144 Fun Facts About Planets

Mars looks red because its dust and rocks are coated in what?

Rust: iron in Martian rock and dust has oxidised, yet that same fine dust scatters light so that sunsets on Mars glow blue.

The five planets visible to the naked eye are named after gods from which mythology?

Roman: when Uranus was discovered, Johann Bode broke the pattern and named it after the Greek god of the sky.

Whose planetarium left out Pluto in 2000?

Neil deGrasse Tyson: his Hayden Planetarium set Pluto apart from the planets years before astronomers demoted it, drawing angry letters from children that he retold in The Pluto Files.

In French and Spanish, which day of the week is named after Mercury?

Wednesday: French mercredi and Spanish miércoles both mean 'Mercury's day', but English swapped in Woden, a Germanic god equated with Mercury.

The word 'planet' comes from a Greek word meaning what?

Wanderer: ancient skywatchers saw planets drift from night to night against the fixed stars, so the Greeks called them wanderers.

Venus is hotter than Mercury, though farther from the Sun, mainly because of what?

Greenhouse effect: Venus's thick carbon dioxide atmosphere traps so much heat that its surface, near 465 °C, is hot enough to melt lead.

What mainly causes Earth's seasons?

Its tilted axis: each hemisphere takes turns leaning toward the Sun, and Earth is actually closest to the Sun in January, during northern winter.

NASA lost the Mars Climate Orbiter in 1999 because its teams' software used different what?

Measuring units: ground software gave thrust figures in English units while the probe's software expected metric, so it flew too close to Mars and was lost.

Which Mars rover, built to last about 90 days, kept working for almost 15 years?

Opportunity: Martian winds kept sweeping dust off its solar panels, until a planet-wide dust storm in 2018 cut off its power for good.

What is the name of Jupiter's giant storm, wider than Earth?

Great Red Spot: it is shrinking, and by 2014 it was well under half the length observers measured in the late 1800s.

The habitable zone around a star, where life-friendly planets might exist, is nicknamed after which fairy-tale character?

Goldilocks: like her porridge, planets in that zone are 'just right', neither too hot nor too cold, which makes them prime targets in the search for life.

What is the name of the largest moon in our solar system?

Ganymede: it is bigger than the planet Mercury, and Hubble's watch on its shifting auroras points to a salty ocean hidden beneath its icy crust.

Pluto lost planet status for failing which test?

Clearing its orbit: Pluto is round and circles the Sun, but other objects cross its path, so it has not swept its neighbourhood clear.

Which astronomer's 1543 book placed the Sun, rather than Earth, at the centre of the universe?

Copernicus: the book appeared as he died, carrying an unsigned preface by Andreas Osiander that called the Sun-centred system merely a useful hypothesis.

Which NASA space telescope, launched in 2009, found thousands of planets by watching stars dim?

Kepler: losing a second pointing wheel in 2013 ended its original survey, yet a reworked mission kept it finding planets until its fuel ran out in 2018.

Which astronomer suggested the 1990 'Pale Blue Dot' photo, which showed Earth as a speck about 6 billion km away?

Carl Sagan: he pushed for years to turn a probe's cameras back toward home, and he later named his 1994 book after the picture.

What distant shell of icy objects, thought to surround the Solar System, is a source of long-period comets?

Oort cloud: it is so far away that it has never been imaged directly, so astronomers know it only through the comets thought to come from it.

Which moon of Saturn has a thick atmosphere and liquid methane lakes on its surface?

Titan: methane falls there as rain and fills rivers, lakes and seas, making it the only world besides Earth known to have liquid on its surface.

Which English word, taken from a planet's name, means cheerful and good-humoured?

Jovial: it comes from Jove, another name for Jupiter, because astrologers believed people born under that planet were naturally merry.

Space probes gain speed by swinging close past a planet, a manoeuvre known as what?

Gravity assist: the planet pays for the boost by losing a tiny, unmeasurable amount of its own orbital energy, while the probe gains a lot.

Which rover landed on Mars in February 2021?

Perseverance touched down in Jezero Crater, an ancient lake-delta site, on February 18, 2021, to drill and cache rock cores for possible return to Earth by a later mission.

