THE SOLAR SYSTEM IN MINUTES
GILES SPARROW
The solar system
1 Sun
Rotation period: 25-35 days Diameter: 1,391,700 km (864,400 miles)
2 Mercury
Orbital period: 88 days Rotation period: 58.6 days Diameter: 4,878 km (3,030 miles)
3 Venus
Orbital period: 224 days Rotation period: 243 days Diameter: 12,104 km (7,518 miles)
4 Earth
Orbital period: 365.25 days Rotation period: 24 hours Diameter: 12,756 km (7,923 miles)
5 Mars
Orbital period: 687 days Rotation period: 24.6 hours Diameter: 6,787 km (4,216 miles)
6 Ceres (dwarf planet within the main asteroid belt) Orbital period: 4.6 Earth years Rotation period: 9.1 hours Diameter: 975 km (606 miles)
7 Jupiter
Orbital period: 11.86 Earth years Rotation period: 9.9 hours Diameter: 142,800 km (88,700 miles)
8 Saturn
Orbital period: 29.5 Earth years Rotation period: 10.6 hours Diameter: 120,500 km (74,800 miles)
9 Uranus
Orbital period: 84.2 Earth years Rotation period: 17.2 hours Diameter: 51,118 km (31,750 miles)
10 Neptune
Orbital period: 164.8 Earth years Rotation period: 16.1 hours Diameter: 49,528 km (30,762 miles)
11 Pluto (dwarf planet)
Orbital period: 247.7 Earth years Rotation period: 6.4 days Diameter: 2,370 km (1,470 miles)
12 Eris (dwarf planet)
Orbital period: 560.2 Earth years Rotation period: c.25.9 hours Diameter: 2,400 km (1,490 miles)
CONTENTS
Introduction
6
What is the solar system? 8 The Sun
50
Mercury
68
Venus
80
Earth
92
Earth’s Moon
120
Mars and its moons
136
Asteroids
160
Introduction 6
What is the solar system? 8
The Sun 50
Mercury 68
Venus 80
Earth 92
Earth’s Moon 120
Mars and its moons 136
Asteroids 160
Jupiter and its moons
198
Saturn and its moons
234
Uranus and its moons
286
Neptune and its moons 306 Comets and centaurs
320
The Kuiper Belt and beyond 346 Exploring the Solar System 374 Glossary
408
Index
412
Acknowledgements
416
Jupiter and its moons 198
Saturn and its moons 234
Uranus and its moons 286
Neptune and its moons 306
Comets and centaurs 320
The Kuiper Belt and beyond 346
Exploring the Solar System 374
Glossary 408
Index 412
Introduction
Our solar system is the region of the Universe about which we know the most – an array of planets, moons and smaller bodies held in orbit around a huge ball of exploding gas that provides them with heat and light. It’s the one region of the cosmos that we can visit directly – currently using robotic probes, but in the future through human exploration as well.
The more we learn, the more complex the solar system becomes – each of its planets and major moons has a unique evolutionary history, preserved in geological scars or internal chemistry. Even smaller objects such as asteroids and comets show surprising variety as we learn more about them. It is also now clear that the solar system is a dynamic place – its objects might be separated by vast gulfs of empty space, but over timescales of millions of years or more, close encounters and even collisions are fairly common. Computer simulations are now revealing how the orbits of the planets and their moons have evolved over time, and how similar processes continue to influence asteroids and comets.
6 INTRODUCTION
The Solar System in Minutes is a concise yet comprehensive guide to the wonders of our cosmic back yard, from bloated giant planets that swallow comets whole, to volcano-tortured moons and deep-frozen outcasts in 10,000-year orbits. Every major object (and a significant number of minor ones of special interest) has an entry – if not a chapter – to itself, broadly in order from the Sun outwards. Planet Earth, famously described as the ‘third rock from the Sun’, of course has its own chapter, but many other worlds we shall visit also have much to tell us about our home. Some preserve episodes in the solar system’s shared history; some reveal secret mechanisms that are obscured on Earth’s ever-changing surface; and others serve as ‘what ifs?’ – cautionary tales of how our planet might have turned out if things had been just slightly different.
A final chapter offers a brief review of our explorations in the solar system so far, and prospects for the future. Crewed missions to other worlds will be expensive and dangerous, but are surely inevitable. They will certainly make scientific discoveries we cannot yet imagine, and by stretching the limits of our technological capability may prove our species worthy of long-term survival – not as residents of one small, vulnerable ball of rock, but as citizens of the wider Milky Way galaxy.
INTRODUCTION 7
What is the solar system?
Most astronomers would agree with a broad definition of the solar system as the region of space dominated by the influence of the Sun. Beyond that, however, differences soon emerge over exactly how to define the Sun’s ‘dominance’. Some take a more limited view that the solar system extends as far as the heliopause, a region where the stream of particles driven out from the Sun
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on the solar wind comes to a halt in the face of pressure from an ‘interstellar medium’ made of stellar winds from countless other stars. By this definition, solar influence only reaches about 123 times further than Earth’s orbit around the Sun, and NASA’s Voyager spaceprobes have therefore already left the solar system.
