Appearance
13.9 — Space, the Solar System and the Earth
Two categories that quizzes treat as one, because the same facts answer both.
The solar system
How many planets are there, and why is Pluto not one? Eight. Pluto fails the third criterion of the 2006 definition — it has not cleared its orbital neighbourhood, sitting among many similar Kuiper Belt objects.
What is the order of the planets from the Sun? Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. The first four are rocky, the last four gas and ice giants.
Which planet is hottest, and why is it not the closest? Venus, at around 465 °C, because its atmosphere is 96 per cent carbon dioxide at ninety times Earth's pressure. Mercury is closer and cooler.
Which planet spins backwards, and which spins on its side? Venus rotates retrograde; Uranus is tilted about 98 degrees, so it effectively rolls along its orbit.
Which planet has the longest day relative to its year? Venus. A Venusian day of 243 Earth days is longer than its 225-day year.
Which is the largest planet, and how large? Jupiter — more massive than all the other planets combined, with a Great Red Spot that is a storm larger than Earth and has been observed for centuries.
Which planet has the most moons? Saturn currently leads, with the count above 140 and rising as small ones are found. Jupiter is close behind.
Which moons should you know? Earth's Moon; Jupiter's Ganymede, the largest moon in the solar system and larger than Mercury; Europa, with a subsurface ocean; Io, the most volcanically active body known; Saturn's Titan, with a thick atmosphere and liquid methane lakes; and Enceladus, which sprays water from its south pole.
Which planets have rings? All four giants — Jupiter, Saturn, Uranus and Neptune. Saturn's are the only ones easily visible, and they are mostly water ice.
Which planet has the largest volcano and the deepest canyon? Mars — Olympus Mons, about 22 km high, and Valles Marineris, over 4,000 km long.
Why is Mars red? Iron oxide dust — rust — across its surface, blown around by planet-wide storms.
What is the asteroid belt, and what is the largest object in it? A ring of rocky bodies between Mars and Jupiter. Ceres, the largest, is classed as a dwarf planet.
What is the Kuiper Belt and the Oort Cloud? A disc of icy bodies beyond Neptune, and a far more distant spherical shell believed to be the source of long-period comets.
What is the difference between a comet, an asteroid, a meteoroid, a meteor and a meteorite? Comets are icy and grow tails near the Sun; asteroids are rocky; a meteoroid is a small fragment in space; a meteor is the streak as it burns in the atmosphere; a meteorite is what reaches the ground.
When does Halley's Comet return? Roughly every 76 years; last seen in 1986, next around 2061. Edmond Halley predicted its return and did not live to see it confirmed.
What is a light year, and how far is the nearest star? The distance light travels in a year, about 9.46 trillion km. Proxima Centauri is 4.25 light years away.
How long does sunlight take to reach Earth? About 8 minutes 20 seconds — so you always see the Sun as it was, never as it is.
What is the Sun made of, and what powers it? About three quarters hydrogen and a quarter helium, powered by fusion of hydrogen into helium in its core at around 15 million degrees.
What will happen to the Sun? In about five billion years it will expand into a red giant, engulfing the inner planets, then shed its outer layers and end as a white dwarf. It is not massive enough to become a supernova.
Stars, galaxies and the universe
Which is the closest star to Earth? The Sun. The nearest other star is Proxima Centauri.
What is a light-year versus a parsec? A parsec is about 3.26 light years, defined by the parallax shift of a star over Earth's orbit.
What is a supernova, a neutron star and a black hole? The explosive death of a massive star; the ultra-dense remnant left behind; and the remnant when even neutron pressure fails and collapse continues.
What is the Chandrasekhar limit? About 1.4 solar masses — the maximum for a white dwarf. Above it collapse continues. Subrahmanyan Chandrasekhar worked it out at nineteen and was publicly ridiculed for it.
What was the first photograph of a black hole? M87*, released in 2019 by the Event Horizon Telescope, an array spanning the whole Earth. Sagittarius A*, at our galaxy's centre, followed in 2022.
What is our galaxy called, and how big is it? The Milky Way, about 100,000 light years across, with somewhere between 100 and 400 billion stars. We sit about 26,000 light years from the centre.
Which galaxy will collide with ours, and when? Andromeda, in around four billion years. Both galaxies are mostly empty space, so almost no stars will actually hit.
How old is the universe, and how big is the observable part? About 13.8 billion years, and about 93 billion light years across — larger than the age suggests, because space expanded while the light travelled.
What is the Big Bang, and who named it? The expansion of the universe from an extremely hot dense state. Fred Hoyle coined the term dismissively while arguing against it.
What is the cosmic microwave background? The afterglow of the early universe, found accidentally by Penzias and Wilson in 1964 while trying to eliminate noise from an antenna. They initially blamed pigeon droppings.
What is redshift, and what does it prove? The stretching of light from receding objects. Hubble showed distant galaxies recede faster, which is the evidence that the universe is expanding.
What are dark matter and dark energy? Dark matter is unseen mass inferred from how galaxies rotate; dark energy is whatever is accelerating the expansion. Together they are about 95 per cent of the universe's content and neither has been directly detected.
What is a nebula, and what is a constellation? A cloud of gas and dust, often a star nursery; and a pattern of stars, of which 88 are officially defined, mostly named from Greek myth.
Which is the brightest star in the night sky, and which the pole star? Sirius, in Canis Major; Polaris, in Ursa Minor, which will not always hold the position because Earth's axis precesses over 26,000 years.
Which constellation contains the Big Dipper, and what is it used for? Ursa Major. The two stars at the end of its bowl point to Polaris, which is how you find north.
Why do stars twinkle and planets not? Stars are effectively points and are wobbled by atmospheric turbulence; planets are small discs, so the wobbles average out.
