Earth’s Place in the Universe and Solar System – Physical Geography (UPSC)
In our solar system, Earth is the third planet from the Sun and the only one known to harbor life. Radiometric dating, a method used to determine the age of rocks or carbon, suggests that Earth was formed approximately 4.5 billion years ago. Earth’s gravity interacts continuously with celestial bodies, particularly the Sun and the Moon, the latter being Earth’s only natural satellite. The Earth revolves around the Sun in 365.26 days (an Earth year) while rotating about its axis approximately 366.26 times during this period.
Looking up from Earth, the sky appears dotted with countless stars that are not uniformly distributed but grouped in clusters called galaxies or nebulae. Each galaxy may contain up to 100 million stars. Earth is part of the Milky Way galaxy, which includes the solar system. Among the theories explaining the universe’s origin, the Big Bang Theory is most prominent. Proposed by Georges Lemaitre, it posits that the universe expanded from a singularity over 13.8 billion years ago, leading to the formation of galaxies, solar systems, and stars. The Milky Way, a spiral-shaped galaxy, lies close to the Andromeda galaxy, and like most galaxies, it houses a central black hole called Sagittarius A.
The solar system comprises the Sun, eight planets, their satellites, asteroids, comets, meteors, and dwarf planets. At its center, the Sun exerts gravitational force, causing the planets to orbit it in elliptical paths. Planets shine by reflecting sunlight. The planets are classified into solid (inner) and gaseous (outer) planets.
Solid or Inner Planets—
Mercury, Venus, Earth, and Mars—are terrestrial planets composed mainly of silicates and metals. Key features of these planets include dense, rocky compositions, smaller sizes, few or no moons, lack of ring systems, and shorter orbital periods around the Sun.
Gaseous or Outer Planets—
Jupiter, Saturn, Uranus, and Neptune—are Jovian planets, often called gas giants due to their immense size and gaseous composition. These planets have rings, numerous moons, and take significantly longer to complete their orbits around the Sun. Saturn’s rings are the most prominent among these.
The composition of planets is linked to the temperature of the early solar system. In the hotter inner solar system, substances with high melting points remained solid, forming rocky planets, while in the cooler outer regions, elements like water and methane could condense, forming gaseous planets. Additionally, the intense solar winds near the Sun swept away lighter gases from the inner planets, while the outer planets retained these elements due to their larger size and gravitational pull.
Dwarf planets, such as Pluto, Ceres, Charon, and Eris, also orbit the Sun but lack the defining properties of full-fledged planets. The International Astronomical Union (IAU) defines dwarf planets as celestial bodies orbiting a star, massive enough to be rounded by gravity, but which have not cleared their neighboring regions of planetesimals and are not satellites.
Which planet is Earth in our solar system?
Earth is the third planet from the Sun in our solar system.
How old is Earth?
Earth is approximately 4.5 billion years old, as determined by radiometric dating.
How long does Earth take to revolve around the Sun?
Earth takes 365.26 days to revolve around the Sun, which is known as an Earth year.
What is the Milky Way?
The Milky Way is the galaxy to which Earth and the solar system belong. It contains billions of stars and lies near the Andromeda galaxy.
What are solid planets in the solar system?
Solid planets, also known as inner planets, include Mercury, Venus, Earth, and Mars. They are composed mainly of silicates and metals.
What are gaseous planets in the solar system?
Gaseous planets, also known as outer planets or gas giants, include Jupiter, Saturn, Uranus, and Neptune. They are mostly composed of gases and do not have solid surfaces.
Why are inner planets rocky while outer planets are gaseous?
Inner planets are rocky due to higher temperatures in the early solar system, which allowed substances with high melting points to remain solid. Outer planets are gaseous because cooler temperatures allowed elements like water and methane to condense.
What are dwarf planets?
Dwarf planets, such as Pluto, Ceres, and Eris, orbit the Sun but do not meet all the criteria of a full-fledged planet. They are defined by the IAU as celestial bodies that have not cleared their neighboring regions of planetesimals.
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