by Paul Murdin
It is likely that Pluto and Charon formed as separate objects in a near-circular, low-inclination orbit beyond Neptune. Neptune perturbed their orbits and they got fed into the same eccentric, steeply inclined orbit, controlled by Neptune. Eventually Pluto and Charon collided and formed the present binary system.
Pluto is an unusual planet; in fact so unusual that in 2006 it was removed from the status of ‘planet’ by the International Astronomical Union and termed a ‘dwarf planet’. It is a member of the Kuiper Belt. Pluto has been visited once, in a fly-by in 2015 by the New Horizons spacecraft. It is covered with mountains of ice and smooth, flat plains of frozen nitrogen. One especially remarkable plain called Sputnik Planitia appears to be a filled-up meteor crater. The meteor impact melted frozen nitrogen and water-ice under Pluto’s surface, the liquidized mixture oozing up and filling the crater to the brim.
The Kuiper Belt
The frontier of the Solar System
The most important thing is not to work on things that other people are working on because otherwise all you’ll do is get the same result as everybody else and you won’t make any discoveries, you’ll just confirm what’s already known.
David Jewitt, On Asteroids – The Good, the Bad and the Ugly, 2014
Following the discoveries of Neptune and Pluto, astronomers began to suspect that the Solar System was much larger than previously thought, and that its frontier extended well beyond the orbits of the known planets. This theory, which originated in the 1950s, was proved forty years later with the discovery of the Kuiper Belt, a belt of asteroids and comets that lies beyond Neptune and is made up of material left over from the formation of the Solar System. Many of the comets that we see from Earth are objects that have travelled from the Kuiper Belt.
Like Pluto and Neptune, the Kuiper Belt was ‘discovered’ in theory long before it was actually seen. American professional astronomer Gerard Kuiper (rhymes with ‘viper’) is conventionally credited with proposing the existence of the Belt, which he put forward at a symposium in 1951, hence the Belt’s name. However, another American astronomer, Frederick Leonard, had mentioned the idea in print as early as 1930, in a publication for amateur astronomers that was largely ignored by professionals.
Irish amateur astronomer Kenneth Edgeworth laid out the idea more clearly in scientific papers that he wrote in the late 1940s. Edgeworth was a soldier and engineer who won the Military Cross in the First World War and had worked in the post and telegraph office in the Sudan, taking up an interest in astronomy when he retired. He wrote articles for amateur and professional journals about the origin of the stars and the Solar System. In 1949 he published an account of the formation process of stars, in which he speculated that the formation of the Sun might have left a debris field beyond Pluto. In its first forty years Edgeworth’s article was referenced only a few times, but it is much better known today – the ‘Kuiper Belt’ is therefore sometimes called the ‘Edgeworth–Kuiper Belt’.
These astronomers argued that there was no reason why the Solar System should end abruptly at Neptune or Pluto. In its modern form, the basic argument would be that the solar nebula, from which the planets were formed, extended well beyond Neptune, but planets as large as Neptune could not form further out in the Solar System. Because of the low gravity in the outermost reaches of the Solar System, everything in that region moves so slowly that collisions in which smaller bodies stick together to form bigger ones are very infrequent, and therefore only small bodies can accumulate. Even some of the original smaller bodies survive. Because it is so cold in those distant regions, these small bodies would be ice-rich asteroids, or comets. In fact, some of the comets that we see from Earth have come from this population, a possibility first proved theoretically by pioneering mathematical work by American astronomer Paul Joss in 1970 and Spanish astronomer Julio Fernández in 1980.
In 1988 the Canadian astronomers Martin Duncan, Tom Quinn and Scott Tremaine used comprehensive computer simulations to show that when individual bodies in the Kuiper Belt encountered each other in near-misses, their orbits might indeed be disturbed dramatically. Many of the Kuiper Belt objects are flung out of the Solar System into interstellar space but some fall inward towards the Sun. Some of these ‘fallen objects’ become trapped into short-period orbits by the massive planets that they encounter as they fall towards the Sun. If this happens, the Kuiper objects become comets, repeatedly passing by the intense heat of the Sun and progressively melting until eventually they break up, the residual dust causing meteor showers. As they studied the properties of such short-period comets, Duncan, Quinn and Tremaine concluded that all of these comets had originated from a single flat disc of material, because their orbits around the Sun were confined to a single plane. The plane of the comets’ orbits was close to the plane of the Earth’s orbit, which suggested that the disc of comet and asteroid material had originated during the formation of the Solar System. This disc is the Kuiper Belt, with Neptune at its inner edge.