A more inclusive definition, however, is based on the dominance of the Sun’s gravity. This extends the solar system almost a thousand times further, out to the most remote comets that orbit the Sun at distances of about one light year. This is the definition used for objects that merit inclusion in this book.
Scale of the solar system
The distances between planets are huge when considered
in everyday terms – if you could get into a car and simply drive to the Sun at an average speed of 100 km/h (60 mph), the journey would take 170 years. For this reason, astronomers use much larger units of measurement when considering distance in the solar system. The astronomical unit (AU) is defined as the average distance between the Earth and the Sun, 149.6 million km (93 million miles). In terms of AU, the orbits of the planets range from innermost Mercury (an average of 0.39 AU from the Sun, though this varies considerably – see page
70), out to Neptune (an average of 30.11 AU from the Sun). The asteroid belt (which divides the solar system into rocky inner planets and giant outer planets) encircles the Sun between 2 and 3.3 AU, while the Kuiper Belt of icy dwarf planets is thought to extend to about 100 AU. The distant Oort Cloud forms a shell around the solar system out to a distance of about 63,000 AU, or 1 ‘light year’ – its outer edges are so far away that light from the Sun takes a year to reach them.
10 WHAT IS THE SOLAR SYSTEM?
page
70
Mars
Uranus
Venus
Sun
Earth
Jupiter
Saturn
Mercury
Neptune
3.3 AU
60 AU
Oort Cloud
Kuiper Belt
Neptune
200 AU
126,000 AU (2 light years)
Orbits and gravity
The Sun’s gravity is the dominant force in the solar system. Physically, gravity is a force exerted by objects with mass that draws other objects towards them. However, in practical terms, the effect of this attraction is often to maintain less massive objects on a more or less stable path around more massive ones – a path called an orbit. All stable orbits take the form of an ellipse – a circle stretched along one axis with two points called ‘foci’ to either side of its centre. In a ‘two-body’ system where one object
is much more massive than the other (for example the Sun and
its planets, or planets and their orbiting satellites or ‘moons’), the more massive object sits at one focus and the less massive one orbits around it (in more evenly balanced systems, both objects orbit around a common ‘centre of mass’). The closest approach between the two objects is known as periapsis (perihelion in the case of orbits around the Sun) and the widest separation as apoapsis (aphelion). Orbiting bodies move faster near periapsis and more slowly at apoapsis, and, as a general rule, the further out an object orbits, the more slowly it moves.
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12 WHAT IS THE SOLAR SYSTEM?
Sun at one focus Planets move faster closer to the Sun.
Planets ‘sweep out’ equal areas of their orbit in equal times.
Centre
Second focus
More distant planets orbit more slowly.
Elliptical shape stretched along one axis
Our place in the Milky Way
Our Sun is just one of about 200 billion stars in an enormous spiral galaxy called the Milky Way. This disc-shaped system is about 100,000 light years across and a few hundred light years thick. It has a pronounced 10,000-light-year bulge at its centre, where stars are closely packed together, ultimately in orbit around a monster black hole with the mass of 2.6 million Suns. Although stars are more or less evenly distributed across the disc, the bright regions where new stars are born form spiral arms that wind their way out across the disc.
The solar system is located about halfway across the galactic disc, on the outer edge of a minor spiral arm called the Orion Spur. At 26,000 light years from the core, the Sun orbits the galaxy every 240 million years. From our location inside the disc, we can see a marked difference in the concentration of stars
in different directions. Where we look across the galactic disc, stars line up behind each other in huge clouds. They are sparsely scattered, however, where we look ‘up’ or ‘down’ out of the disc.
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14 WHAT IS THE SOLAR SYSTEM?
Location of solar system
Defining planets
The word ‘planet’ comes from the Greek for ‘wanderer’, an indication of the way that certain bright objects in the night sky – our neighbouring worlds Mercury, Venus, Mars, Jupiter and Saturn – first stood out through their movements against the more distant ‘fixed’ stars. When Uranus, Neptune and Pluto were discovered following the invention of the telescope, they were all classed as planets. By the late 20th century, however, astronomers realized that Pluto was just one of many icy objects orbiting beyond Neptune. The discovery of a large object now called Eris (see page 362) raised the dilemma of either adding dozens of new planets in the coming decades of discovery, or agreeing to a more formal, scientific definition. Hence today, according to a ruling agreed in 2006, a planet
is officially defined as a body in an independent orbit around the Sun, with sufficient mass to be spherical and enough gravitational influence to ‘clear its orbit’ of large interlopers. Worlds such as Pluto, Eris and the large asteroid Ceres, which meet only the first two criteria, are termed ‘dwarf planets’.
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16 WHAT IS THE SOLAR SYSTEM?
page 362
‘Retrograde’ loop of planet in sky.
