Solar System in Minutes
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26 WHAT IS THE SOLAR SYSTEM?
The ice dwarf Chariklo is the first Kuiper Belt Object known to have its own ring system.
The birth of solar systems
Much of what we know about the origin of our solar system is based on observations of others that are still in the process of formation. Solar systems begin life in collapsing clouds of star-forming gas and dust called ‘nebulae’. The process of collapse may be triggered by tides from passing stars, by
the shockwave of a nearby supernova (exploding star), or by
the stellar winds blown out by other newborn stars. Eventually, random knots within the nebula become dense enough to exert their own gravitational pull, drawing in more and more gas and dust and separating into dark clouds called ‘Bok globules’.
As matter falls towards the centre of a globule, it starts to spin faster (thanks to conservation of angular momentum – the same principle that causes a pirouetting skater to spin faster when they pull their arms in) and flatten out in a broad disc. The centre of the disc becomes a dense and hot protostar, while the surrounding disc contains material that will go on to form its planetary system.
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28 WHAT IS THE SOLAR SYSTEM?
The Eagle Nebula (Messier 16) is a site of present-day star formation.
Origins of our solar system
stronomers base their ideas about our own solar system’s
origins on studies of the present-day Sun, models of stellar evolution and analysis of material found in meteorites that have changed little since their formation. Hence, we
can be reasonably certain that the Sun itself became a
fully fledged star about 4.6 billion years ago. Analysis of the elements found within meteorites even suggests that the nebula out of which the Sun formed was enriched with material from recent supernovae – our solar system may owe its existence to the shockwave from the death of another star.
The oldest meteorites found so far, dating to 4.57 billion years ago, show that solid material was condensing around the Sun at this time. Competing theories differ over just how long the planets took to come together, with estimates ranging from just a few million to around 100 million years. However, studies of rocks from the Moon and Mars suggest these worlds had formed by 4.50 billion years ago at the latest.
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30 WHAT IS THE SOLAR SYSTEM?
The expanding gas clouds of supernova remnants scatter heavy elements that enrich later generations of stars and planets.
Rocky planet formation
Modern ideas about planet formation are broadly based on a ‘solar nebula disc model’ proposed by Soviet astronomer Viktor Safronov in the 1960s. According to this model, the original ‘protoplanetary’ disc was a mix of gas, ice (chemicals with low melting points), and heat-resistant dust. Heat from the young Sun caused ice to evaporate, while fierce solar winds drove both gas and vapour out of the inner solar system, leaving only dust behind.
Over the next few million years, dust particles collided at random and stuck together in a process called collisional accretion. The traditional view is that some dust clumps eventually grew large enough to exert their own gravity, pulling in material from their surroundings and snowballing in size to become Moon-scale bodies called planetesimals. These eventually collided, melting and coalescing to become the planets we know today. A recent theory known as ‘pebble accretion’, however, argues that the planets did not grow in this piecemeal way – instead, they formed abruptly as huge drifts of orbiting dust became gravitationally unstable and underwent sudden collapse.
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32 WHAT IS THE SOLAR SYSTEM?
1 Gas and dust orbits Sun in protoplanetary disc.
2 Solid bodies grow by random collisions.
3 Gravity becomes strong enough to pull in more material.
4 Larger planetesimals collide to form planets.
5 Planetary surfaces are bombarded by asteroids and comets.
Formation of giant planets
Beyond the region of the present-day asteroid belt, the heat
of the newborn Sun and the pressure of its solar wind were weaker. This allowed large amounts of ice to survive without melting, and for lightweight gases to linger in a vast, doughnut-shaped ring around the Sun. These materials vastly outweighed the relatively small amounts of dust in the outer solar system and, as a result, gas and ice became the raw materials of the outer planets and their moons.
One possible model for their formation mirrors that suggested for their inner neighbours. Dust and ice clumped together by chance collision, eventually forming cores with sufficient gravity to pull in a huge envelope of ice and gas. A problem with this theory lies in growing the cores quickly enough to pull gas from their surroundings before it dissipates into interstellar space. An alternative theory requires the planets to be created much more rapidly, perhaps as a result of collapsing eddies in a mostly uniform protoplanetary nebula, and to develop their structure later.
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34 WHAT IS THE SOLAR SYSTEM?
Two models of giant planet formation
Gas dominates outer part of protoplanetary nebula.
Solid cores form in the same way as rocky planets, then pull in gas from surroundings.
Large clumps of gas separate out into protoplanets which then collapse under their own gravity.
The Late Heavy Bombardment
Some time after the main era of planet formation had come to an end, worlds across the inner solar system seem to
have undergone a violent bombardment from space. The oldest surfaces on planets and moons are saturated with impact craters of all sizes, caused by asteroids and comets that rained down at a rate seen neither before nor since. Younger landscapes, wiped clean by geological activity such as volcanoes, have since endured crater formation at a greatly reduced rate.
