Astronomy · Rung 2 of 12

The Solar System

Eight planets and two belts around an ordinary star. Four are small and rocky (Mercury, Venus, Earth, Mars) and four are giants: two of gas (Jupiter, Saturn) and two of ice (Uranus, Neptune), which is not the same thing, because under the atmosphere of the ice giants there is water, methane and ammonia at brutal pressures. One rule explains almost everything: the closer to the Sun, the harder gravity pulls, the faster you must orbit and the shorter the year. Mercury takes 88 days; Neptune, 165 years.

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Exercises from this rung

Generated by the app, from easiest to hardest

  1. This orbit takes 11.86 years to complete.

    • Mercury
    • Jupiter
    • Uranus
    • Venus

    Why Jupiter is 5.2 AU from the Sun and its year lasts 11.86 years. It weighs two and a half times more than the other seven planets combined, and its day lasts under ten hours.

    easier

  2. This planet goes around the Sun in 88 days.

    • Venus
    • Jupiter
    • Mars
    • Mercury

    Why Mercury. The closer to the Sun, the harder gravity pulls and the faster you must travel not to fall in: hence the shortest year. The odd part is its day, 59 Earth days, exactly two thirds of its year.

    in between

  3. Venus is further from the Sun than Mercury and yet far hotter. Why?

    • It is closer than it looks
    • It spins very slowly and heats up
    • It has constant volcanic activity
    • It has a CO₂ atmosphere that traps the heat

    Why A runaway greenhouse effect. Its atmosphere is 96 % carbon dioxide and ninety times denser than ours: it lets light in and will not let infrared out. The surface sits at 464 °C, enough to melt lead, while on Mercury, with no atmosphere, the night side drops to −170 °C.

    harder