In 1987 British-born astronomer David Jewitt was becoming increasingly puzzled by the emptiness of the outer Solar System: ‘It was just freaky.’ He persuaded Jane Luu, a colleague at the University of Hawaii, to help him find what was beyond Neptune and Pluto, because, as he told her, ‘If we don’t, nobody will.’ They set out on a search using the same technique used by David Tombaugh to find Pluto, repeatedly imaging an area of the sky to find moving objects in the Solar System – though using digital electronic detectors called Charge Coupled Devices (CCDs) instead of photography. In August 1992, after a five-year search, they found the first member of the Kuiper Belt, which was catalogued as ‘1992 QB1’. This was the archetypical Kuiper Belt Object, and gives its ‘QB1’ catalogue number to the name of similar objects in the Kuiper Belt, which are collectively called cubewanos.
The small planets or comets in the Kuiper Belt are called Kuiper Belt Objects, or perhaps more neutrally Trans-Neptunian Objects or TNOs. Thousands of TNOs have been discovered, and there are probably over 100,000 of them with diameters larger than 100 kilometres. The cubewanos, which account for most of these, have nearly circular orbits that lie in the same plane as the major planets. They are the original planetesimals and have always orbited this way. But some of the objects have orbits that have been strongly disturbed. A few are in large, eccentric orbits, perhaps put there by a long-ago interaction with Neptune or disturbed by a passing star.
About a quarter of the known TNOs are in resonance with Neptune. This means that the object makes a certain number of orbits around the Sun for every orbit completed by Neptune. Pluto does this, making three orbits for every two that Neptune makes, so it is in the ‘3 to 2’ resonance. A large number of TNOs are also in the same ‘3 to 2’ resonance, and are consequently called Plutinos. Because it orbits together with a large group of TNOs, and is physically distinct from the other planets of the Solar System, Pluto is considered the largest known object in the Kuiper Belt, not a planet. This is the stance that informed the decision of the International Astronomical Union in 2006 to revoke Pluto’s status as a planet in 2006.
Pluto was investigated in 2015 by the New Horizons spacecraft, which flew by and on into the Kuiper Belt. The spacecraft’s future trajectory was examined by the Hubble Space Telescope, to see what TNOs were within its reach. The target was detected and chosen, and named Ultima Thule, after the mythological island to the north of Britain that became the generic name in the Middle Ages for a distant and unseen land. It orbits with a period of 298 years at an average of 44.5 times the Earth–Sun distance from the Sun. New Horizons flew by Ultima Thule in 2018–19. It proved to be two lumps in contact. Seen from some angles it looks much like a snowman. It is thought that two TNOs collided gently and fused together. It has few craters, presumably because potential colliding objects are so scarce at this distance from the Sun.
Ultima Thule is the most distant object in the Solar System that has been visited by a spacecraft. Even further from the Sun are three spacecraf
t (Voyagers 1 and 2 and Pioneer 10), some comets that have very eccentric orbits, and further TNOs, one of them at a distance of 120 times the distance of the Earth from the Sun. The largest is Eris, a TNO as large as Pluto (2,400 kilometres in diameter) and has a satellite, Dysnomia. It has a very eccentric orbit with a period of 557 years. As of 2018, it lies 96 times further from the Sun than the Earth. This is the frontier of our Solar System.
Meteors and Meteorites
The sky is falling!
All you that do behold my stone
O: think how quickly I was gone:
Death does not always warning give
Therefore be careful how you live.