Geological dating evidence from Moon rocks suggests the socalled ‘Late Heavy Bombardment’ occurred between about 4.0 and
3.8 billion years ago, since this is when the vast majority of ‘impact melt’ rocks seem to have formed. Popular models of solar system evolution suggest that the bombardment happened as a result of disturbances among the giant planets of the outer solar system. Some scientists are sceptical about whether the bombardment ‘peaked’ in the way most believe, however, or whether impact rates simply tailed off steadily from the birth of the solar system.
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36 WHAT IS THE SOLAR SYSTEM?
Solar system evolution
Until recently, astronomers believed that our solar system had changed little since the end of the Late Heavy Bombardment 3.9 billion years ago. The orderly, near-circular orbits of the major planets suggested they had followed these tracks without alteration for most of their history. Since the mid-1990s, however, the discovery of solar systems around other stars and huge advances in computer modelling have painted a different picture of our system’s surprisingly dynamic history. Many of these alien solar systems show very different arrangements of planets and wildly elliptical orbits, and while they are inevitably ‘snapshots’ of a single moment in their development, together they undermine the belief that orderly orbits are the only way for a solar system to survive in the long term. The idea that our own system has gone through traumatic changes in its past also helps to answer some otherwise puzzling features of individual planets, such as the arrangement of the gas giants, the location of the Oort Cloud and the tilted axis of Uranus.
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38 WHAT IS THE SOLAR SYSTEM?
Exchanges of orbital energy with swarms of comets may have caused the planets to shift their locations in the early solar system.
Evolving orbits
Traditional models of orbits (see page 12) treat them as an interaction between just two bodies, but the reality is far more complex – every massive body has its own gravity that effects e
verything else. So while a two-body model may describe an orbit at one instant, this doesn’t mean the orbit will remain the same when extrapolated forwards or backwards in time.
One example of such complexities, seen across the solar system on many different scales, is ‘orbital resonance’. Objects in different orbits travel at different speeds, so the distance between them (and their gravitational pull on each other) can vary hugely. Usually, this means that their influence on each other can be ignored. However, if objects happen to have ‘resonant’ orbits (with periods that are simple fractions or multiples of each other), they will return to the same alignments quite frequently. In this situation, the effects of even tiny gravitational tugs is magnified – over time a resonant object can pull another’s orbit out of shape, or even disrupt it completely.
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page 12
Ganymede
Europa
Io
Jupiter
Start
After 1 Io orbit After 2 Io orbits After 4 Io orbits
Three of Jupiter’s large ‘Galilean’ moons display a pattern of orbital resonance, with
Io orbiting the planet twice as fast as Europa and four times faster than Ganymede. Io and Europa’s orbits were both driven into this arrangement long ago by the tidal tug-of-war between Ganymede, the largest moon in the solar system, and Jupiter itself.
The Nice Model
One of the most influential models of orbital evolution in the solar system was developed by astronomers at France’s Nice Observatory in the early 2000s. The Nice Model suggests that shortly after their formation, the four giant planets were tightly packed together, with near-circular orbits inside the current orbit of Uranus, and that Neptune, now the outermost planet, started life closer to the Sun than Uranus. Beyond the major planets lay a ‘proto-Kuiper Belt’ of small, icy objects. Computer simulations suggest this arrangement would have been stable for about 500 million years, before close encounters between Uranus and Neptune disrupted their orbits. At first, they moved closer to Jupiter and Saturn, but soon these giants’ more powerful gravity flung the slightly smaller worlds out towards their present orbits. As they ploughed through the proto-Kuiper Belt, smaller worlds were either thrown further out (see page 366) or pushed inwards to bombard the worlds of the inner solar system. The model may even explain Uranus’s sharply tilted axis (see page 291).
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page 366
page 291
1 Giant planets orbit close together, surrounded by a dense ‘proto-Kuiper belt’.
2 Jupiter and Saturn’s orbits become resonant, pushing Neptune and Uranus outwards.
3 Ice giants plough into the proto-Kuiper belt, swapping positions as they do so.
4 Planets reach their present configuration, with surviving KBOs widely scattered.
Lost planets?
Some theories of solar system evolution do more than just reshuffle the orbits of the existing planets. They suggest the existence of other planet-sized bodies that have since been lost – either crashed into the Sun, exiled to the edges of the solar system or ejected completely into interstellar space. For example, one explanation for the Late Heavy Bombardment proposes the existence of a fifth rocky planet that originally formed between Mars and the proto-asteroid belt. This world’s orbit would have become unstable after a few hundred million years, sending it ploughing through the asteroids and disturbing their orbits before the planet itself plunged into the Sun.