What is the name of the asteroid belt's only dwarf planet?

Ceres: NASA's Dawn found that its bright spots are salt deposits, left where briny liquid seeped up from below and dried out.

Which moon of Jupiter is one of the most promising places to search for life beyond Earth?

Europa: its smooth, young crust hides an ocean that may touch a rocky seafloor, and NASA launched the Europa Clipper probe in 2024 to investigate.

Venus, the only planet named after a female god, honours the goddess of what?

Love: fittingly, most surface features on Venus are named after women from history and myth.

An 11-year-old Oxford schoolgirl suggested naming Pluto after the god of which realm?

The underworld: Venetia Burney first put the idea to her grandfather, and astronomers adopted the name in 1930.

What is the main component of Jupiter's atmosphere?

Hydrogen: deep inside Jupiter the pressure squeezes it into a liquid that conducts like metal, and that churning layer generates the planet's huge magnetic field.

Astronomer Percival Lowell was convinced that Mars was criss-crossed by what, built by intelligent beings?

Canals: spacecraft photos in the 1960s finally showed a cratered, barren world, and the straight lines turned out to be an optical illusion.

What feature on Mars is the largest canyon system in the solar system?

Valles Marineris: rather than being carved by a river, it likely formed as the Martian crust cracked apart and the canyon walls collapsed.

Which spacecraft flew past both Uranus and Neptune?

Voyager 2: planners sent it on toward Uranus and Neptune only after its twin had completed its own mission at Saturn.

What phenomenon causes Io, Jupiter's moon, to be the most volcanically active body in the solar system?

Tidal heating: Jupiter's gravity and the precisely timed tugs of two neighbouring moons keep flexing Io's interior, and that heat feeds hundreds of volcanoes.

How long does it take light from the Sun to reach Earth?

About 8 minutes: so the Sun we see in the sky is always the Sun as it was about 8 minutes earlier.

NASA's MAVEN orbiter found that Mars has been losing its air to space, stripped away by what?

The solar wind: Mars once had a thick, warm atmosphere, but after losing so much gas to space its air today is thin and cold.

How long is a Mercury year — one lap of the Sun?

A Mercury year is 88 Earth days, the shortest orbital period of any planet, because Mercury circles the Sun at about 47 km/s on the innermost orbit.

Uranus and Neptune are classed apart from Jupiter and Saturn as what kind of planet?

Ice giants: most of their mass is a hot, dense fluid of 'icy' compounds, which is what sets them apart from the two giants nearer the Sun.

Which moon of Saturn shoots geysers of water ice into space from its south pole?

Enceladus: its jets are fed by a global ocean beneath the ice, and the spray supplies most of the material in Saturn's E ring.

What makes Neptune appear blue?

Methane in its atmosphere: the gas absorbs red light and reflects blue, and the same gas gives Uranus its blue-green tint.

Jupiter's volume is about how many times Earth's?

About 1,300: yet Jupiter packs only about 318 times Earth's mass into all that space, so it is far less dense than our planet.

Ancient skywatchers who saw Venus shining at dawn and again at dusk believed it was what?

Two separate stars: they named a morning star and an evening star, because Venus never strays far from the Sun in our sky.

What is the Great Dark Spot?

A storm on Neptune: the first one seen, big enough to hold Earth, had vanished by 1994, and new dark spots keep forming and fading.

What did a Viking orbiter photo of a Martian hill seem to show in 1976, fuelling alien theories?

A human face: sharper images in 2001 showed an ordinary mesa, and the 'face' was a trick of low resolution and shadow.

What is the Roche limit?

The distance within which tides tear a moon apart: Saturn's rings may be the remains of moons or comets shattered this way before they reached the planet.

Which country's space agency put a probe into orbit around Mars on its first attempt, in 2014?

India: its Mars Orbiter Mission was built to last six months but kept working for more than seven years.

What is the Kuiper Belt?

A disc of icy bodies beyond Neptune: they are leftovers from the Solar System's birth that never came together into a planet, and Pluto is the best-known.

