Why this chapter matters for UPSC: The solar system is the starting point of physical geography, and the same ideas (planets, the Moon, asteroids, the Sun's energy) return in GS3 science and technology through India's lunar and solar missions. In the rationalised 2023-24 edition this is still Chapter 1, "The Earth in the Solar System", with the same text.

Contemporary hook: Moon counts keep changing. NCERT gives Saturn "about 53" moons; NASA now lists 293 confirmed moons for Saturn as of August 2026, far more than any other planet. Counts like these must always carry a date.

🧠 First Principles — Read This First

NCERT opens with the night sky: countless shining objects, some twinkling and some glowing steadily like the Moon. The Sun, the Moon and all the objects shining in the night sky are celestial bodies. We cannot see them by day because the very bright light of the Sun hides them.

The chapter sorts them into kinds:

  • Stars are very big, hot bodies made of gases, with their own heat and light. The Sun is a star, our nearest one. Other stars look tiny because they are very far away.
  • Planets have no light of their own; they are lit by a star. "Planet" comes from the Greek planetai, "wanderers". The Earth is a planet.
  • Satellites move around planets as planets move around the Sun. The Moon is the Earth's satellite.
  • Asteroids and meteoroids are smaller bodies that also move around the Sun.

The Sun, eight planets, satellites, asteroids and meteoroids together form the solar system, a "solar family" with the Sun as its head. It belongs to the Milky Way galaxy, and millions of galaxies make up the universe.

PART 1 — Quick Reference

The Eight Planets (NCERT, with NASA's current moon counts)

PlanetOne orbit (NCERT)One spin (NCERT)Moons (NCERT)Moons (NASA, 2026)
Mercury88 days59 days—0
Venus255 days (see note)243 days—0
Earth365 days1 day11
Mars687 days1 day22
JupiterAbout 12 years9 h 56 minAbout 53115
Saturn29 years 5 months10 h 40 minAbout 53293 (as of Aug 2026)
Uranus84 years17 h 14 minAbout 2729 (as of Aug 2026)
Neptune164 years16 h 7 min1316

Note on Venus: NCERT's table prints 255 days for one orbit. NASA gives about 225 Earth days for Venus's year (and 243 Earth days for one spin), so Venus spins more slowly than it orbits. Use 225 days.

NCERT groups the planets as inner planets, very close to the Sun and made of rocks, and outer planets, very far from the Sun and huge, made of gases and liquids. NCERT's mnemonic for the order from the Sun: My Very Efficient Mother Just Served Us Nuts.

Key Numbers

ItemFigureSource
Earth–Sun distanceAbout 150 million kmNCERT
Speed of lightAbout 300,000 km per second; sunlight takes about 8 minutes to reach the EarthNCERT
Earth–Moon distanceAbout 3,84,400 kmNCERT; NASA
Moon's diameterAbout one-quarter of the Earth'sNCERT
Moon's orbit and spinAbout 27 days each (NASA: 27.3 days); new moon to new moon 29.5 daysNCERT; NASA
Earth's size rankFifth largest planet; third nearest the SunNCERT
Milky WayAbout 100,000 light years across; the Sun lies about 8 kiloparsecs (about 26,000 light years) from its centreNASA
Age of the universe13.8 billion yearsPlanck Collaboration (2018 results)

PART 2 — Concepts & Narrative

Stars, Constellations and the Pole Star

Key Term

Constellation: a pattern formed by a group of stars. Ursa Major (Big Bear) is one. The Saptarishi ("seven sages") is a group of seven stars that forms part of Ursa Major.

Pole Star (North Star): indicates the north. NCERT says it "always remains in the same position in the sky". Draw an imaginary line through the two pointer stars of the Saptarishi and extend it: it points to the Pole Star. In ancient times people found directions at night this way.

NCERT's "Let's Do" activity shows why stars vanish by day: a torch covered with pin-pricked paper throws dots of light on a wall in a dark room, and the dots almost disappear when the room lights are switched on, just as the stars do once the Sun rises.

The Planets

Explainer

All eight planets move around the Sun in fixed paths called orbits, which are elongated. Mercury is nearest the Sun and takes only about 88 days to go round it. Venus is called the "Earth's twin" because its size and shape are very similar to the Earth's.

What NASA adds (Science pages, 2026):

  • Venus is the hottest planet, with a surface temperature of about 467 °C, even though Mercury is closer to the Sun.
  • Venus and Uranus spin in the opposite direction to most planets. Uranus's axis is tilted about 98°, so it spins nearly on its side.
  • Jupiter is about 11 times wider than the Earth.
  • Saturn is the only planet whose average density is less than that of water.
  • Neptune has winds of about 2,000 km/h. It is not simply "the coldest planet": Uranus's atmosphere reaches a minimum of 49 K (−224.2 °C), which NASA notes is colder than Neptune in some places.
  • Rings: NCERT says Jupiter, Saturn and Uranus have rings, belts of small debris visible through powerful telescopes. Neptune has rings too: NASA counts at least five main rings.

