Planetary Geography is one of the most scoring areas of the NDA Geography section because the facts are fixed and rarely change. If you remember the order of the planets, a few record-holders, and how eclipses work, you can lock in easy marks. This page builds the full picture of the Solar System—the Sun, the eight planets, the Moon, and small bodies—in plain language you can revise fast.
Why Planetary Geography matters for NDA
The NDA General Ability Test mixes Physical Geography with Astronomy, and Planetary Geography appears almost every year. Questions are usually direct and fact-based—for example, naming the hottest planet, the planet with the most moons, or the cause of a solar eclipse. Unlike map-based questions, these have one clear answer, so a little memorisation pays off heavily.
Astronomy facts almost never change between editions of NCERT, so the time you invest here protects marks year after year. Treat this chapter as a quick, reliable scorer.
In this page we move outward from the centre—the Sun first, then the planets in order, then the Moon and the small bodies, and finally eclipses. Studying it in this spatial order makes the facts stick.
What is the Solar System?
The Solar System is the Sun together with everything bound to it by gravity: eight planets, their moons (natural satellites), dwarf planets, asteroids, comets, meteoroids, and clouds of dust and gas. It formed about 4.6 billion years ago from a giant rotating cloud of gas and dust called the solar nebula.
The two families of planets
- Inner (terrestrial) planets: Mercury, Venus, Earth, Mars. They are small, rocky, dense and have few or no moons.
- Outer (Jovian / gas-giant) planets: Jupiter, Saturn, Uranus, Neptune. They are huge, made mostly of gases, have rings and many moons.
Order of planets from the Sun: Mercury → Venus → Earth → Mars → Jupiter → Saturn → Uranus → Neptune. A common memory line is “My Very Educated Mother Just Served Us Noodles.”
Between Mars and Jupiter lies the asteroid belt, which neatly separates the four inner rocky planets from the four outer giants.
Planets do not move in perfect circles; they travel in slightly stretched paths called elliptical orbits, a fact first explained by the astronomer Johannes Kepler. All eight planets revolve around the Sun in the same direction (anticlockwise when viewed from above the North Pole) and lie roughly in the same flat plane, which is why we see them strung along a single band in the sky. Each planet also spins on its own axis, producing its day, while its journey around the Sun produces its year.
The Sun: heart of the system
The Sun is a medium-sized star and the centre of our system. It contains about 99.8% of the total mass of the Solar System, which is why its gravity holds everything in orbit. It is made mostly of hydrogen and helium.
Energy is produced in the Sun’s core by nuclear fusion, where hydrogen nuclei fuse to form helium, releasing enormous heat and light. The Sun’s visible surface is the photosphere; above it lie the chromosphere and the outermost corona, which is seen as a glowing ring during a total solar eclipse.
- Average distance Sun to Earth ≈ 149.6 million km (1 Astronomical Unit, AU).
- Light from the Sun takes about 8 minutes 20 seconds to reach Earth.
- Surface temperature ≈ 5500°C; core temperature is millions of degrees.
The inner (terrestrial) planets
Mercury
The smallest planet and the closest to the Sun. It has almost no atmosphere, so it swings between scorching days and freezing nights. It also has the shortest year (about 88 Earth days to orbit the Sun).
Venus
The hottest planet because of a thick carbon-dioxide atmosphere that traps heat (a runaway greenhouse effect). It is the brightest planet in our sky and is often called the Morning Star or Evening Star. Venus is also Earth’s nearest planetary neighbour and rotates from east to west (retrograde).
Earth
The only planet known to support life, thanks to liquid water, a protective atmosphere and the right temperature. It is sometimes called the Blue Planet.
Mars
The Red Planet, coloured by iron oxide (rust) in its soil. It hosts Olympus Mons, the tallest volcano in the Solar System, and has two small moons, Phobos and Deimos.
Mercury = smallest & nearest; Venus = hottest & brightest; Earth = only life; Mars = red, tallest volcano. These four one-line tags answer most inner-planet questions.
The outer (gas-giant) planets
Jupiter
The largest planet. Its famous Great Red Spot is a giant storm bigger than Earth. Jupiter spins fastest, giving it the shortest day of any planet (about 10 hours).
Saturn
Famous for its bright, broad rings made of ice and rock. Saturn has the lowest density of all planets—less than water—so it would float if placed in a giant ocean.
Uranus
An ice giant that is tilted on its side (its axis is almost in the plane of its orbit), so it appears to roll along its path. It looks blue-green due to methane.
Neptune
The farthest planet from the Sun and the windiest, with the fastest winds in the Solar System. Like Uranus, its blue colour comes from methane.
Mnemonic links: Jupiter = biggest, Saturn = rings/least dense, Uranus = sideways tilt, Neptune = farthest/windiest. NDA loves “which planet is the…” questions, so memorise these superlatives.
The Moon: Earth's natural satellite
The Moon is Earth’s only natural satellite. It has no atmosphere and no water in liquid form, so its surface is covered with craters formed by meteorite impacts. Because there is no air to scatter sunlight, its sky is black even by day.
The Moon takes about 27.3 days to orbit the Earth and roughly the same time to spin once on its axis. This is why the same side of the Moon always faces Earth; the far side is never seen from the ground.
- Average Earth–Moon distance ≈ 3,84,400 km.
- Moonlight takes about 1.3 seconds to reach Earth.
- Gravity on the Moon is about 1/6th of Earth’s.