Which NASA spacecraft radar-mapped about 98% of Venus's surface in the early 1990s?

Magellan: it rode into space aboard the shuttle Atlantis in 1989, and later skimmed Venus's upper air to reshape its orbit, the first probe to try that.

Pluto, once counted as the ninth planet, is smaller across than which world?

Earth's Moon: Pluto is only about two-thirds as wide as our Moon, yet it has five moons of its own.

What is the largest impact basin on Mercury called?

Caloris Basin: about 1,550 km across, it formed in an impact so violent that shock waves may have converged on the far side of Mercury and jumbled the land there.

In which direction does Triton, Neptune's largest moon, orbit its planet?

Retrograde, against the host's spin: Triton circles opposite to Neptune's rotation, which suggests Neptune captured it rather than forming it.

How does Jupiter's mass compare with all the other planets combined?

Over twice as much: Jupiter is so massive that it and the Sun circle a balance point lying just outside the Sun's surface.

What name did William Herschel propose for Uranus, the planet he discovered?

Georgium Sidus: it honoured King George III, who made Herschel his royal astronomer, but Johann Bode's name, Uranus, later won out everywhere.

Which is the largest moon of Uranus?

Titania: William Herschel found it and Oberon on the same night in 1787, and it is named after the fairy queen in A Midsummer Night's Dream.

Verona Rupes, a towering cliff many kilometres high, is on which moon of Uranus?

Miranda: its gravity is so weak that a rock dropped from the top of its highest cliff would take many minutes to hit the bottom.

Uranus's moons are named after characters from Shakespeare and which poet?

Alexander Pope: Ariel and Umbriel take their names from his poem The Rape of the Lock. John Herschel began the Shakespeare-and-Pope naming tradition in 1852, breaking with Greek and Roman mythology.

Which was the first spacecraft to visit Mercury, flying past it in the mid-1970s?

Mariner 10 flew past Mercury three times in 1974–75; it was also the first probe to visit two planets, using a Venus gravity assist and steering partly with sunlight pressure on its solar panels.

Which NASA probe became the first to orbit Mercury, before crashing into it in 2015?

MESSENGER: it found ice hiding in polar craters that never see sunlight, on the planet closest to the Sun.

The BepiColombo mission to Mercury is a joint project of ESA and which agency?

JAXA: Japan's Mio orbiter rides stacked on Europe's orbiter for the long trip, and the pair split apart only on reaching Mercury.

Before 1960s radar, astronomers believed Mercury did what?

Kept one side sunward: radar revealed that it actually turns once every 59 Earth days, not once per orbit as astronomers had assumed.

How long is a solar day on Mercury, from one sunrise to the next?

About 176 Earth days: Mercury's slow spin and speedy orbit nearly cancel each other out, stretching the time from one sunrise to the next.

Craters on Mercury are named after what kind of people?

Artists, musicians and authors: in 2015 the IAU adopted names picked in a public contest, giving craters to Egyptian singer Umm Kulthum and Irish composer Turlough O'Carolan.

Venus's planet-wide cloud layer is made mostly of what?

Sulfuric acid: those clouds cover the whole planet and hide its surface from view, which is why the ground had to be mapped with radar.

What did Venus's surface do to landers?

Destroyed them within hours: the surface pressure is over 90 times Earth's, like being nearly a kilometre deep in the ocean.

Which country landed the first spacecraft on another planet, in 1970?

The Soviet Union: Venera 7 came down much faster than planned, yet it still radioed a faint signal back from the surface.

What is the highest mountain on Venus, rising about 11 km above the mean surface?

Maxwell Montes: it breaks Venus's rule of naming features after women, honouring physicist James Clerk Maxwell, whose work made radar possible.

Venus's northern highland, Ishtar Terra, is about the size of which landmass?

Australia: its highest peaks shine brightly on radar, possibly because a metallic 'snow' coats the rock where the air is cooler.

What is the largest clearly exposed impact basin on Mars, about 2,300 km across?

Hellas Planitia: its light colour against the rest of Mars made it one of the first Martian features astronomers spotted through telescopes.

Viking 1 landed on Mars exactly seven years after which mission?

