, yet this is enough to eventually block our view of the newly forming star at ordinary wavelengths of light. As astronomers look into the dusty cocoons with infrared telescopes they see something remarkable: each collapsing clump transforms itself from a nearly spherical shape into a disk surrounding the nascent star. These flat disks can be anything from 100 to 1000 times larger than the Earth’s orbit, and it is in these that planets form.
Planet-forming disks are flat because of centrifugal force. This is not a force based upon an intrinsic property, such as electrical charge, but is instead created when an object begins to spin. The faster something rotates, the greater the centrifugal force it generates, pushing objects outward: so a ball placed on a merry-go-round will roll toward the outer edge as the merry-go-round begins to spin. It is also the reason a car will leave the road if the driver takes a bend too fast.
Consider the clump of gas and dust that formed our Solar System. As the spherical clump contracted under its own gravity, it naturally spun faster, in the same way that a spinning ice dancer speeds up when she pulls in her arms. This would have boosted the centrifugal force felt in the equatorial plane of the sphere, opposing the pull of gravity on the gas and dust and slowing the collapse in this region. Away from the equator, where the effects of centrifugal force were not so great, the gas and dust would have fallen inwards more or less freely, so the overall result was that the cloud pancaked into a flat disk. Within this disk the planets subsequently materialized, and therefore they occupy almost identical orbital planes rather than whizzing around the Sun in random orientations. This provides the solution to the first mystery.
The second mystery was why the composition of our Solar System planets changes so drastically from the inner regions to the outer regions. In the inner Solar System, the planets are like the Earth—small, rocky, and with thin atmospheres—whereas in the outer Solar System they are more like Jupiter—large, gaseous, and with thick atmospheres. This difference comes about because, as the newly forming Sun shrank to its modern size and density, it released energy; nowhere near as much energy as would be released later when nuclear fusion ignited in its core, but nevertheless enough to heat the surrounding disk. The heat prevented certain chemicals from forming by driving particular atoms into frantic motion. Other atoms and molecules were not so affected by the heat. They bumped into one another and bound together, creating dust of different chemical compositions. The chemicals that formed depended on the temperature, which in turn depended on the distance from the forming Sun. Near the Sun, in what would eventually be Mercury’s orbit, the temperature would have been several thousands of degrees and only metallic and some silicate atoms could condense into dust (hence Mercury’s metallic core takes up a large proportion of is volume); other chemicals would have been vaporized back into gas by the heat from the young Sun. In the somewhat lower temperatures near the Earth’s eventual location, more silicates would have been able to condense (giving us a smaller metallic core in comparison with the rest of our planet). Further out still, the lower temperatures enabled other elements to form dust.
INSIDE ROCKY PLANETS: THE INTERIOR STRUCTURES OF EARTH AND MERCURY GIVE THE BIGGEST CLUE TO THEIR FORMATION
A distinct boundary in the planet-forming disk occurred at five times the Earth’s distance from the Sun. Called the snow line, it is where Jupiter orbits today. At the snow line, when the planetary material was condensing, the temperature would have been around 90 K, low enough for molecules such as water, ammonia and methane to form ice. (Astronomers measure temperature in kelvin, K. Zero on the kelvin scale is the temperature at which atoms and molecules all cease to transfer energy between one another, known as “absolute zero,” this is the equivalent of approximately –273 degrees Celsius.) At 40 times the Earth’s distance from the Sun, roughly where Pluto orbits, the temperature would have been just 20 K and almost every chemical element could condense; the only exceptions were hydrogen and helium, which remained in a gaseous state. Consequently the dust grains developed different compositions throughout the planet-forming disk, leading to the variety of planets. The much larger size of the outer planets can be explained by the much greater reservoir of matter beyond the snowline.
In the inner Solar System, the dust gradually accumulated into objects resembling small asteroids; these “planetesimals” would have populated the early Solar System in vast numbers. To build Mercury, Venus, Earth and Mars would have required ten billion or more planetesimals of 10 kilometers (6 miles) in diameter. These lumps of rock continued to grow by basically colliding and sticking together, but the process was subtler than it may at first sound.
Head-on collisions were no use because they released too much energy and would have shattered the planetesimals, blasting the debris into space. In any case such energetic clashes would have been rare because the planetesimals were all rotating in the same direction. Sometimes an impact was just enough to melt the planetesimals together, at other times although it broke them into fragments, the pieces remained together and continued to orbit as a pile of rubble.
