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Earth and its pale lunar companion are the only worlds with human footprints. They are special in many other ways, too. Nowhere else, for example, does such a huge moon orbit such a small planet. That could be connected to another phenomenon, unique to Earth as far as we know: the presence of small quantities of matter that organise themselves into complex, dynamic forms, apparently defying the laws of thermodynamics. And besides the curiosity we call life, these two worlds could hold the key to understanding the formation of the whole solar system.…

AFTER Earth, the moon is the most studied object in our solar system. More than 70 successful missions have unlocked its geological history, determined its internal structure and measured its surface composition. The conclusions of those explorations stretch well beyond the barren lunar surface. The same astronomical processes that have influenced Earth have also been felt by the moon. Yet while weathering and the restless shifting of the continents on our planet have largely erased the most ancient events from our geological record, that isn’t true of moon rocks. Decoding the lunar record began in earnest 50 years ago, when the first moon rocks were collected by Apollo 11’s Neil Armstrong and Buzz Aldrin. During a 2-hour-and-36-minute moonwalk, they pocketed 22 kilograms of the lunar surface, then brought it back…

The three closest planets to Earth are all small, solid worlds. All of them have iron cores, bound with rock. Yet they are all spectacularly different from one another. Mercury is a black and blasted plain. Venus is a sweltering world beset by rain of pure acid. Mars is cold and arid, with tantalising hints of alien life. Understanding the origin of these differences could cast light on the nature of our own world and the diversity of rocky planets throughout the universe – and so prevent us from being too blinkered in our search for life.…

QUESTIONS about Mars start with why it is so small. Models of the solar system’s formation suggest that it should be between 1.5 and 2 times Earth’s mass. Instead, it weighs in at a mere one-tenth the mass of our world. The reason why still isn’t understood, but may well be connected with the antics of Jupiter, whose gravity could have scattered much of the material available for planet-building where Mars formed. Whatever the reason, the diminutive size of Mars gives it its character. Its gravity is too weak to hold onto a thick atmosphere that might have kept the surface cosy and damp. It lacks Earth’s big, hot, liquid core, which here generates a magnetic field that shields us from space radiation. There is still plenty of ground to…

TO GET to Mars, we will need to blast off from Earth with more supplies than we have ever put in space before, traverse millions of kilometres of deadly interplanetary nothingness and land safely at the other end. The good news is that we don’t need to dream up new types of engine or worry about things like sunlight-powered solar sails, which accelerate very slowly. All we need is a big rocket pointed in the right direction. With existing rocket technology, the most plausible trajectory would take about nine months, a little longer than an astronaut’s standard six-month stint on the International Space Station (ISS). Seven types of rocket in operation could make it to Mars. The most powerful of these, SpaceX’s Falcon Heavy, could shuttle about 18.5 tonnes there.…

Beyond the asteroid belt, four heavyweight planets reign. Unimaginably deep oceans of gas conceal their mysterious interiors, where strange states of matter may lurk. Their powerful gravity has remodelled the solar system. Like the classical gods whose names they bear, they have both nurtured and toyed with life on Earth. And the giants are surrounded by a panoply of fascinating moons. These solid, approachable little worlds have a remarkable diversity – and perhaps their own alien inhabitants.…

WE ONCE thought that Titan was the largest moon in the whole solar system, bigger even than Jupiter’s mighty Ganymede. Astronomers at the time didn’t realise that, alone among moons, it is shrouded in a thick atmosphere, which increases its apparent size. This rich atmosphere gives the moon many of its unique attributes, and also hides them from us. Solar ultraviolet radiation drives reactions between nitrogen and methane molecules in the atmosphere that create an opaque orange-brown smog. Even when Voyager 1 passed Titan in 1980, it couldn’t see the surface. The purpose of the Huygens lander, built by the European Space Agency and carried by the Cassini mission, was to find out what lay beneath. Huygens descended to Titan’s surface in 2005, giving us our first clear view. Photos…

Arrival dates (closest approach or orbital insertion) JUPITER Pioneer 10 4 December 1973 Pioneer 11 3 December 1974 Voyager 1 5 March 1979 Voyager 2 9 July 1979 Ulysses 8 February 1992 Galileo 7 December 1995 Cassini 29 December 2000 New Horizons 28 February 2007 Juno 4 July 2016 SATURN Pioneer 11 1 September 1979 Voyager 1 12 November 1979 Voyager 2 25 August 1981 Cassini 1 July 2004 URANUS Voyager 2 24 January 1986 NEPTUNE Voyager 2 25 August 1989 PLUTO New Horizons 14 July 2015 SOURCE: NASA/JPL PIONEER 10 LAUNCHED: 3 MARCH 1972 Pioneer 10 was the first probe to cross the asteroid belt, between July 1972 and February 1973. Arriving at Jupiter in December 1973, it passed some 132,000 kilometres from the planet’s cloud tops and obtained fuzzy images of the four large Galilean moons, Ganymede, Europa, Callisto…