Apollo 11: Viking 1 then worked on Mars for over six years, until a faulty command sent from Earth cut contact in 1982.

What was the first wheeled vehicle to operate on a world other than Earth or the Moon?

Sojourner: it was named after the abolitionist Sojourner Truth, and it kept exploring for about 83 days instead of the planned week.

NASA's Curiosity rover landed in 2012 inside which Martian crater?

Gale: from its landing spot on the crater floor, Curiosity has driven up the layered slopes of Mount Sharp, the mountain at the crater's centre.

What did NASA's Ingenuity achieve on Mars in April 2021?

First powered flight on another planet: planned as five test hops in air less than 1% as dense as Earth's, Ingenuity went on to fly 72 times.

Which 1965 mission returned the first close-up pictures of the Martian surface?

Mariner 4: its photos showed a cratered, Moon-like world, crushing hopes of finding intelligent life on Mars.

How does a day on Mars compare with a day on Earth?

About 40 minutes longer: mission teams call a Martian day a sol to avoid confusing it with an Earth day.

Seen from the surface of Mars, Phobos does what that no moon does on Earth?

Phobos rises in the west and sets in the east because it orbits faster than Mars rotates, circling in just 7 hours 39 minutes — it crosses the sky 'backwards' about twice each sol.

Which Jupiter probe is named after the goddess who saw through Jupiter's clouds?

Juno: its three huge solar panels had to be that big because Jupiter gets only about 4% as much sunlight as Earth.

Which spacecraft was the first to orbit an outer planet, arriving at Jupiter in 1995?

Galileo: its main antenna failed to open fully, so engineers rewrote its software to send data home through a small backup antenna.

Comet Shoemaker–Levy 9 was found in 1993 already doing what?

Orbiting Jupiter: the planet had probably captured it years before, and in July 1994 more than 20 of its fragments crashed into Jupiter.

Which probe was the first to cross the asteroid belt and reach Jupiter?

Pioneer 10: it carries a plaque with drawings of a man and a woman and a map of Earth's place, meant for any aliens who might find it.

Which moon of Jupiter is the most heavily cratered object in the Solar System?

Callisto: beneath that ancient, battered crust it may hide an ocean of salty liquid.

Jupiter's faint ring system was discovered in 1979 by which spacecraft?

Voyager 1: the faint rings are mostly dust, likely kicked up when meteoroids smash into Jupiter's small inner moons.

Saturn is the only planet less dense than what?

Water: Saturn's average density is lower than water's, so in a big enough bathtub the whole planet would float.

Which moon of Saturn bears the giant crater Herschel, giving it a Death Star look?

Mimas: the crater spans about a third of the moon's width, and the impact that made it probably came close to breaking Mimas apart.

Which moon of Saturn has one dark hemisphere and one bright one?

Iapetus: Giovanni Cassini deduced its two-tone surface in the 1670s because the moon vanished from view on one side of Saturn. It also has a ridge running along its equator.

The Cassini Division, the largest gap in Saturn's rings, separates which two rings?

A and B: the rings are lettered in the order astronomers found them, which is why the main rings run C, B, A outward from Saturn.

Which probe was the first to reach Saturn, flying past it in 1979?

Pioneer 11 flew past Saturn in September 1979, discovering the F ring and a small new moon; its flight path tested whether later probes could safely pass through the ring plane.

How did NASA end the Cassini mission on September 15, 2017?

Burned it up in Saturn's atmosphere: NASA did it on purpose, so the probe could never crash into and contaminate a moon that might hold life.

Which sponge-looking moon of Saturn tumbles chaotically, so its rotation is unpredictable?

Hyperion: about 40% of the moon may be empty space, which helps explain its deep, sponge-like craters.

Triton was found by William Lassell how soon after Neptune itself was discovered?

17 days: yet it was commonly called just 'the satellite of Neptune' until 1949, when Neptune's second moon was found.

Which NASA probe, launched while Pluto was still a planet, flew past it in 2015?

New Horizons: Pluto was so far away that sending all the flyby data home took more than 15 months.

What is Pluto's largest moon, with a diameter just over half of Pluto's own?