These close encounters were repeated along entirely random lines until eventually some larger planetesimals began generating enough gravity to pull smaller ones onto themselves. Throughout the disk, these major planetesimals began to outpace their lesser companions and, the bigger they became, the more efficient they grew at drawing in smaller bodies. Astronomers call these planetesimals “oligarchs,” because they controlled their surroundings. Essentially they were small rocky planets, each containing between the mass of the Moon and Mars; computer simulations show that 20 to 30 of them must have ultimately smashed together to build the Solar System’s four terrestrial planets of today.
The Solar System’s gas giants—Jupiter, Saturn, Uranus and Neptune—were probably formed in a similar way to the inner planets but from bigger oligarchs, bolstered by the astronomical ices. Once the forming Jupiter and Saturn reached between three and five times the mass of the Earth, they generated so much gravity that they began pulling in gas from their surroundings and this gave them their thick atmospheres that mirror the cosmic abundance of elements. Uranus and Neptune formed in a similar fashion although, being less massive, they were not so good at attracting the hydrogen and helium, so they display a greater proportion of astronomical ices in their atmospheres.
A number of astronomers have put forward the alternative suggestion that the gas giants formed in the same way as stars do. In this scenario, a region of the disk around the Sun would have reached a critical density and gravity simply pulled it all together. There were no oligarchs building up and colliding, just a sudden collapse of gaseous matter into a giant planet. At present there is no way to determine which of these gas giant formation scenarios actually took place in the Solar System. Both theories correctly predict that the giant planets surround themselves with their own mini-disks, which subsequently coalesce into extensive moon systems.
At the distance of Pluto, the density of matter orbiting the young Sun was thinner, so the bodies that formed there were consequently smaller. Pluto itself is only two-thirds the size of Earth’s Moon, and has an orbital plane significantly inclined to that of the other planets. In 2006, these factors, together with the discovery of a number of other Pluto-like objects in the outer reaches of the Solar System, led to the International Astronomical Union voting to downgrade Pluto from the status of planet to dwarf planet. The newly observed bodies included the icy celestial objects Haumea and Makemake, but it was the body cataloged as 2003 UB313 that really triggered the debate. Observations showed that it was at least the size of Pluto, probably bigger. Hence, astronomers faced a choice: downgrade Pluto or name 2003 UB313 as the tenth planet in the Solar System. A heated discussion ensued, during which 2003 UB313 was nicknamed Xena, after the television heroine Xena: Warrior Princess. Eventually, the decision was made: 2003 UB313 was not a planet and as a result Pluto was downgraded. Xena’s name was changed to Eris—rather appropriately—after the Greek goddess of strife and discord.
“I’m expecting planets 10, 11, 12 and many more to be found in the distant outer Solar System, all larger than Mars and possibly even than the Earth.”
ALAN STERN CONTEMPORARY PLANETARY SCIENTIST
There are undoubtedly many more dwarf planets yet to be found in the Solar System. Astronomers estimate that there could be hundreds or even thousands of them beyond Pluto, and possibly a few fully-fledged planets. In fact, according to computer simulations, a whole second Solar System’s worth of planets may be lurking at thousands of times the distance of Earth from the Sun. These could turn out to be as big as Mars or even Earth—not formed in situ, but thrown there by the gravity of the gas giant planets. If a planetesimal were traveling sufficiently fast near a giant planet, the giant’s gravity would be unable to pull it into a collision. As the smaller object sped by, the near miss could result in it gaining considerable speed and being boosted into a larger orbit. In this way, Jupiter could have scattered rocky planets out to between 25 and 250 times further from the Sun than Pluto.
At such a distance from the Sun, the scattered planets would be very faint indeed and so extremely difficult to spot. Add to this that Jupiter could have catapulted them into any orbital plane, and the only way astronomers will be able to search for them is to trawl the whole sky with a powerful telescope. There are a number of such instruments on the drawing board at the moment, all due to begin searching within ten years.
By 4.6 billion years ago the Solar System looked almost as it does now; the familiar planets and their moons had formed. The space between the planets, however, remained home to countless, smaller leftovers. These tiny objects ranged from pebbles and rocks to planetesimals that had so far escaped the planets’ gravitational clutches. Jupiter’s gravity trapped many of them between itself and Mars to form the asteroid belt, but most whizzed about the Solar System. During the next 700 million years these celestial vagabonds collided with the planets and their moons, blasting out craters of all sizes. Old planetary surfaces are easily identified today because of their heavily pockmarked appearance: our Moon being the classic example. Its scarred face has taught astronomers much of what they know about this last phase of planet formation, referred to as the “heavy bombardment” period. On Earth, the early craters have been eroded away; today, less than 200 craters are known, and all of them are from comparatively recent impacts.
