- What counts as a planet?
- Three tests, set by the IAU in 2006: it orbits the Sun, its own gravity has pulled it round, and it has cleared other bodies out of its orbital neighbourhood. Pluto passes the first two and fails the third.
- Why are there two kinds?
- Distance from the young Sun. Close in it was too warm for ice, so only rock and metal were available and the planets stayed small; beyond the line where water ice could survive there was far more material, and four cores grew heavy enough to hold on to hydrogen and helium.
- How far apart are they really?
- Much further than any diagram shows. If the Earth sat one metre from the Sun, Neptune would be thirty metres away, the Sun would be a bead nine millimetres across, and the Earth would be smaller than a grain of sand.
- How were the outer ones found?
- Uranus by accident in 1781, at the eyepiece. Neptune by arithmetic in 1846: Uranus was not where Newton's laws said it should be, two astronomers independently worked out where an unseen eighth planet would have to be to explain the discrepancy, and it was found within a degree of the prediction.
- What is a day and a year on another planet?
- A year is one trip around the Sun and a day is one turn on the axis, and the two have nothing to do with each other. Venus turns once every 243 Earth days, backwards, and completes an orbit in 225 — so its day is longer than its year.
- Why is Pluto not a planet?
- It meets two of the three conditions the IAU set in 2006 — it orbits the Sun and it is round — and fails the third, because it shares its region with a great many similar bodies in the Kuiper Belt. The decision followed the discovery of comparable objects out there: either Pluto was reclassified, or the count of planets became open-ended. It is a dwarf planet, which is a category rather than a demotion.
- Which planet is hottest?
- Venus, at about 464 °C at the surface, though Mercury is closer to the Sun. A carbon dioxide atmosphere ninety times the pressure of Earth's traps heat so effectively that Venus is hotter day and night than Mercury ever gets, and hot enough to melt lead.
- Can you see planets without a telescope?
- Five of them, easily: Mercury, Venus, Mars, Jupiter and Saturn have been known since antiquity for exactly that reason. They look like bright stars that do not twinkle and that move against the fixed pattern from night to night — which is what "planet" originally meant, a wanderer. Uranus is borderline under perfect conditions, and Neptune needs optics.
- Is there a Planet Nine?
- It is a hypothesis, not a discovery. Several distant Kuiper Belt objects have orbits that cluster in a way that would be explained by an unseen body of several Earth masses far beyond Neptune. Surveys have not found it, and the clustering may also be a selection effect in how those objects were discovered.