Planets stand out from the more distant stars because of their complex motion in Earth’s skies.
Slower orbit of more distant planet
Orbit of Earth
Rocky planets
The planets of the solar system are broadly divided into rocky inner worlds and giant outer ones (see page 20). The rocky planets Mercury, Venus, Earth and Mars (as well as Earth’s large natural satellite, the Moon) are all composed principally of materials with high melting points (atmospheric gases, surface water and ice on Earth and Mars are minor when compared to the bulk of the planets).
The collisions that formed these worlds heated most of their rocks to melting point, allowing them to separate into layers with the heaviest elements (predominantly iron and nickel) sinking to the centre and the lightest rising upwards to form a crust. In between lies an intermediate mantle region that transfers heat from the core towards the surface (either through simple conduction or through a slow process of convection in which hot rocks actively rise through their surroundings). Broadly speaking, the smaller worlds have cooled more rapidly while the larger ones retain more heat and, therefore, show more geological activity on their surfaces.
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18 WHAT IS THE SOLAR SYSTEM?
page 20
Giant planets
he planets of the outer solar system are much larger and much more widely separated than their inner neighbours.
Dominated by materials with relatively low melting points (which were unable to persist closer to the heat of the Sun), they have relatively small solid cores surrounded by huge envelopes of gas, liquid or slushy materials.
The giants are divided into two pairs – closer to the Sun lie enormous Jupiter and Saturn, true ‘gas giants’ composed mostly of the lightweight gas hydrogen, which transforms into vast liquid seas a few thousand kilometres beneath their visible cloudtops. Further out lie Uranus and Neptune, considerably smaller (though still much larger than Earth) and rich in relatively complex chemical ‘ices’ – not only water (H2O) but also ammonia (NH3), methane (CH4) and others. Although the term ‘gas giants’ is still casually used to encompass all four planets, scientists increasingly make the distinction of calling Uranus and Neptune ‘ice giants’.
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20 WHAT IS THE SOLAR SYSTEM?
Moons and rings
Not every object in the solar system directly orbits the Sun – many instead orbit the major planets or smaller worlds. These range from substantial moons in their own individual orbits to ring systems – swarms of smaller particles that follow concentric circular orbits in a single narrow plane. Moons that follow close-to-circular orbits, near the plane of a planet’s equator and in the same direction as the planet’s own rotation, are known as ‘regular’ satellites – with a few exceptions, such as Earth’s own Moon, they are thought to have condensed from material left behind as their parent planet formed. Moons in more elliptical or highly tilted orbits, or those which orbit the ‘wrong way’ around a planet, are called irregular satellites – they mostly started out as comets or asteroids in independent orbits around the Sun, before being captured by their planet’s gravity. The particles that form ring systems, meanwhile, are generally thought to be the shattered remnants of earlier moons that fragmented due to interplanetary collision or gravitational stress.
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22 WHAT IS THE SOLAR SYSTEM?
Saturn’s large satellites Dione and Titan, with the smaller moon Prometheus (centre, on the edge of Saturn’s rings).
Asteroids and comets
In addition to planets and the moons and rings in orbit around them, the solar system is filled with countless other objects, most of which are classed as asteroids and comets. Asteroids are small, mostly rocky bodies made from debris left over after the formation of the inner solar system
. They originated in the region between the orbits of Mars and Jupiter (where Jupiter’s gravity disrupted the formation of larger bodies) and this is where the main asteroid belt is still found today (see page 161). However, interactions between asteroids have since scattered many into orbits that bring them into the inner solar system or send them out among the giant planets. The largest asteroid, Ceres, is also defined as a dwarf planet.
Comets, meanwhile, are small icy bodies, formed in the region where the giant planets orbit today but subsequently exiled to the distant edges of the solar system. From here, they occasionally fall back towards the Sun, where they can heat up and release gas that forms spectacular tails.
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24 WHAT IS THE SOLAR SYSTEM?
page 161
Asteroid 21 Lutetia
Centaurs and ice dwarfs
Orbiting between and beyond the outer planets, centaurs and ice dwarfs are ice-rich bodies that are related to comets.
Ice dwarfs are found in a region called the Kuiper Belt, stretching from the orbit of Neptune to perhaps 100 AU from the Sun. They vary in size from fairly substantial worlds like Pluto and Eris to smaller comet-like bodies, but are thought to have stayed in roughly this region of the solar system for most of their history. The doughnut-shaped ‘classical’ Kuiper Belt is surrounded by a more diffuse ‘Scattered Disc’ whose members have more elongated and wildly tilted orbits. As the name suggests, these objects are thought to have been ejected or scattered into their present orbits by close encounters with Neptune or other giant planets. Centaurs, meanwhile, orbit between the giant planets. They probably originate in the Kuiper Belt, and spend only a relatively short time in this region of the solar system before the gravity of the planets disrupts their orbits further, perhaps kicking them out to the scattered disc, or sending them into more elliptical orbits where they become fully fledged comets.
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