THE twin detectors of the Panoramic Survey Telescope and Rapid Response System, Pan-STARRS, sit atop a 3000-metre peak in Hawaii, constantly scanning the sky for space rocks straying too close. On the night of 19 October 2017, they spotted a dim trail of light, fast moving against the starry backdrop. This object wasn’t orbiting the sun, it was paying us a visit from far outside our solar system. Asteroids or comets orbiting the sun generally follow closed, elliptical paths. This object’s trajectory was an open curve, a hyperbola, meaning it couldn’t be orbiting the sun. It had already made its closest approach, and was racing away at 38 kilometres per second, fast enough to escape back into interstellar space. There are plenty of things that could throw debris out of…

The sun dominates the solar system. It is our centre, our source of warmth, the origin of the word “solar” in “solar system”, of course; and, in terms of sheer mass, it is overwhelming. If the contents of this Essential Guide were in proportion to the mass of their subject matter, this chapter would run for well over 30,000 words, leaving a total of four words to be shared between the other four chapters on our solar system. Yet for all its dominance, our home star still holds many mysteries.…

IN 1956, Eugene Parker was a young postdoc at the University of Chicago. He was investigating cosmic rays arriving at Earth from far off in the galaxy when an idea struck him. “We knew cosmic rays were correlated with the sun’s magnetic activity, but the timing of the cosmic rays on our detectors during one particularly violent solar flare showed that the particles were moving very freely from sun to Earth. Around the same time, astronomers were noting that comet tails always pointed away from the sun, and that, too, was very difficult to explain. “One day in 1958, it occurred to me this was all very simple. The sun’s atmosphere, the corona, is not tightly bound. Stuff can escape, and the whole thing acts like one big gaseous outward…

THE risk of an asteroid collision is the price we pay for living in our crowded bit of space. There are millions of rocky and icy vagabonds jostling for room among the moons and planets of our solar system. So far, astronomers have spotted more than 21,000 asteroids with orbits that are set to bring them close to our world. Every day, another one of these near-Earth objects is discovered. At the small end of the scale, they aren’t worth worrying about. An asteroid the size of a car is likely to burn up in the atmosphere, putting on a light show but not causing any destruction on the ground. At the other extreme are giants such as the one that hit what is now Chicxulub in Mexico about 66…

MERCURY is not a hospitable place. Its airless and lifeless surface is seared by the sun, reaching up to 430°C on the dayside equator. Because there is barely any atmosphere to hold and spread heat, the temperature drops to about −175°C under the blackness of night after the sun sets, in a daily cycle lasting twice as long as the planet’s year (which itself lasts just 88 Earth days). The tortured landscape is raked with ionising radiation and scarred by billions of years of explosive impacts. Beneath it lies only a thin 500-kilometre mantle, just a thin rock shell around a huge iron core, reaching out 80 per cent of the whole planet’s radius. Why is Mercury so heavy in metal, unlike the other inner planets of the solar system?…

Beyond Mars lie the asteroids – a huge diffuse belt of rocky material left over from the formation of the solar system. It is estimated that there are some 1.9 million asteroids in the asteroid belt larger than 1 kilometre across. Ceres, around 1000 kilometres across, is the greatest among them. Owing to the quirks of planetary classification, it also has the distinction (for now) of being the solar system’s smallest recognised dwarf planet. → See page 79 for more on the dwarf planet controversy Ceres was once thought to be a dead and static rock, but when NASA’s Dawn spacecraft orbited it between 2015 and 2018, it found an active and curious little world. In 2015, it beamed back pictures of a mountain called Ahuna Mons, with bright streaks…

THE importance of finding life beyond Earth cannot be overstated. To find it elsewhere in our solar system would surely mean it is widespread throughout the entire galaxy. It would allow us to study the chemical composition of life, and perhaps find out whether it must be based on DNA, or even on carbon chemistry. → Page 91 has more on the search for life beyond our solar system Just 40 years ago, we never would have suspected that the secrets of how life formed on Earth, and whether it exists elsewhere, may lie in the icy moons of the outer solar system. Then came two seemingly unconnected events in 1977. Off the Galapagos Islands, oceanographers Jack Corliss at Oregon State University and Tjeerd van Andel at Stanford University in…

