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Dwarf planets, comets and missions

Dwarf planets, comets and missions: a chapter of The Solar System Book, with planets you watch move on their orbits and the facts behind them.

12 cards · Science

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Each Zap teaches up to 5 new cards, so you learn the chapter a few at a time.

Checked by the Brain Zap team · reviewed 27 Oct 2026

Sources: Solar System (NASA Science), Dwarf planets (IAU)

Preview

  • Largest object in the asteroid belt, a dwarf planet

    Show answer: Largest object in the asteroid belt, a dwarf planet

    Ceres

    Learn more: Found in 1801 and first called a planet, it holds about a third of the asteroid belt's mass.

  • Dwarf planet whose discovery got Pluto demoted

    Show answer: Dwarf planet whose discovery got Pluto demoted

    Eris

    Learn more: Found in 2005 and about as big as Pluto, it made astronomers decide what a planet is.

  • Dwarf planets the IAU officially recognises

    Show answer: Dwarf planets the IAU officially recognises

    5

    Learn more: Ceres, Pluto, Eris, Haumea and Makemake; many more probably wait in the outer Solar System.

  • What a comet's nucleus is mostly made of

    Show answer: What a comet's nucleus is mostly made of

    Ice and dust

    Learn more: Near the Sun its ice turns to gas, making a glowing coma and tails millions of kilometres long.

  • Way a comet's tail always points

    Show answer: Way a comet's tail always points

    Away from the Sun

    Learn more: Sunlight and the solar wind push gas and dust away, so a comet can even travel tail first.

About this topic

Beyond the eight planets, the Solar System is full of smaller worlds. Ceres rules the asteroid belt, Eris forced astronomers to decide what a planet is, and comets, balls of ice and dust, grow tails when they near the Sun. Robots have now visited most of them: Rosetta landed on a comet, DART knocked an asteroid off course, and missions such as Cassini, Juno, Perseverance and BepiColombo keep studying the planets you can watch glowing on their orbits.

This is a chapter of The Solar System Book. Each card asks one question, with a hint before you answer and a short reason after; on some, the planet glows on its orbit.

All 12 cards

  1. Largest object in the asteroid belt, a dwarf planet

    Ceres

    Learn more: Found in 1801 and first called a planet, it holds about a third of the asteroid belt's mass.

  2. Dwarf planet whose discovery got Pluto demoted

    Eris

    Learn more: Found in 2005 and about as big as Pluto, it made astronomers decide what a planet is.

  3. Dwarf planets the IAU officially recognises

    5

    Learn more: Ceres, Pluto, Eris, Haumea and Makemake; many more probably wait in the outer Solar System.

  4. What a comet's nucleus is mostly made of

    Ice and dust

    Learn more: Near the Sun its ice turns to gas, making a glowing coma and tails millions of kilometres long.

  5. Way a comet's tail always points

    Away from the Sun

    Learn more: Sunlight and the solar wind push gas and dust away, so a comet can even travel tail first.

  6. Streaks of light from a comet's leftover dust

    A meteor shower

    Learn more: When Earth crosses a comet's dusty trail, the grains burn up high in the air.

  7. First mission to land a probe on a comet, in 2014

    Rosetta

    Learn more: ESA's Rosetta followed comet 67P for two years; Philae bounced twice before it settled.

  8. NASA mission that crashed into an asteroid in 2022

    DART

    Learn more: It hit the small moonlet Dimorphos and shortened its orbit, proving we can nudge an asteroid.

  9. Glowing on the orbit: the planet Cassini studied

    Saturn

    Learn more: Cassini spent 13 years there and found geysers of water on its moon Enceladus.

  10. Glowing on the orbit: the planet Juno orbits

    Jupiter

    Learn more: Since 2016 Juno has swooped over its poles, mapping its storms and deep interior.

  11. Glowing on the orbit: where Perseverance roves

    Mars

    Learn more: It collects rock samples that may one day be brought back to Earth, and carried the helicopter Ingenuity.

  12. Glowing on the orbit: BepiColombo's target

    Mercury

    Learn more: A mission of Europe and Japan, launched in 2018, slowed by years of flybys to reach it.

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