Meanwhile, a variation on the Nice Model (see page 42) argues that the presence of a lost giant planet (an ice giant similar to Uranus and Neptune) would ultimately have made it more likely for the other worlds to settle in their current orbits. This fifth planet might also have triggered the Late Heavy Bombardment before being expelled from the solar system.
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44 WHAT IS THE SOLAR SYSTEM?
page 42
This artist’s impression shows a hypothetical Neptune-like planet at the edge of our solar system.
Life beyond Earth?
century ago, many astronomers were willing to entertain
the idea of life existing on other worlds in our solar system. Discoveries over the following decades, however (culminating
in the first space-probe flybys of the 1960s), revealed that our neighbouring planets Venus and Mars were far more hostile to life than had previously been suspected. In recent decades, however, the prospect of alien life on our cosmic doorstep
has returned. Potentially hospitable environments have been found in surprising locations (most notably on icy moons,
such as Europa and Enceladus – see pages 216 and 256), and Mars may well have reservoirs of liquid water, essential to the development of life as we know it, just beneath its surface (see page 153). Meanwhile, our understanding of life’s ability to thrive in hostile conditions has been transformed by the discovery of ‘extremophile’ organisms on our own planet. Some astronomers have even speculated that primitive life could have started out elsewhere in the solar system and was subsequently carried to Earth by comets (the so-called ‘Panspermia’ theory).
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pages 216 and 256
page 153
Undersea volcanic vents called ‘black smokers’ host flourishing ecosystems on Earth. Could similar environments on moons such as Europa and Enceladus be home to alien life within our solar system?
The future of the solar system
Mathematical limitations mean it is impossible to make predictions about the orbits of individual planets more than 50 million years beyond the present, so models of the future solar system tend to focus on the fate of the star at its centre. The Sun is currently about halfway through its ‘main sequence lifetime’ of about 10 billion years (during which it generates
fuel by the nuclear fusion of hydrogen in its core). At the end
of this stage in its life, with its principal fuel source exhausted, structural changes will see the Sun brighten considerably (perhaps by a factor of a thousand or more) and swell hugely in size, becoming a “red giant” star whose outer layers will extend beyond the orbit of Venus and perhaps engulf Earth itself.
Although we have 5 billion years until its core hydrogen runs out, the Sun will grow brighter long before that, rendering Earth uninhabitable in about a billion years. On a similar timescale, reduction in the Sun’s mass due to solar wind (see page 64) is likely to see all the planets spiral slowly outwards in their orbits.
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48 WHAT IS THE SOLAR SYSTEM?
page 64
Planetary nebulae are short-lived but spectacular ‘smoke rings’ created when a dying Sunlike star throws off its outer layers.
Our local star
By any conceivable measure, the Sun dominates everything else in the solar system. With a diameter of 1.4 million km (865,000 miles), it is large enough to contain all objects that orbit it almost 600 times over. It also contains 99.8 per cent of the solar system’s total mass.
Like all stars, the Sun is essentially a huge ball of gas, predominantly hydrogen with small amounts of helium and a few other trace elements. High temperatures strip atoms
of their outer electron particles, leaving them with exposed atomic nuclei that carry electrical charges. As huge masses of gas rise and fall inside the Sun, vast electromagnetic fields are generated that affect the Sun’s outward appearance (see pages 60–63) and make themselves felt across the solar system. Streams of charged particles blown out of the atmosphere form a supersonic ‘solar wind’ whose influence defines a vast region of space called the heliosphere, often considered to be the limit of the solar system (see page 372).
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50 THE SUN
pages 60–63
page 372
The visible Sun
From a distance, the Sun’s visible surface looks like a blazing, sharply defined sphere, but appearances can be deceptive – in reality, the incandescent ‘photosphere’ that we consider to be
the surface of the Sun is one layer among many. The photosphere appears solid because it is the region in which the Sun’s gases finally become tenuous enough, and temperatures low enough (at around 5,500°C/9,900°F), for light to escape into interplanetary space. This change actually occurs across a zone about 1,000 km (600 miles) deep, so up close the photosphere would appear more like a thinning bank of fog than a solid surface. Although the Sun’s surface appears uniformly bright in visible light except for occasional dark sunspots (see page 60), filtered views reveal a more complex pattern – the photosphere is covered in granular cells with bright, hot centres and cooler, darker edges. The bright cores of these planet-sized regions mark areas where hot gas from inside the Sun reaches the surface and sheds energy, while the darker edges are created where cooled gas, pushed aside, begins to sink back downwards.
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52 THE SUN
page 60