John Shipley’s headstone in Wold Newton churchyard, 1829
Meteors are what are popularly called ‘shooting stars’. An interplanetary piece of rock or dust, called a meteoroid, may fall onto the Earth if its orbit crosses the Earth’s orbit. As the meteoroid enters the Earth’s atmosphere, its surface melts because of friction with the air; its outer layers vaporize, and the gases glow. Small meteoroids completely disintegrate in the atmosphere. If the meteoroid is large or robust, it can survive the fall and reach the ground as a rock or iron lump, in which case it is called a meteorite. It may hit the ground so hard that it makes a crater.
Meteors are relatively common, but meteorites are much rarer. For centuries, people have recorded unusual stones that fell from the sky, sometimes regarding them as sacred. Some meteorites have been seen to fall, accompanied by the sound and flash of an atmospheric explosion. In 1492 a huge triangular stone noisily made a metre-deep hole in a wheat field outside the small town of Ensisheim, Alsace, witnessed by a young boy. Because the Emperor Maximilian decided that the fall was a good omen, the ‘Thunderstone of Ensisheim’ is preserved in the Regency Palace there. One of the best-documented early falls occurred near the village of Wold Newton near Scarborough in England, where in 1795 a 17-year-old ploughman, John Shipley, saw and heard a 25-kilogram meteorite impact on the ground 8 metres away from him, and was showered by earth from the resulting 50-centimetre-deep crater. This meteorite is now in London’s Natural History Museum, and a pillar marks the spot where it fell. In 1992 a meteorite damaged a Chevrolet Malibu car in Peekskill, New York, which was later put up for sale at a premium price – one of the few cases where a dent in a car’s bodywork elevated its value.
The fourth-century BCE Greek philosopher Aristotle explained meteors as a wholly atmospheric phenomenon. He thought that everything was composed of different proportions of four basic elements: ‘earth’, ‘water’, ‘air’ and ‘fire’, the latter being something like ‘inflammable material’. In his book Meteorologica Aristotle suggested that thin streams of a mixture of ‘fire’ and ‘air’ rose to the top of the atmosphere. Ignited by the rotating motion of heavenly bodies turning around the stationary Earth, the exhalations burst into a flame, like sparks off a grinding machine, making a ‘shooting star’. This point of view about the origin of meteors is the reason why they have a name that seems to connect them with weather. Variations of the explanation persisted until the end of the eighteenth century CE, though Aristotle considered meteorites unrelated phenomena caused by bits of volcanoes that had been launched into the sky by distant explosions. Aristotle’s view was that stones could not originate from the sky, and for over two millennia, learned people dismissed accounts of meteor strike as peasants’ fables. However, at the beginning of the nineteenth century this view was suddenly replaced by a new paradigm in the face of the overwhelming scientific evidence.
In 1794 the German physicist Ernst Chladni laid out the connection between meteors and meteorites. He studied a 700-kilogram iron meteorite that had been found in Siberia. Its surface was blackened and had been melted. Its composition was similar to other meteorites that had been found in widely distributed areas around the world. These meteorites were mainly iron, but had been found in places that had no iron deposits. Chladni concluded that meteorites must have the same source and that this source must cover the whole Earth – they had fallen from space.
Chladni’s hypothesis was confirmed by the French scholar Jean-Baptiste Biot, who was sent by the French Academy of Sciences to investigate reports of many stones falling from the sky at L’Aigle, Basse-Normandie, in 1803. Biot had firmly believed in the Aristotelian explanation for meteorites, but two pieces of evidence changed his mind. One was the number of reports by respected people who had actually witnessed the ‘fall of a rain of stones thrown by the meteor’. The other was the sudden appearance across the area of stones that had no similarity with any kind of mineral or human artefact from the region.
The stone in the Temple of Apollo at Delphi, Greece, known to the ancients as the omphalos or ‘navel of the world’, was a meteorite said to have been thrown to earth by the god Cronus when he created the Universe. In the Great Mosque of Mecca, the hadschar al aswad is a sacred ‘Black Stone’ kept in the Kaaba, the axis of the Islamic world. Although the stone has never been examined scientifically, it is thought to be a meteorite, said to have been given to Abraham by the archangel Gabriel and at one time possessed by the prophet Mohammed. The 14-tonne Willamette iron meteorite, now on display in the American Museum of Natural History in New York, was originally used by the tribes of the native Clackamas people in Oregon in pre-hunting rituals to harden their weapons.
