A planet is a large celestial body that orbits a star and reflects its light. Planets are different from stars, which produce their own light and heat through nuclear fusion. Planets are also different from smaller objects like asteroids, comets, and moons, which have not cleared their orbital region of other similar-sized bodies.
The word planet comes from the Greek word planḗtai, meaning "wanderers". Ancient astronomers observed that some points of light in the sky moved against the background of fixed stars. They called these points planets and identified seven of them: Mercury, Venus, Mars, Jupiter, Saturn, the Sun, and the Moon. Later, with the development of the heliocentric model of the solar system, the Sun and the Moon were recognized as not being planets, but rather the central star and a satellite of Earth. Earth itself was also recognized as a planet.
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The invention of the telescope allowed astronomers to discover more planets beyond the ones visible to the naked eye. Uranus was discovered in 1781 by William Herschel, and Neptune was discovered in 1846 by Johann Galle and Urbain Le Verrier. Pluto was discovered in 1930 by Clyde Tombaugh and was initially considered the ninth planet of the solar system. However, in 2006, the International Astronomical Union (IAU) adopted a new definition of a planet that excluded Pluto and reclassified it as a dwarf planet. According to the IAU, a planet must meet three criteria:
- It must orbit a star (in our case, the Sun).
- It must have enough mass and gravity to pull itself into a spherical shape.
- It must have cleared its orbital neighborhood of other similar-sized objects.
By this definition, there are eight planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. These planets can be divided into two groups: terrestrial planets and giant planets. Terrestrial planets are rocky and have thin atmospheres. They are Mercury, Venus, Earth, and Mars. Giant planets are mostly made of gas and have thick atmospheres. They are Jupiter, Saturn, Uranus, and Neptune. Some giant planets also have rings of dust and ice particles around them.
Besides our solar system, there are thousands of other planetary systems in our galaxy and beyond. These are called exoplanets or extrasolar planets. Exoplanets are planets that orbit other stars than the Sun. The first confirmed exoplanet was discovered in 1992 by Aleksander Wolszczan and Dale Frail around a pulsar called PSR B1257+12. The first confirmed exoplanet around a main-sequence star (like the Sun) was discovered in 1995 by Michel Mayor and Didier Queloz around 51 Pegasi. Since then, more than 4,000 exoplanets have been detected by various methods such as radial velocity, transit, microlensing, direct imaging, and astrometry.
Exoplanets are very diverse in their sizes, masses, orbits, compositions, temperatures, and atmospheres. Some exoplanets are similar to Earth in size and distance from their stars, which makes them potentially habitable for life as we know it. These are called Earth-like or habitable exoplanets. However, none of these exoplanets have been confirmed to harbor life yet.
Planets are fascinating objects that reveal a lot about the origin and evolution of our universe. By studying planets both within and outside our solar system, we can learn more about how they form, how they interact with their stars and other bodies, how they develop atmospheres and climates, how they support life or not, and how they compare to our own home planet.
References:
- What is a Planet? | Planets – NASA Solar System Exploration
- Planet - Wikipedia
- What is a Planet? - Definition, Facts - BYJU'S
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