UKPSC Prelims 2025 Q95: Which of the following planets takes the longest time to

Q: What is the answer to UKPSC Prelims 2025 Q95?

C. Correct — C, Neptune. Of the four planets on offer, **Neptune** orbits farthest from the Sun, at roughly 30 astronomical units. Kepler's third law ties distance to time: the square of a planet's orbital period is proportional to the cube of its mean orbital radius. A more distant planet therefore has both a longer path to travel and a lower average orbital speed. Neptune's year runs to about 165 Earth years, against about 84 for Uranus, 29.5 for Saturn and 11.9 for Jupiter. A useful anchor: Neptune was discovered in September 1846 and came back round to that discovery position only in July 2011 — one Neptunian year. Carry away that **a planet's year is set by its distance from the Sun, not by its size**.

UKPSC Prelims 2025 General Studies Paper-I · Question 95 of 149

Verified against the official question booklet. Answer as per UKPSC's provisional answer key.

Answers from UKPSC's provisional answer key. The Commission publishes a provisional key for UKPSC Prelims 2025 and invites objections before issuing a final key. Every answer shown here is the Commission's own provisional answer (key PDF), not our derivation. If the final key changes an answer, this page is updated to match.

Which of the following planets takes the longest time to complete one revolution around the Sun?

(a)Saturn
(b)Uranus
(c)Neptune
(d)Jupiter

Answer

Why

Correct — C, Neptune.

Of the four planets on offer, Neptune orbits farthest from the Sun, at roughly 30 astronomical units. Kepler's third law ties distance to time: the square of a planet's orbital period is proportional to the cube of its mean orbital radius.

A more distant planet therefore has both a longer path to travel and a lower average orbital speed. Neptune's year runs to about 165 Earth years, against about 84 for Uranus, 29.5 for Saturn and 11.9 for Jupiter.

A useful anchor: Neptune was discovered in September 1846 and came back round to that discovery position only in July 2011 — one Neptunian year.

Carry away that a planet's year is set by its distance from the Sun, not by its size.

Why the others are wrong

(a)
Saturn — Saturn's revolution takes about 29.5 Earth years from a mean distance near 9.6 AU — long, but Neptune's year is more than five times longer from about three times the distance.
Saturn is the right answer to which planet visible to the unaided eye takes the longest to circle the Sun: it is the outermost of the planets that ancient observers could track without a telescope.
(b)
Uranus — Uranus needs about 84 Earth years for one revolution, orbiting near 19 AU. That is the second-longest year among these four, but Neptune lies farther out and takes roughly twice as long again.
Uranus is the right answer to which planet was first identified with a telescope — William Herschel picked it out in 1781, pushing the known Solar System past Saturn.
(d)
Jupiter — Jupiter completes a revolution in about 11.9 Earth years, the shortest of the four options, because it is the closest of them to the Sun at roughly 5.2 AU.
Jupiter is the right answer to which is the largest planet in the Solar System. Size is the association that makes it look plausible here, but a planet's own diameter does not enter the relation that fixes the orbital period — distance does.

Concept

Kepler's third law states that for bodies going round the Sun, the square of the orbital period is proportional to the cube of the mean orbital radius. Doubling the distance more than doubles the length of the year.

The physical reason is twofold. Gravity weakens with distance, so a far planet holds its orbit at a lower speed, and it also has a far longer circuit to complete. Both effects lengthen the period together.

In years and astronomical units the law reduces to a clean working form: period equals distance raised to the power three-halves.

This sits inside the basic architecture of the Solar System — the eight planets ordered outward as Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune, with the inner four rocky and the outer four giant.

Once that ladder is fixed in memory, a whole set of questions collapses into reading it in one direction or the other: longest year, shortest year, fastest orbital speed, nearest to or farthest from the Sun.

Key facts

Neptune's orbital period is about 165 Earth years, the longest among the eight planets.
Uranus takes about 84 Earth years, Saturn about 29.5 and Jupiter about 11.9 to complete one revolution.
Mean distances from the Sun: Jupiter about 5.2 AU, Saturn about 9.6 AU, Uranus about 19.2 AU, Neptune about 30 AU.
Kepler's third law: the square of the orbital period varies as the cube of the mean orbital radius.
Neptune was discovered in September 1846 and completed its first full orbit since discovery in July 2011.
Mercury, nearest the Sun, has the shortest year at about 88 Earth days and the highest orbital speed of the eight planets.
Revolution is the circuit round the Sun; rotation is the spin about the planet's own axis, and Jupiter's rotation takes under 10 hours.

Mean distance from the Sun against length of one revolution

Jupiter — about 5.2 AU→about 11.9 Earth years

Saturn — about 9.6 AU→about 29.5 Earth years

Uranus — about 19.2 AU→about 84 Earth years

Neptune — about 30 AU→about 165 Earth years

The four options, ordered outward from the Sun; the farthest of them carries the longest year.