The best place to discover meteorites is Antarctica. They are relatively easy to find on the white surface of ice, with the nearest terrestrial rock 3,000 feet underneath. Moreover, the flow of ice down valleys under the snow cover concentrates the meteorites into particular places. The first Antarctic meteorite was discovered in 1912, by a member of Douglas Mawson’s Australian expedition. In 1969 Japanese glaciologists discovered nine meteorites within 3 kilometres of each other – the meteorites were of five different types and therefore not fragments from the same fall. This find emphasized the importance of Antarctica as a place to discover meteorites. The Japanese National Institute of Polar Research and the University of Pittsburgh set up expeditions to Antarctica in the mid-1970s, which led to the establishment of the Japanese Antarctic Meteorite Research Center in Tokyo and the US Antarctic Search for Meteorites programme (ANSMET), now led by Scott Sandford. Tens of thousands of meteorites have since been collected from the continent.
Meteorites are valuable sources of information about the makeup of other planets in the Solar System. Roberta (Robbie) Score of ANSMET discovered ALH84001 (its number signifies that it was found in the Alan Hills icefield in North Victoria Land in 1984), which originated from the planet Mars, as shown by a comparison of the gases trapped in it with measurements of the Martian atmosphere made by the Viking lander spacecraft. ALH84001 is one of about two hundred meteorites that have been found that were ejected from the surface of Mars by the impact of a comet or asteroid. They often contain minerals or molecules that do not naturally occur on Earth. It is the hope that they will one day offer clear evidence that there is, or has been, life on Mars. Other meteorites come from the Moon and from the asteroid Vesta.
Meteor Showers
‘In the middle of the night, stars fell like rain’
In the gloomiest period of the war, [Abraham Lincoln] had a call from a large delegation of bank presidents. In the talk after business was settled, one of the big dons asked Mr. Lincoln if his confidence in the permanency of the Union was not beginning to be shaken—whereupon the homely President told a little story: ‘When I was a young man in Illinois, I boarded for a time with a deacon of the Presbyterian church. One night [in 1833] I was roused from my sleep by a rap at the door, and I heard the deacon’s voice exclaiming “Arise, Abraham, the Day of Judgement has come!” I sprang from my bed and rushed to the window, and saw the stars falling in great showers! But looking back of them in the heavens I saw all the grand old constellations with which I was so well acquainted, fixed and true in their places. Gentlemen, the world did not come to an end then, nor will the Union now.’
Walt W
hitman, Prose Works, III. Notes Left Over, 17. A Lincoln Reminiscence, 1892
On any night it is possible to see sporadic meteors, but regularly, on certain days of the year, meteors come in showers. The first record of a meteor shower dates from 16 March 687 BCE, when astronomers of the Chinese Chou dynasty noted that: ‘In the middle of the night, stars fell like rain.’ Mistaken for the apocalyptic collapse of the heavens in ancient times, and for V-2 rockets during the Second World War, meteor showers are caused by clouds of dust and rock that have regular orbits and are closely associated with comets.
The meteors of a shower have a common origin in a single comet (or in one known case, an asteroid). Meteoroids released from the parent body spread along its orbit and form a meteoroid stream, which might intersect the Earth’s orbit. When the Earth passes through the stream, lots of the meteoroids shower into the Earth’s atmosphere. These meteoroids travel in parallel. Seen in perspective from the Earth’s surface, the meteors appear to radiate from the same point, just as parallel railway tracks do. The radiant (that is, the vanishing point of the paths of the individual meteors, observed from Earth) lies in a given constellation or near a given star.
There might be half a dozen sporadic meteors per hour on a normal night, but anything up to tens of thousands of meteors per hour during a shower. The number during an annual shower varies from year to year because the meteoroids travel around their orbit in clumps, with the main clump closely associated with the parent comet or asteroid, and in a given year the Earth might or might not pass through a clump. Also, the meteors’ orbit may move or split into sub-streams, so the Earth might pass closer or further from the centre of the stream or between two.