Charon: it is so large next to Pluto that the pair is sometimes called a double dwarf planet rather than a world and its moon.

Who discovered Pluto in 1930?

Clyde Tombaugh: he sent Lowell Observatory drawings of Mars and Jupiter made through his homemade telescopes, and the observatory hired him.

Pluto's large bright region, photographed up close in 2015, is nicknamed after what shape?

A heart: the frozen nitrogen in its western half slowly churns and renews the surface, which is why no craters show there.

Which elongated dwarf planet was found in 2017 to have its own ring system?

Haumea: it spins about once every four hours, so fast that it is stretched into a football shape, and an ancient collision may have set it spinning.

Which was the first exoplanet found orbiting a Sun-like star, in 1995?

51 Pegasi b: a huge world circling its star in about four days, it sat so close in that it clashed with the planet-formation theories of the day.

TRAPPIST-1, host of seven Earth-sized planets, is what kind of star?

Red dwarf: it gives off so little energy that planets orbiting closer to it than Mercury is to the Sun can still be temperate.

Worldwide expeditions timed Venus's 1769 transit to measure what?

Earth-Sun distance: timings from far-apart places could reveal it, so Captain James Cook's 1769 voyage went to Tahiti to watch.

In the leading theory of the Moon's origin, what is the Mars-sized impactor called?

Theia: Moon rocks match Earth's own too closely to be chance, evidence that the Moon formed from debris of that collision.

Which Martian volcano, about two and a half times the height of Mount Everest, is the tallest on any planet?

Olympus Mons rises about 21 km above the Martian plains and sprawls across an area roughly the size of Italy.

Venus spins so slowly, once every 243 Earth days, that a single rotation lasts longer than a Venusian what?

Year is the surprise: Venus laps the Sun in about 225 Earth days, quicker than it completes a single turn. It also spins the opposite way to most planets.

A jet stream circling Saturn's north pole, with sides longer than Earth is wide, forms what shape?

A hexagon crowns the pole, each side about 14,500 km long, and it has held its shape since it was first spotted in images from 1981.

Daytime skies on Mars are a dusty butterscotch, but what colour do they turn around the setting Sun?

Blue glows around the Sun at Martian dusk and dawn, the mirror image of Earth, where a blue day ends in a red sunset.

Uranus's axis is tipped nearly 98 degrees, so the planet circles the Sun spinning in what odd way?

On its side is how Uranus travels, like a rolling ball, possibly after a collision with an Earth-sized object long ago. Each pole gets about 42 years of sunlight, then 42 of darkness.

Which gas makes up about 78% of the air on Earth, far more than any other?

Nitrogen dominates Earth's air at 78%, with oxygen a distant second at about 21%. Argon, at just under 1%, is the surprise third.

In which month does Earth pass closest to the Sun, a fact that surprises many shivering northerners?

January brings Earth nearest the Sun, about two weeks after the December solstice. The gap changes too little to drive the seasons, which is why the north is cold at the time.

Which English composer wrote the orchestral suite The Planets, opening with 'Mars, the Bringer of War'?

Gustav Holst based the suite on astrology rather than astronomy, giving each of seven planets a character. Earth is left out, and Pluto had not been found when he finished in 1917.

What did Mercury do as its interior cooled over billions of years, pushing up mile-high cliffs?

Shrank is the word: Mercury contracted as it cooled, and its crust buckled into cliffs hundreds of miles long. They cut across older craters, a sign they formed late.

Because their light reaches us as a single point, stars do what that the bright planets usually do not?

Twinkle is what stars do, as moving air bends their pinpoint light. Planets are close enough to show tiny discs, which smooths the shimmer out.

Scientists think the crushing pressure deep inside Uranus and Neptune creates a rain of what?

Diamonds may fall like hailstones through the mantles of both planets, and the heat released as they sink could help drive Neptune's magnetic field.

A gas confirmed in the cloud tops of Uranus in 2018 gives off the stink of which rotten food?

Eggs gone bad give off the same sulfur gas that astronomers confirmed in Uranus's cloud tops, settling a long debate. The cold and lack of air would kill a visitor long before the stink did.