OUT in the solar system’s liminal zone, there may be more bodies than there are stars in the entire Milky Way. Bit by bit, we are building up a picture of what is out there. The first object discovered here – besides Pluto and its moon Charon – showed up in 1992, and is still known only as (15760) 1992 QB1. Since then, we have charted the orbits of more than 1000 Kuiper belt objects. They include several confirmed dwarf planets, including Eris, Makemake and Haumea, along with their tiny moons. But at some point, the Kuiper belt just… stops. This “Kuiper cliff” occurs some 40 astronomical units (AU) out, and from there on, it is, as far as we can make out, a whole load of nothing until you…

TO SOME people, the sheer size of the universe makes it unlikely that life formed only once. To others, the remarkable complexity of life on Earth is testament to its uniqueness. The huge uncertainties in estimating the likelihood of life are encapsulated in a famous equation formulated by astronomer Frank Drake (see page 92). But we may be in sight of a real answer, thanks to new telescopes that will let us study exoplanet atmospheres and surfaces – and, if we are lucky, reveal the first signs of life beyond Earth. To work out what we should be looking for, it helps to reverse our point of view. If alien astronomers were observing Earth from a remote star system, would anything about it grab their attention? Compared with our rocky…

OUR planet’s abundant life is invariably constructed from carbon and reliant on liquid water. There are good reasons to believe that, as it is on Earth, so it is in the heavens. Carbon and water are two of the most common substances in the universe. In tandem, they provide an extravagance of durable chemical products unmatched by any other obvious combination of elements. If life does need liquid water, then any habitable planet must occupy a slim sliver of the space surrounding a star. Too close to the thermonuclear furnace and water will evaporate away. Too far and it will freeze, consigning life to a frigid fate. Where exactly these boundaries are in a given planetary system depends on a star’s brightness. Earth seems to be snugly sandwiched in the…

WE THOUGHT we had the origins of the solar system more or less worked out. Some 4.6 billion years ago, a vast cloud of dust and gas in some corner of an unremarkable galaxy began to collapse into a dense ball of matter. As more and more surrounding material was pulled towards it, the temperature and pressure at its core increased, to the point where nuclear fusion kicked in. This released vast quantities of energy and marked the moment our sun became a star. ← Turn back to page 6 for more on how the sun works As the newborn star slowly began to spin, smaller bodies started to coalesce in orbit around it. Close in, vast quantities of water ice were boiled away, leaving metallic and silicate compounds behind…

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BY CONTRAST to the paucity of our missions to Mercury, we have visited Venus dozens of times. Many Venus missions have even put landers onto the planet’s surface – although no lander has survived longer than 90 minutes. Venus is even more of a hellhole than Mercury, with surface temperatures averaging more than 460 °C. It is so hot because of a runaway greenhouse effect. Its thick carbon-dioxide atmosphere traps heat near the surface, and also means that the heat gets distributed all around the world instead of radiating away into the chill of space at night. Unlike Mercury, there is no cold side. For some unknown reason, one layer of the atmosphere whips around the planet in only about four days, rotating 60 times faster than the solid planet.…

PERSEVERANCE, lowered to the surface from a hovering sky crane in February 2021, is the fifth rover that humans have landed on Mars. It is exploring Jezero crater, once a Martian lake and a fantastic place to hunt for traces of life, with just the right kinds of rocks for preserving fossils. The rover carries 43 sample tubes, to be filled and left for a subsequent mission to collect and launch back into orbit around Mars. The plan is for these to be picked up and returned to Earth as early as 2031. Why has there been so much excitement about the Perseverance rover? This is the first opportunity for us to get samples from a really well-understood geologic context on Mars. We picked a site that once had water,…

GANYMEDE Diameter – 5268 km Distance from Jupiter – 1,070,000 km All four of Jupiter’s Galilean moons are big and round enough that they would be at least dwarf planets if they were orbiting the sun. Ganymede is the biggest of all, outsizing even the planet Mercury. As with Earth, but uniquely for a solar system moon, Ganymede has a strong field, generated by an internal dynamo in a liquid iron core. Its magnetism even creates a visible aurora. The Hubble Space Telescope has shown that Ganymede’s aurora is modulated by yet another magnetic field, and the best guess is that this comes from a briny, electrically conducting subsurface ocean. Models suggest that there could be several such oceans in concentric layers sandwiched between shells of rock. CALLISTO Diameter –…

We have learned in the past few years that Proxima Centauri, the nearest star to the sun, has its own system of planets. The most exciting among them is Proxima b, which seems to be a little larger than Earth, and orbits in the habitable zone of its star – although that doesn’t necessarily mean the planet really is habitable. Any life there would somehow have to cope with the nasty outbursts of X-rays and ultraviolet from the star. Then there is Proxima c, with a few times Earth’s mass, probably a small gas giant. It is much brighter than expected, suggesting that it is surrounded by a huge disc of dust or a shining system of rings bigger than Saturn’s. A third, tentatively detected, planet appears to have a…