The Solar System & Universe

The Solar System & Universe

The Solar System

Overview

• The solar system consists of the Sun, the eight planets and their satellites (or moons), and thousands of other smaller heavenly bodies such as asteroids, comets, and meteors.

• The Sun is at the centre of the solar system, and all these bodies are revolving around it.

• The gravitational pull of the Sun keeps all the planets and other objects revolving around it. Thus, the motion of all the members of the solar system is governed mainly by the gravitational force of the Sun.

• Planets revolve around the Sun in elliptical orbits.

• In the solar system, the planet nearest to the Sun is Mercury, and the planet farthest from the Sun is Neptune (not Pluto).

• The size of the solar system has been estimated to be about 105 A.U.

• The solar system is dominated by the Sun, which accounts for almost 99.9% of the matter in the whole solar system.

• The Sun is also the source of all the energy in the solar system.

• Pluto is a dwarf planet.

• Mercury, Venus, Earth, and Mars are called terrestrial planets, and Jupiter, Saturn, Uranus, and Neptune are called gaseous planets.

Heliocentrism

Definition

• Heliocentrism, a cosmological model in which the Sun is assumed to lie at or near a central point (e.g., of the solar system or the universe), while the Earth and other bodies revolve around it.

Historical Development

• In the 5th century BC, Greek philosophers Philolaus and Hicetas suggested Earth revolved around a "central fire." Later, Aristarchus of Samos proposed that Earth and other planets orbited the Sun.

• In the 2nd century AD, Ptolemy proposed a geocentric model with Earth at the center, and celestial bodies orbiting it. This view dominated science for about 1,400 years.

• In 1444, Nicholas of Cusa supported Earth’s rotation, but heliocentrism gained ground with Copernicus's 1543 work. Galileo later backed this model, leading to his trial by the Inquisition in 1633.

Geocentric Model

Definition and Historical Context

• The Geocentric Model is a theory where Earth is considered the center of the universe. The most developed version was by Ptolemy in the 2nd century CE.

• This model was widely accepted until the 16th century, when Copernicus’s heliocentric model replaced it.

Members of the Solar System

The Sun

• The Sun is at the centre of the Solar System.

• Its size is thirteen lakh times that of the Earth.

• It is the nearest star to the Earth.

• It is the ultimate source of energy for life on Earth.

• Its diameter is 14 lakh km (approx.).

• Composed of 71% Hydrogen, 26.5% Helium, and 2.5% other elements.

• Hydrogen and Helium are the main gases present in the Sun.

• Nuclear fusion inside the Sun converts hydrogen into helium, releasing tremendous heat and light.

• Surface temperature: 5778 K / 5504.85°C / 9940.73°F.

Solar Phenomena

• Solar Flares: Storms of hot atoms escaping the Sun's gravity into space.

• When they reach Earth, they cause colourful effects:

  • Aurora Borealis in the North Pole.

  • Aurora Australis at the South Pole.

• Sun Spots: Dark spots that are cooler regions (~1500°C).

  • Cause strong Magnetic Radiations, disrupting wireless communication.

  • Appear and disappear every 11 years: Sun Spot Cycle.

Sun’s Structure

• Core temperature: ~15,000,000°C (1.571 x 10⁷ K).

• Photosphere: Shining surface that radiates energy.

• Corona: Outer atmosphere, visible during a total eclipse or with a Coronograph.

• The Sun travels with planets through the galaxy at 70,000 km/h.

• Distance to Earth: 150 million km.

• Sunlight takes ~8.5 minutes to reach Earth.

The Planets

General Characteristics

• Planets are opaque bodies that revolve around and are illuminated by the Sun.

• According to the IAU General Assembly (2006), a planet:

  • Orbits the Sun,

  • Has enough mass for gravity to form a round shape,

  • Has cleared its orbit of other objects.

• Thus, the solar system has eight planets:

  • Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

• A new class, "Dwarf planets," was introduced. Members include:

  • Ceres, Pluto, 2003 UB.

• A ninth planet has been recently discovered by NASA, named Carla.

Planetary Order

• By Distance from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune

• By Size (biggest to smallest): Jupiter, Saturn, Uranus, Neptune, Earth, Venus, Mars, Mercury

• Jupiter is the biggest, and Mercury is the smallest.

Classification of Planets

By Composition

• Terrestrial (Rocky) Planets: Mercury, Venus, Earth, Mars

• Jovian (Gaseous) Planets: Jupiter, Saturn, Uranus, Neptune

By Position: Inner vs. Outer Planets

Separated by the asteroid belt:

Final Thoughts

The Solar System is a vast and dynamic system centered around the Sun, which dominates it in both size and influence. From the rocky inner planets to the massive gas giants and distant icy bodies, each component plays a unique role in shaping the system's structure and behavior.

Scientific models, like heliocentrism, have helped us better understand our place in the universe and replaced older geocentric beliefs. Advancements in astronomy continue to uncover new insights, including dwarf planets and even possible new planetary bodies.

The ongoing study of solar phenomena, planetary composition, and orbital dynamics not only deepens our understanding of space but also highlights the delicate balance that sustains life on Earth.