CDS (II) 2020 General Knowledge · Question 63 of 120

Correct — A, terrestrial planets. Mercury, Venus, Earth and Mars are the four inner planets, and they are grouped together because they are alike in the ways that matter: small, dense, rocky, with solid surfaces you could stand on, thin atmospheres and few or no moons. 'Terrestrial' simply means earth-like. They sit inside the asteroid belt because the young Sun's heat drove the light gases outward, leaving only metals and silicates able to condense close in. Beyond the belt the same process left Jupiter, Saturn, Uranus and Neptune — the Jovian or giant planets, far larger, far less dense, with deep gaseous envelopes, ring systems and dozens of moons apiece.

• (b)giant planets — The giant planets are the four OUTER ones, Jupiter through Neptune. Jupiter alone is more than 1,300 times the volume of the Earth, which is exactly why the inner four are not described this way.
• (c)dwarf planets — A dwarf planet is a body that orbits the Sun and is round but has not cleared its orbital neighbourhood — Pluto, Ceres, Eris, Haumea and Makemake. None of the four inner planets qualifies; all of them are full planets.
• (d)gas planets — Gas planets means Jupiter and Saturn, with Uranus and Neptune increasingly called ice giants. The inner four have solid rocky surfaces and only thin atmospheric skins.

The eight planets split into two families. The four inner or terrestrial planets — Mercury, Venus, Earth and Mars — are rocky, dense and small, with average densities around 4-5.5 grams per cubic centimetre. The four outer or Jovian planets — Jupiter, Saturn, Uranus and Neptune — are much larger and far less dense; Saturn's mean density is lower than that of water. The dividing line is the asteroid belt between Mars and Jupiter. Pluto was reclassified as a dwarf planet by the International Astronomical Union in 2006.

The word 'terrestrial' is doing all the work in this question. Once you read it as 'earth-like' rather than as a technical term, the answer follows from the ordering of the planets, which almost every candidate already knows. The three wrong options are all real astronomical categories, which is why the item is worth more than it looks — 'giant', 'dwarf' and 'gas' each describe a genuine class of body, just not this one. Earth is the densest planet in the solar system, at about 5.52 grams per cubic centimetre, and that single fact captures why the inner four are grouped apart.

• The terrestrial planets are Mercury, Venus, Earth and Mars — rocky, dense, small, with thin atmospheres and few moons.
• The Jovian or giant planets are Jupiter, Saturn, Uranus and Neptune, separated from the inner four by the asteroid belt.
• Earth has the highest mean density of any planet, about 5.52 g per cubic centimetre.
• The Sun holds roughly 99.8 per cent of the mass of the solar system, and its diameter is about 109 times that of the Earth.
• Pluto was reclassified as a dwarf planet in 2006 because it has not cleared its orbital neighbourhood.

• Reading 'terrestrial' as meaning Earth alone rather than all four earth-like planets.
• Calling all four outer planets gas giants — Uranus and Neptune are better described as ice giants.
• Still counting Pluto among the planets, or assuming a dwarf planet is simply a very small planet.

As a definition item on the two planetary families, or as a statements set mixing facts about individual planets.

• practice — not a real PYQ

  Which one of the following is not a terrestrial planet?

  • (a)Mercury
  • (b)Venus
  • (c)Mars
  • (d)Neptune

  Answer(d) Neptune — it is one of the four outer giant planets; the terrestrial group is Mercury, Venus, Earth and Mars.

• practice — not a real PYQ

  The asteroid belt of the solar system lies between the orbits of which two planets?

  • (a)Earth and Mars
  • (b)Mars and Jupiter
  • (c)Jupiter and Saturn
  • (d)Saturn and Uranus

  Answer(b) Mars and Jupiter — the belt marks the boundary between the terrestrial and the Jovian planets.