Which gas, the second most plentiful in both Jupiter and Saturn, is named after the Greek word for the Sun?

Helium was first spotted as a mystery yellow line in sunlight during an 1868 eclipse, nearly three decades before chemists isolated it on Earth.

Which gas makes up about 95% of the thin air on Mars and freezes onto its poles each winter?

Carbon dioxide freezes into slabs of dry ice a metre or two thick over the winter pole, then turns straight back into gas when summer returns.

Cleopatra has a crater on Venus, where by international rule every crater is named after which group?

Women lend their names to every crater on Venus: some honour famous figures such as Jane Austen and Billie Holiday, others simply carry female first names.

Venus is dotted with broad, flat lava domes that geologists nicknamed after which breakfast food?

Pancakes fit the look: each dome is thought to come from one slow ooze of thick, sticky lava, and they run 10 to 100 times larger than lava domes on Earth.

Which novelist sent Martians in towering tripods marching on London in The War of the Worlds?

H. G. Wells ended the invasion with a twist: the Martians are killed not by armies but by earthly germs, to which they had no immunity.

Earth's daily spin makes it bulge, so the globe is about 43 km wider across which imaginary line?

The Equator is where the bulge peaks, so the summit farthest from Earth's centre is Chimborazo in Ecuador rather than Mount Everest.

Which planet, once thought to orbit closer to the Sun than Mercury, was hunted for decades but never existed?

Vulcan was proposed to explain a quirk in Mercury's orbit, which Einstein's general relativity later accounted for with no extra planet needed.

Saturn's main rings stretch hundreds of thousands of kilometres across, yet how thick are they in most places?

About 10 metres is typical for the main rings, which are made of billions of chunks ranging from dust-sized grains to pieces as big as a house.

Which English singer asked 'Is there life on Mars?' on his 1971 album Hunky Dory?

David Bowie conceived it as a parody of 'My Way', made famous by Frank Sinatra, after his own English lyrics for the same French tune were rejected.

In the film The Martian, which crop does stranded botanist Mark Watney grow in Martian soil to stay alive?

Potatoes were grown for real on a sound stage next to the set, while on screen Watney fertilises his crop with the crew's own waste.

Which scientist's 1687 book, the Principia, showed that gravity keeps the planets in their orbits?

Isaac Newton wrote it only after Edmond Halley came asking how the planets move, and Halley then paid for the printing out of his own pocket.

Which metal, alloyed with nickel, is thought to make up most of Earth's solid inner core?

Iron also dominates the liquid outer core, where swirling currents in the molten metal are the main source of Earth's magnetic field.

Earth takes about 365 and a quarter days to circle the Sun, a mismatch calendars fix by adding what?

A leap day lands on 29 February every fourth year, but century years skip it unless they divide by 400, so 1900 had none and 2000 did.

Neptune's astronomical symbol shows which weapon, carried by the sea god who shares the name?

A trident was put forward together with the name itself, by the mathematician whose sums had shown astronomers where to find the new planet.

The astronomical symbol for Mars, a circle with an arrow, is used in biology as the sign for what?

Male has been its meaning since the botanist Carl Linnaeus borrowed the planet signs in the 1750s, pairing it with the Venus symbol for female.

What nickname was given to the sunlit photograph of the whole Earth taken by Moon-bound astronauts in December 1972?

The Blue Marble became one of the most reproduced images in history, though the original frame has the South Pole at the top and was flipped for publication.

Astronomers long assumed planets moved in perfect circles, but their orbits actually trace which shape?

An ellipse emerged in 1609 from years of wrestling with Mars, whose measured positions refused to fit any circle.

Which small moon of Mars, companion to Phobos, takes its name from the Greek figure of dread?

Deimos and Phobos, Dread and Fear, are summoned by the war god Ares in the Iliad, the passage that inspired both names.

The closest known planets beyond the Solar System orbit which star, about 4.2 light-years away?

Proxima Centauri is the Sun's nearest stellar neighbour, yet it is too faint to be seen with the naked eye and went unnoticed until 1915.

Which Latin name, meaning 'light-bringer', was given to Venus when it shone before sunrise?