Pluto and the dwarf planets. Until August 2006 Pluto was considered a planet. At its General Assembly in Prague, the International Astronomical Union (IAU) defined a planet as a body that (a) orbits the Sun, (b) is massive enough to be nearly round, and (c) "has cleared the neighbourhood around its orbit". Pluto fails the third test. The IAU named Ceres, Pluto and 2003 UB313 (now Eris) as the first members of a new class, the dwarf planets (IAU, 24 August 2006). NCERT records the same decision.

The Earth: a Unique Planet

The Earth is the third nearest planet to the Sun and the fifth largest. It is slightly flattened at the poles, so its shape is called a geoid, meaning "an earth-like shape".

NCERT explains why it is unique: it is neither too hot nor too cold, and it has water and air, with life-supporting gases such as oxygen. Conditions favourable to life are probably found only on the Earth. From outer space it looks blue, because water covers about two-thirds of its surface, so it is called the blue planet.

Note

Aryabhata. NCERT notes that the ancient Indian astronomer Aryabhata said the Moon and the planets shine because of reflected sunlight. People who study celestial bodies and their movements are called astronomers.

The Moon

Key Term

The Moon is the Earth's only natural satellite. Its diameter is only about one-quarter of the Earth's; it looks big because it is much nearer to us than other celestial bodies, about 3,84,400 km away. It goes round the Earth in about 27 days and takes exactly the same time to complete one spin, so only one side of the Moon is ever visible from the Earth. It has mountains, plains and depressions that cast shadows, and no conditions favourable for life.

Phases: the full moon (Poornima) comes about once a month; a fortnight later is the new moon (Amavasya), when the night sky is best for watching stars.

Human-made satellites are built by scientists to gather information or for communication, carried by rockets and placed in orbit. NCERT names India's INSAT, IRS and EDUSAT. Neil Armstrong was the first person to step on the Moon, on 20 July 1969.

UPSC Connect

India's missions to the Moon and the Sun (dated)

MissionWhat happenedSource
Chandrayaan-1India's first mission to the Moon, launched 22 October 2008; it carried NASA's Moon Mineralogy Mapper among its payloadsISRO
Chandrayaan-3The Vikram lander soft-landed near the lunar south pole on 23 August 2023, making India the fourth country to land on the Moon and the first near its south pole. The landing site is named Shiv ShaktiPIB, 12-Aug-2024; ISRO
Pragyan roverIts LIBS instrument confirmed sulphur on the lunar surface and detected aluminium, calcium, iron, chromium, titanium, manganese, silicon and oxygen; the search for hydrogen was continuingISRO, 28-Aug-2023
National Space DayThe Government has declared 23 August National Space Day to mark Chandrayaan-3's landingPIB, 12-Aug-2024
Aditya-L1India's solar observatory, launched by PSLV-C57 on 2 September 2023; inserted into a halo orbit around the Sun-Earth L1 point, about 1.5 million km from the Earth, on 6 January 2024ISRO
Mars Orbiter MissionLaunched on 5 November 2013; completed seven years in Mars orbit on 24 September 2021ISRO

Gaganyaan, India's human spaceflight programme, aims to send a crew of three to an orbit of 400 km for a three-day mission and bring them back to land in Indian waters (ISRO). Its first uncrewed orbital flight is still awaited: on 4 September 2026 ISRO's chairman said work was on to launch it within the calendar year (PTI).

Space reforms and security. On 24 June 2020 the Union Cabinet approved reforms to open the space sector to private companies and created IN-SPACe (Indian National Space Promotion and Authorization Centre) to give them a level playing field in using Indian space infrastructure (PIB). The Indian Space Policy 2023 was approved by the Cabinet Committee on Security and released in the public domain in 2023 (PIB, Department of Space year-end review). In Mission Shakti, on 27 March 2019, India tested an anti-satellite missile against one of its own satellites, joining "an exclusive group of space faring nations consisting of USA, Russia and China" (MEA); the Ministry of Defence describes India as "the fourth nation in the world" with this capability (PIB, 9-Nov-2020).

Asteroids, Meteoroids and the Milky Way

Asteroids are numerous tiny bodies that move around the Sun, found between the orbits of Mars and Jupiter. NCERT says scientists think they are parts of a planet that exploded long ago. That is an old idea. NASA describes asteroids as "rocky remnants left over from the formation of our solar system about 4.6 billion years ago", most of them orbiting in the main belt between Mars and Jupiter.

Meteoroids are small pieces of rock that move around the Sun. When one comes near the Earth and falls through the air, friction heats it and it burns, causing a flash of light. Sometimes one does not burn up completely and falls on the Earth, creating a hollow.

India's own impact crater. Lonar Lake in Buldhana district, Maharashtra, is a saline lake that "was formed by meteorite impact in the Deccan Trap region"; it was designated a Ramsar site on 22 July 2020 (Ramsar Information Sheet). Its age is uncertain: the Buldhana district administration gives the traditional estimate of 52,000 ± 6,000 years and notes that a 2010 study put it at 570,000 ± 47,000 years.