The Moon’s gravity, with the Sun’s, is the main cause of ocean tides on Earth—a favourite link-up question in the exam. When the Sun, Earth and Moon line up at new moon and full moon, their combined pull produces the highest spring tides; when they are at right angles, we get the weaker neap tides.
The changing shapes of the Moon we see through the month—new moon, crescent, half, gibbous and full—are called phases. They occur because we see different amounts of the Moon’s sunlit half as it orbits Earth; the Moon does not make its own light but only reflects sunlight.
Asteroids, comets, meteors and dwarf planets
Asteroids
Small rocky bodies, mostly found in the asteroid belt between Mars and Jupiter. They are sometimes called minor planets.
Comets
Bodies made of ice, dust and rock. As a comet nears the Sun it heats up and forms a glowing tail that always points away from the Sun because of solar wind and radiation pressure. Halley’s Comet returns roughly every 76 years.
Meteoroids, meteors and meteorites
- Meteoroid: a small rock travelling in space.
- Meteor (shooting star): a meteoroid burning up in Earth’s atmosphere.
- Meteorite: a piece that survives and reaches the ground.
Dwarf planets
Pluto was reclassified as a dwarf planet in 2006 because it does not clear its orbital path. Other dwarf planets include Ceres, Eris, Haumea and Makemake.
Pluto is no longer counted among the eight planets. If a question asks for the number of planets, the answer is eight, not nine.
Solar and lunar eclipses
An eclipse happens when one body’s shadow falls on another. The two important ones for NDA are the solar and lunar eclipse.
Solar eclipse
Occurs on a new moon day when the Moon comes between the Sun and the Earth, blocking the Sun’s light. The order is Sun → Moon → Earth. It can be total, partial or annular.
Lunar eclipse
Occurs on a full moon day when the Earth comes between the Sun and the Moon, so Earth’s shadow falls on the Moon. The order is Sun → Earth → Moon.
Easy memory hook: in a Solar eclipse the Son (Moon) blocks the parent (Sun). Eclipses do not happen every month because the Moon’s orbit is tilted about 5° to Earth’s orbit.
High-yield record facts to memorise
NDA examiners love superlatives. Lock these in and you can answer most one-line questions instantly.
- Largest planet: Jupiter · Smallest planet: Mercury
- Hottest planet: Venus · Coldest / farthest: Neptune
- Brightest planet: Venus · Brightest star in night sky: Sirius
- Densest planet: Earth · Least dense planet: Saturn
- Fastest spin / shortest day: Jupiter · Shortest year: Mercury
- Most moons / largest moon (Ganymede): Jupiter system
- Planet tilted on its side: Uranus
Earth is the densest planet, while Saturn is the least dense. Students often mix these up—keep them as a contrasting pair.
Worked example: planet identification
Let’s see how a typical NDA reasoning question is solved by elimination.
A planet is the second from the Sun, is the hottest in the Solar System, and is the brightest object in our night sky after the Moon. Identify it.
Notice that each clue independently points to Venus. In the exam, you only need one reliable clue you are sure about to lock the answer.
Previous-year style practice
Try this question the way it would appear in the NDA paper, then check the worked answer.
Q. A solar eclipse occurs when which of the following alignments takes place?
Answer: A solar eclipse occurs when the Moon comes between the Sun and the Earth on a new moon day, i.e. the order is Sun → Moon → Earth. The Moon’s shadow then falls on Earth, blocking sunlight. (Do not confuse this with a lunar eclipse, where the Earth lies between the Sun and the Moon on a full moon day.)
Many students reverse the order. Anchor it like this: Solar = Moon in the middle; Lunar = Earth in the middle.
Quick revision
- Solar System = Sun + 8 planets + moons + asteroids, comets and dust; formed ~4.6 billion years ago.
- Planet order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
- Inner = small, rocky; Outer = large gas giants with rings and many moons; asteroid belt sits between Mars and Jupiter.
- Sun holds 99.8% of system mass; light reaches Earth in ~8 min 20 s.
- Venus = hottest & brightest; Jupiter = largest; Saturn = least dense; Neptune = farthest; Uranus = sideways tilt.
- Moon: no air/water, same face to Earth, drives tides; gravity 1/6th of Earth’s.
- Solar eclipse = Sun–Moon–Earth (new moon); Lunar eclipse = Sun–Earth–Moon (full moon).
- Pluto is a dwarf planet, so there are eight planets, not nine.
Frequently asked questions
How many planets are there in the Solar System and why is Pluto excluded?
There are eight planets. Pluto was reclassified as a dwarf planet in 2006 because, although it orbits the Sun and is round, it has not cleared other objects from its orbital path.
Which is the hottest planet, and why is it not Mercury?
Venus is the hottest planet even though Mercury is closer to the Sun. Venus has a thick carbon-dioxide atmosphere that traps heat through a runaway greenhouse effect, while Mercury has almost no atmosphere to hold warmth.
What is the difference between a solar and a lunar eclipse?
In a solar eclipse the Moon comes between the Sun and the Earth on a new moon day. In a lunar eclipse the Earth comes between the Sun and the Moon on a full moon day, casting its shadow on the Moon.
Why does the same side of the Moon always face the Earth?
The Moon takes about the same time (around 27.3 days) to rotate once on its axis as it does to orbit the Earth. This synchronised motion, called tidal locking, keeps one face permanently turned towards us.
Which planet facts are most likely to be asked in the NDA exam?
Superlatives are the most common: largest (Jupiter), smallest (Mercury), hottest (Venus), farthest (Neptune), least dense (Saturn) and the sideways tilt of Uranus, along with the cause of eclipses.
Related NDA Geography topics
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