Space exploration
What was the first artificial satellite, and when? Sputnik 1, 4 October 1957. It transmitted a beep any amateur radio operator could hear, which was the point.
Who was the first human in space, and the first woman? Yuri Gagarin, 12 April 1961, one orbit; Valentina Tereshkova, 1963.
Who was the first person on the Moon, and who else was on the mission? Neil Armstrong, 20 July 1969, with Buzz Aldrin on the surface and Michael Collins in orbit alone.
How many people have walked on the Moon? Twelve, all between 1969 and 1972. Nobody has been back, for reasons of funding rather than capability.
Which was the first animal in orbit, and what happened to it? Laika, a dog, on Sputnik 2 in 1957. She did not survive; there was no return plan.
What was Apollo 13, and what saved it? A 1970 mission crippled by an oxygen tank explosion. The crew survived by using the lunar module as a lifeboat.
Which probes have left the solar system? Voyager 1, which crossed into interstellar space in 2012, and Voyager 2 in 2018. Both carry the Golden Record.
What is on the Golden Record? Greetings in 55 languages, music including Bach and Chuck Berry, sounds of Earth, and diagrams showing where it came from.
What are the major space telescopes? Hubble, launched 1990 and famously launched with a flawed mirror that had to be corrected by a servicing mission; James Webb, launched 2021, which observes in infrared from a point 1.5 million km away; and Chandra, the X-ray observatory named after Chandrasekhar.
What is the International Space Station? A continuously crewed laboratory in low Earth orbit since November 2000, built by fifteen nations, orbiting about every 90 minutes.
Which country was first to land on the Moon's far side, and which near the south pole? China with Chang'e 4 in 2019; India with Chandrayaan-3 in 2023.
What did Chandrayaan-1 discover? Water molecules on the lunar surface, in 2008-09.
What is a geostationary orbit, and how high? About 35,786 km above the equator, where the orbital period matches Earth's rotation so the satellite stays over one spot. Arthur C. Clarke proposed the idea in 1945.
What is escape velocity from Earth? About 11.2 km/s, roughly 40,000 km/h. From the Moon it is only 2.4 km/s, which is why the return vehicle could be small.
Why do astronauts float? Not because gravity is absent — it is about 90 per cent of surface gravity at station altitude. They are in continuous free fall, falling around the Earth rather than into it.
The Earth itself
What are the layers of the Earth? Crust, mantle, liquid outer core and solid inner core. We know because earthquake waves bend and because S waves cannot cross liquid.
How deep is the deepest hole ever drilled? The Kola Superdeep Borehole, 12,262 m — about 0.2 per cent of the way to the centre. It stopped because the rock became plastic at 180 °C.
What is plate tectonics, and how fast do plates move? The Earth's shell is broken into about fifteen major plates moving a few centimetres a year, roughly the rate fingernails grow.
What causes earthquakes, and what is the magnitude scale? Stuck plates slipping. The moment magnitude scale is logarithmic — each whole number is about 32 times more energy released.
What are P waves and S waves, and why does the difference matter? Compression waves that arrive first and shear waves that arrive second and shake harder. The gap between them is what early warning systems exploit.
What causes a tsunami? Usually undersea earthquakes displacing water. In open ocean the wave is low and fast; it piles up on reaching shallow water.
What is the Richter scale replacing, and what replaced it? It replaced qualitative damage descriptions and has itself been replaced by moment magnitude for large events.
Why do we have seasons? The 23.5-degree axial tilt, not distance from the Sun. Earth is actually closest to the Sun in early January.
What causes tides, and why two a day? The Moon's gravity, which pulls the near side more than the centre and the centre more than the far side, producing bulges on both sides.
What causes the Coriolis effect, and what does it not do? Earth's rotation deflecting moving air — right in the north, left in the south. It does not control bathroom drains; that is basin shape and how the water was poured.
How old is the Earth, and how do we know? About 4.54 billion years, from radiometric dating of meteorites and the oldest Earth minerals.
What are the five mass extinctions? End-Ordovician, Late Devonian, End-Permian — the worst, killing perhaps 90 per cent of species — End-Triassic, and End-Cretaceous, which ended the non-avian dinosaurs 66 million years ago.
What killed the dinosaurs, and what is the evidence? An asteroid impact at Chicxulub, evidenced by a worldwide iridium layer, shocked quartz, glass droplets and a 180 km buried crater of exactly the right age.
What is the atmosphere made of, and what are its layers? 78 per cent nitrogen, 21 oxygen, 1 argon, with trace carbon dioxide. Layers upward: troposphere, stratosphere, mesosphere, thermosphere, exosphere.
Where is the ozone layer, and what does it do? In the stratosphere, absorbing most ultraviolet radiation. The hole over Antarctica is recovering after the Montreal Protocol.
What is the greenhouse effect, and which gases cause it? Gases absorbing outgoing infrared and re-radiating it downward — water vapour, carbon dioxide, methane and nitrous oxide. Without it the Earth would average about −18 °C.
What is the water cycle, and how much of Earth's water is fresh? Evaporation, condensation, precipitation, collection. About 2.5 per cent is fresh, and most of that is locked in ice and groundwater.
What percentage of Earth is covered by water, and how much of the ocean is mapped? About 71 per cent water. A large fraction of the seafloor is still unmapped at high resolution — better maps exist of Mars.
What is the aurora, and why is it near the poles? Charged particles from the Sun funnelled by Earth's magnetic field into the upper atmosphere, exciting gases that glow — green from oxygen, red and blue from oxygen and nitrogen.
What is the magnetic field, where does it come from, and does it flip? From convection of molten iron in the outer core. It reverses on an irregular schedule, most recently about 780,000 years ago.
What comes next
The next page is technology and inventions — the category that changes fastest and where the origin stories are the most surprising.