The Milky Way. On a clear night a whitish, glowing band crosses the sky: a cluster of millions of stars, the Milky Way galaxy, of which our solar system is a part. Ancient India imagined it as a river of light and named it Akash Ganga. A galaxy is a huge system of billions of stars and clouds of dust and gas, and millions of galaxies make up the universe.

Note

A light year is a distance, not a time: the distance light travels in a year. At 299,792 km per second over a year of 365.25 days, that is about 9.46 trillion km (arithmetic from the defined speed of light). By the same arithmetic, light from the Sun, about 150 million km away, takes about 500 seconds, a little over 8 minutes, which matches NCERT.

PART 3 — UPSC Integration

The chapter connects physical geography to science and technology. The Earth's place in the solar system and its water, air and temperature explain why it supports life, the theme GS1 builds on in later chapters on the atmosphere and oceans. India's space missions turn the same topics into GS3 material: Chandrayaan-3's south-polar landing, Aditya-L1 at the L1 point, and Gaganyaan's human-spaceflight goal. Asteroids appear in GS3 as a planetary hazard. In answers, give each mission's date and its source, because launch plans change and only completed events count as achievements.

Exam Strategy

Prelims traps:

  • Hottest planet is Venus, not Mercury, though Mercury is nearest the Sun.
  • Most moons: Saturn, 293 as of August 2026 (NASA); Jupiter has 115. NCERT's "about 53" for each is out of date.
  • Venus's year is about 225 days, not NCERT's printed 255.
  • Coldest: Uranus records the lowest atmospheric temperature (49 K); do not assume Neptune because it is farthest.
  • Pluto became a dwarf planet at the IAU General Assembly of August 2006.
  • Chandrayaan-3 (23 August 2023): fourth country to land on the Moon, first near the south pole.
  • Aditya-L1 is in a halo orbit around L1, about 1.5 million km from the Earth.
  • A light year measures distance.

Mains angle: Explain why the Earth is the only known planet with life, and assess what India's lunar and solar missions have achieved, with dates.

Practice Questions

Practice (UPSC-pattern, not past papers). Real UPSC questions on this chapter are listed in the PYQ box above.

Prelims:

  1. Which of the following is the hottest planet in the solar system?
    (a) Mercury
    (b) Venus
    (c) Mars
    (d) Jupiter

  2. Chandrayaan-3's lander touched down near which part of the Moon?
    (a) The equatorial region
    (b) The north pole
    (c) The south pole
    (d) The far side's equator

  3. According to NCERT, the Pole Star can be located by extending a line through the pointer stars of:
    (a) Orion
    (b) the Saptarishi
    (c) the Southern Cross
    (d) Cassiopeia

  4. Consider the following statements:

    1. Asteroids are found between the orbits of Mars and Jupiter.
    2. A meteoroid that does not burn up completely can fall on the Earth and create a hollow.
    3. The Moon takes the same time to spin once as to go round the Earth once.
      Which of the statements given above are correct?
      (a) 1 and 2 only
      (b) 2 and 3 only
      (c) 1 and 3 only
      (d) 1, 2 and 3

  5. Aditya-L1 was placed in a halo orbit around which Lagrange point?
    (a) L2
    (b) L1
    (c) L4
    (d) L5

  6. As of August 2026, which planet has the most confirmed moons (NASA)?
    (a) Jupiter
    (b) Saturn
    (c) Uranus
    (d) Neptune

Mains:

  1. Why is the Earth called a unique planet? Discuss the conditions that make life possible on it. (150 words)

📦 Revision Capsule

Revision Capsule

Hard Facts

  • Celestial bodies: stars (own heat and light; the Sun), planets (lit by a star), satellites, asteroids, meteoroids.
  • Planets in order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune; Earth third nearest, fifth largest; Venus = Earth's twin.
  • Moon: about 3,84,400 km away; one-quarter of Earth's diameter; spin = orbit (about 27 days), so one side faces us.
  • Moons (NASA, 2026): Saturn 293, Jupiter 115, Uranus 29, Neptune 16, Mars 2.
  • Missions: Chandrayaan-3 landed 23 Aug 2023 (south pole); Aditya-L1 at L1 from 6 Jan 2024; National Space Day 23 August.

Core Concepts

  • The Earth is unique: right temperature, water, air with oxygen.
  • Solar system → Milky Way (Akash Ganga) → universe of millions of galaxies.
  • Pluto reclassified as a dwarf planet (IAU, 2006).

Confused Pairs

  • Star (own light) vs planet (reflected light) vs satellite (orbits a planet)
  • Asteroid (between Mars and Jupiter) vs meteoroid (small rock; burns as it falls)
  • Inner planets (rocky) vs outer planets (gases and liquids)
  • Hottest (Venus) vs nearest the Sun (Mercury)

PYQ Pattern

  • Mains GS3 asked about Chandrayaan-3's goal of a soft landing (2023), India's unmanned missions and the case for a manned mission (2017), and the threat from asteroids (2024). See the PYQ box.

Sources

Sources