Study next

Kepler's three laws of planetary motion and the working form T² = a³ in years and AUOrder and mean distances of the eight planets, in AUTerrestrial planets against gas giants and ice giantsRotation period versus revolution period for each planetOrbital speed and why it falls with distance from the SunDwarf planets, Pluto's reclassification, and the eccentric orbit that brings it inside Neptune's

Common traps

Reading 'revolution' as the daily spin. Revolution is the circuit round the Sun; rotation is the spin on the axis, and the two rank the planets differently.
Ranking the options by size. Jupiter is the largest of the four yet has the shortest year of them, because it is the nearest of them to the Sun.
Swapping Uranus and Neptune. Neptune is the outer of the two, so the longer year belongs to Neptune.
Carrying an answer across option sets. With Earth, Mars, Venus and Jupiter on the menu, Jupiter has the longest year; add Neptune and it no longer does.
Treating the distance-to-period link as linear. Neptune is about six times as far out as Jupiter but its year is close to fourteen times as long.

The idea turns up as a plain ranking — longest or shortest year, fastest or slowest orbital speed — where the only work is placing the named planets in order of distance from the Sun.

It also comes in numerical dress: a distance in astronomical units with the period to be found, or a ratio of two orbital radii with the ratio of periods to be worked out.

A third form is the assertion-reason pair, which offers a planet's diameter, mass or number of moons as the cause of a longer or shorter year and asks whether that reasoning holds.

Related PYQs

UPSC_2003_GS1_Q1262003

The same question with a different menu — which planet takes the maximum time for one revolution. Identical reasoning: pick the option orbiting farthest from the Sun. The option set differs, since Earth, Mars and Venus all lie inside Jupiter's orbit, so Jupiter carries the longest year there while Neptune outranks it here.

UPSC_2006_GS1_Q182006

Comes at the same confusion from the other side, offering a planet's smaller diameter as the reason for a shorter year — while Mars takes about 687 days against Earth's 365. Same principle that distance rather than size fixes the period; the format differs, an assertion-reason pair rather than a ranking.

UPSC_2008_GS1_Q822008

Tests the ladder that this question rests on — which planets lie between Mars and Uranus in order of distance, namely Jupiter and Saturn. The ordering knowledge is shared; that item stops at the order, while UKPSC asks what the order implies for the length of the year.

NDA_GAT_2020_I_II_Q1282020

The same law stated quantitatively: two orbits of radius R and 4R, with periods scaling as the three-halves power, giving a ratio of 1 to 8. UKPSC asks only for the direction of the effect, so recognising that farther means slower suffices; the NDA item needs the exponent itself.

Practice

practice — not a real PYQ
Which of the following planets completes one revolution around the Sun in the shortest time?
- (a)Mercury
- (b)Venus
- (c)Earth
- (d)Mars
Answera — Mercury is the closest planet to the Sun, so by Kepler's third law it has the shortest orbital period, about 88 Earth days.
(b) Venus takes about 225 days and (c) Earth about 365 days, both longer because both orbit farther out. (d) Mars, farther still, takes about 687 days, nearly two Earth years.
practice — not a real PYQ
A body revolves around the Sun at a mean distance of 9 astronomical units. Its orbital period is closest to:
- (a)9 years
- (b)18 years
- (c)27 years
- (d)81 years
Answerc — In years and astronomical units, Kepler's third law reads T² = a³, so T = 9 raised to the power three-halves, which is 27 years.
(a) 9 years treats the period as rising in step with distance, the linear guess. (b) 18 years simply doubles that guess. (d) 81 years comes from squaring the distance rather than cubing it and then taking the square root.
practice — not a real PYQ
Which one of the following arranges the planets correctly in order of increasing distance from the Sun?
- (a)Jupiter, Saturn, Neptune, Uranus
- (b)Saturn, Jupiter, Uranus, Neptune
- (c)Jupiter, Saturn, Uranus, Neptune
- (d)Uranus, Neptune, Jupiter, Saturn
Answerc — Outward from Mars the sequence runs Jupiter (about 5.2 AU), Saturn (about 9.6 AU), Uranus (about 19.2 AU) and Neptune (about 30 AU).
(a) puts Neptune inside Uranus, reversing the outermost pair. (b) starts with Saturn ahead of Jupiter. (d) places the two ice giants inside the two gas giants, inverting the whole set.
practice — not a real PYQ
A planet farther from the Sun takes longer to complete one revolution mainly because
- (a)it is larger in diameter
- (b)it covers a longer orbital path at a lower average orbital speed
- (c)it is attended by a greater number of moons
- (d)it spins more slowly about its own axis
Answerb — Distance lengthens the circuit and, because the Sun's pull weakens with distance, the planet holds that orbit at a lower speed. A longer path at a slower pace makes the year longer.
(a) fails because diameter does not enter the period relation — Jupiter is the largest planet and still orbits the Sun faster than Saturn does.
(c) fails because the number of moons does not follow distance from the Sun. (d) fails because axial rotation is a separate quantity from the orbital circuit.