Lucifer was simply the morning star, often pictured as a figure carrying a torch, long before Christian tradition attached the name to Satan.

In Chinese and Japanese, the name for Mars means the star of which traditional element?

Fire is Mars's share of an ancient Chinese scheme of elements, which also makes Jupiter the 'wood star' across East Asia.

Thousands of asteroids share Jupiter's orbit in two swarms, known by what name borrowed from Greek legend?

Trojans in the swarm ahead of Jupiter are named after Greek heroes such as Achilles, while those trailing behind honour the defenders of Troy.

Which organisation's members voted in Prague in 2006 to adopt an official definition of a planet?

International Astronomical Union members passed it with only 424 astronomers present, out of a membership of about 9,000, a turnout critics seized on.

When Earth passes directly between the Sun and Mars, so Mars shines all night, astronomers say Mars is at what?

Opposition puts a planet on the far side of the sky from the Sun, rising around sunset and setting around sunrise, at roughly its closest and brightest.

The planets all keep close to one line across the sky, the Sun's apparent yearly path, called what?

The ecliptic got its name because the ancients noticed that eclipses happen only when the Moon is crossing it.

Teams driving rovers on Mars count time in Martian days, each one known by what short name?

A sol borrows the Latin word for the Sun, and many rover team members wore wristwatches calibrated to Martian time rather than Earth time.

What name is given to the small satellites whose gravity keeps the edges of Saturn's rings sharp?

Shepherd moons herd ring particles like a flock: tiny Pan keeps the Encke Gap swept clear, and Uranus has a pair of its own, Cordelia and Ophelia.

Which astronomer of ancient Alexandria set out the Earth-centred model of the planets used for over 1,500 years?

Ptolemy had each planet ride a small circle called an epicycle, a device that explained why the planets sometimes appear to reverse across the sky.

Which French mathematician predicted where Neptune would be found, using only wobbles in the orbit of Uranus?

Urbain Le Verrier posted his sums to Berlin, where an astronomer found the planet the very night the letter arrived, within one degree of the predicted spot.

Which Dutch scientist worked out in the 1650s that the odd bulges seen beside Saturn were really a ring?

Christiaan Huygens described 'a thin, flat ring, nowhere touching' the planet, and in the same decade he invented the pendulum clock.

Which Dane, who lost part of his nose in a duel, made the most accurate planet records before the telescope?

Tycho Brahe was 20 when a cousin's sword took the bridge of his nose in a duel fought in the dark, and he wore a false nose for the rest of his life.

Three years before any were found around a Sun-like star, the first confirmed exoplanets turned up circling what?

A pulsar, the fast-spinning remnant of a dead star, hosts them, and the three planets were later given the ghostly names Draugr, Poltergeist and Phobetor.

Uranium, neptunium and plutonium honour distant worlds; which element is named after Earth, via a Latin word?

Tellurium comes from 'tellus', Latin for earth, and when a similar element turned up in 1817 its discoverer named it selenium, after the Moon, to match.

Sunlight pushes atoms off Mercury to form a comet-like tail, made mostly of which element?

Sodium streams more than 24 million km behind Mercury, shoved along by sunlight whose pressure is far stronger there than it is near Earth.

What name do astronomers give to a straight-line arrangement of three bodies, such as the Sun, Earth and a planet?

Syzygy happens at every new and full moon, when Sun, Earth and Moon fall into line, which is why those phases bring the highest tides.

Venus's clouds bounce back about 70% of the sunlight that hits them, a reflectivity measure known as what?

Albedo comes from the Latin for whiteness, and that mirror-like cloud deck makes Venus bright enough to be regularly misreported as a UFO.

Which German astronomer, also known for a rule about planet spacing, proposed the name Uranus?

Johann Bode wanted a name from mythology to match the other planets, and his colleague Martin Klaproth backed him by naming a newly found element uranium.

Jupiter and the Sun circle a shared balance point just outside the Sun's surface, known by what name?

The barycentre of every other planet's pairing with the Sun sits inside the Sun itself; only Jupiter is massive enough to pull it above the surface.