The Solar System

The planets in order, their stranger moons, and what lies past Neptune.

Start with the order of the planets and build out from there: why Venus is hotter than Mercury, which moons might hide oceans, what the Kuiper Belt is, and how long sunlight takes to reach us. Cards mix typed answers, multiple choice and true-or-false, with a short explanation after each answer so the fact has something to hang on.

All 49 cards

  1. Which planet is first from the Sun?

    Mercury

  2. Which planet is second from the Sun?

    Venus

  3. Which planet is third from the Sun?

    Earth

  4. Which planet is fourth from the Sun?

    Mars

  5. Which planet is fifth from the Sun?

    Jupiter

  6. Which planet is sixth from the Sun?

    Saturn

  7. Which planet is seventh from the Sun?

    Uranus

  8. Which planet is eighth from the Sun?

    Neptune

  9. What is the largest planet in the Solar System?

    Jupiter

    More than twice the mass of all the other planets put together.

  10. Which planet is the hottest?

    Venus

    Its thick carbon dioxide atmosphere traps heat: the surface is about 465 °C, hotter than Mercury, which is closer to the Sun.

  11. Which planet is called the Red Planet?

    Mars

    Iron oxide (rust) in its dust gives it the colour.

  12. Which planet has the most known moons?

    Saturn

    Saturn passed 270 confirmed moons in 2025, far ahead of Jupiter.

  13. Which planet spins on its side, with its axis tilted about 98°?

    Uranus

    Probably knocked over by a giant collision long ago. Each pole gets about 42 years of sunlight, then 42 of darkness.

  14. Which planet has the Great Red Spot, a storm wider than Earth?

    Jupiter

    It has been watched for at least 190 years, and it is slowly shrinking.

  15. Which planet has the fastest winds in the Solar System?

    Neptune

    Up to about 2,000 km/h (1,200 mph).

  16. Which planet has recorded the coldest temperatures?

    Uranus

    Down to about −224 °C, colder than Neptune even though Neptune is farther out: Uranus gives off very little internal heat.

  17. True or false: A day on Venus is longer than its year.

    True

    Venus takes about 243 Earth days to turn once, but only about 225 to orbit the Sun. It also spins backwards compared with most planets.

  18. True or false: Saturn is less dense than water.

    True

    Its average density is about 0.69 g/cm³. In a big enough bathtub, it would float.

  19. True or false: Pluto is officially classed as a planet.

    False

    The International Astronomical Union reclassified it as a dwarf planet in 2006, because it has not cleared its orbit of other objects.

  20. True or false: Jupiter was only a little too small to become a star.

    False

    It would need roughly 80 times its mass to fuse hydrogen.

  21. How many planets in the Solar System have rings?

    4

    Jupiter, Saturn, Uranus and Neptune. Saturn’s are just the brightest by far.

  22. What is the largest moon in the Solar System?

    Ganymede

    Jupiter’s moon Ganymede is bigger than the planet Mercury.

  23. Which moon has a thick atmosphere and lakes of liquid methane?

    Titan

    Saturn’s largest moon. NASA’s Dragonfly drone is due to arrive there in the 2030s.

  24. What is the most volcanically active body in the Solar System?

    Io

    Jupiter’s moon Io has hundreds of active volcanoes, heated by tides as Jupiter squeezes it.

  25. Europa, thought to hide a salty ocean under its ice, is a moon of which planet?

    Jupiter

  26. What does Saturn’s moon Enceladus spray from geysers at its south pole?

    Water ice and vapour

    The plumes come from an ocean under the ice, and they feed one of Saturn’s rings.

  27. What is the tallest known volcano in the Solar System?

    Olympus Mons

    On Mars, about 22 km (13.6 miles) high: roughly two and a half times Everest.

  28. What lies between the orbits of Mars and Jupiter?

    The asteroid belt(also Asteroid belt, Main belt, Asteroids)

  29. What is the largest object in the asteroid belt?

    Ceres

    A dwarf planet about 940 km across, holding roughly a third of the belt’s mass.

  30. What is the region of icy bodies beyond Neptune, Pluto included, called?

    The Kuiper Belt(also Kuiper belt)

  31. Which dwarf planet, found in 2005, is about Pluto’s size but more massive?

    Eris

    Its discovery pushed astronomers to define "planet", which is how Pluto lost the title.

  32. What is the Oort Cloud?

    A vast, distant shell of icy bodies thought to surround the Solar System.

    It may reach a light-year or more from the Sun, and is where long-period comets are thought to come from.

  33. The Sun holds about 99.8% of all the mass in the Solar System.

  34. How long does sunlight take to reach Earth?

    About 8 minutes 20 seconds.

    So you always see the Sun as it was a little over eight minutes ago.

  35. What is an astronomical unit (AU)?

    The average distance from Earth to the Sun: about 150 million km (93 million miles).

  36. What causes Earth’s seasons?

    The tilt of Earth’s axis, about 23.4°.

    Not our distance from the Sun. The hemisphere tilted toward the Sun gets more direct light and longer days.

  37. True or false: Earth is closest to the Sun in early January.

    True

    Perihelion is around January 3, in the middle of the northern winter, which shows distance is not what drives the seasons.

  38. What is the point in an orbit closest to the Sun called?

    Perihelion

    The farthest point is the aphelion.

  39. Why does the Moon always show Earth the same face?

    It is tidally locked: it turns exactly once each time it orbits Earth.

  40. True or false: The far side of the Moon is always dark.

    False

    It gets as much sunlight as the near side. It is only "dark" in the sense of unseen from Earth.

  41. The Moon orbits Earth about every 27.3 days, but its phases repeat about every 29.5 days.

    The difference: Earth moves along its own orbit meanwhile, so the Moon has to go a bit further to line up with the Sun again.

  42. Who was the first person to walk on the Moon?

    Neil Armstrong(also Armstrong)

    On July 20, 1969, with Buzz Aldrin, during Apollo 11.

  43. In what year did Apollo 11 land on the Moon?

    1969

  44. What happens in a solar eclipse?

    The Moon passes between the Sun and Earth and blocks the Sun’s light.

  45. What happens in a lunar eclipse?

    Earth passes between the Sun and the Moon, and Earth’s shadow falls on the Moon.

    Sunlight bent through Earth’s atmosphere often turns the Moon a coppery red.

  46. Which spacecraft is the farthest human-made object from Earth?

    Voyager 1

    Launched in 1977, it left the Sun’s bubble of solar wind in 2012 and is still sending data.

  47. Which spacecraft flew past Pluto in 2015?

    New Horizons

  48. With almost no atmosphere, Mercury swings from about 430 °C by day to about −180 °C at night.

  49. About how many Earth years does Neptune take to orbit the Sun?

    165

    It completed its first full orbit since its 1846 discovery in 2011.