The position of the Earth inside the snowline meant that it was formed without any water; the heat from the young Sun would have vaporized any water molecules that formed. This is another puzzle that astronomers have had to address: how we came to have oceans. The “late bombardment” suggests a way; planetesimals that formed in the outer Solar System, incorporating water and other ices, rained down on Earth and the other planets of the inner Solar System, supplying them with water and other volatile substances that they lacked. On Earth this material was swiftly transformed into life (see Are We Made from Stardust?).
During the bombardment period, many of the planetesimal remnants would have been ejected from the solar vicinity by near misses with Jupiter, in the same way that Jupiter is thought to have scattered planets. Because they were much smaller than planets, Jupiter could have lofted them much further, throwing a trillion or more of them into incredibly large orbits, reaching 10,000 to 100,000 times the Earth’s distance from the Sun. This distant collection is called the “Oort Cloud.” Occasionally one of its members returns to the inner Solar System, and we call these distant visitors “comets.” Being composed of ice they begin to melt when they approach the Sun, and leave a trail of gases in space that we see illuminated as the comet’s tail. Dust released from the melting ice litters interplanetary space, and if the dust falls into the Earth’s atmosphere it burns up and creates meteor showers, or shooting stars. Now and then, if a conglomerate of dust is large enough, it might not burn up entirely but plummet all the way to strike the ground as a meteorite. Fragments chipped from asteroids can also fall as meteorites.
THE OORT CLOUD: THIS CONTAINS THE DEBRIS FROM THE FORMATION OF THE PLANETS
After some 700 million years the bombardment petered out, and the formation of the Earth and the rest of the Solar System as we know it was complete. But even now we catch a glimpse of what the late bombardment must have been like. In 1993, fragments of a comet called Shoemaker-Levy 9 repeatedly struck Jupiter. The 21 fragments, some of which were 2 kilometers (1.2 miles) wide, slammed into the giant planet over a six-day period. Each struck with a speed of approximately 60 kilometers per second (130,000 miles per hour), causing tremendous explosions that left plumes of debris, some larger than the Earth, visible in the planet’s atmosphere for weeks afterward.
Orbiting spacecraft have shown that comets the size of a two-story house regularly approach the Earth, but fortunately they break up in the atmosphere and present no danger. However, occasionally a large meteorite hits the ground: notably the strike on the Tunguska region of Siberia in 1908, which razed an area of uninhabited forest the size of a modern city such as London.
Today, a constant watch is kept for threatening asteroids. Although most asteroids are safely corralled in the main belt between Mars and Jupiter, an increasing number of “near-Earth objects” are being discovered. Astronomers track almost 800 nearby objects with diameters of greater than 1 kilometer (0.6 miles). Of these, none presently pose a threat to Earth although calculations suggest that, on average, something of this size hits us every half a million years. There may be many thousands of nearby asteroids less than 1 kilometer across, of which only a small percentage is currently being tracked. One, cataloged as 2007VK184 and about 130 meters (520 feet) wide, is already known to present a slight danger. While tiny in asteroid terms, nevertheless should it hit the Earth it would release energy equivalent to 10,000 Hiroshima atomic bombs. It currently has a 1-in-3000 chance of hitting Earth in 50 years’ time, but this probability is expected to decrease to a zero as further observations allow astronomers to refine its orbit. Better and better survey equipment is constantly being developed to reveal and track such near-Earth objects.
Inevitably, one day an asteroid will be discovered to be on a collision course with Earth. As soon as its path can be reliably predicted, plans will be put into action to attempt to deflect it, and astronomers have a number of methods in mind for doing this. Blowing the asteroid to pieces with nuclear weapons is not the answer because this would not alter the orbit of the fragments—instead of coming toward us as cannonball, it would be transformed into buckshot. Exploding nuclear weapons some way above the asteroid’s surface might be a more successful, if tricky option; some of the rocks would be melted and would release gases creating a natural “rocket” engine to nudge the asteroid onto a safer orbit. It seems somewhat ironic that the very objects that brought water to Earth and made life possible now threaten to destroy it. When it comes to safeguarding the Earth against dangerous impacts, it is literally a case of “watch this space.”