Our solar system consists of the Sun, which is a star, eight planets, 146 moons, numerous comets, asteroids, and other celestial objects like space rocks and ice. Additionally, there are several dwarf planets, including Pluto. The eight planets in order of their distance from the Sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune, with Mercury being the closest and Neptune the farthest. These celestial bodies, including planets, asteroids, and comets, follow elliptical orbits around the Sun.
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What is Planet?
A planet, in astronomical terms, is a celestial body that lacks its own light, orbits a star, and possesses enough mass for its gravity to mold it into a spherical shape. The International Astronomical Union (IAU) differentiates planets from dwarf planets based on their ability to clear their orbit of other significant objects like asteroids and comets.
Planet or dwarf planet classification is not based on their inherent nature or scientific relevance. Astronomers have not reached a consensus on the planet’s definition, but the IAU sets criteria that include orbiting a star, having sufficient mass for a rounded shape, and clearing its orbit of debris. Under this definition, Pluto and Titan don’t qualify as planets due to their orbital characteristics.
Alternative scientific perspectives introduce various criteria for defining planets, such as the presence of moons, position in the solar system, mass, or other considerations. The term “planet” originates from ‘wanderer,’ reflecting the planets’ apparent wandering in the night sky compared to the fixed stars. A planet is defined as an astronomical body orbiting a star, massive enough for gravitational rounding but not for thermonuclear fusion, and having cleared its nearby space of planetesimals.
In essence, a planet is a sizable celestial body that revolves around a star, reflecting sunlight as it lacks its own light. Unlike stars, planets do not twinkle in the night sky due to their closer proximity. Earth, as a planet, stands out as the only known place in the universe capable of supporting life.
Planets in the Solar System
In the vast expanse of our solar system, excluding dwarf planets, smaller asteroids, and distant icy trans-Neptunian objects (TNOs), a total of eight planets are widely recognized, with the possibility of a yet-to-be-discovered ninth planet.
Arranging them in order of increasing distance from the Sun, the planets are outlined as:
- Mercury
- Venus
- Earth
- Mars
- Jupiter
- Saturn
- Uranus
- Neptune
These planets exhibit diverse characteristics, prompting their classification into distinct groups:
Terrestrial Planets: Mercury, Venus, Earth, and Mars are rocky entities with solid surfaces.
Gas Giants: Dominated by Jupiter and Saturn, these planets are primarily composed of gas, representing the largest members of our solar system.
Ice Giants: Uranus and Neptune, residing at considerable distances from the Sun, are frozen solid, comprising the category of ice giants.
The Sun’s gravitational pull governs the orbital mechanics of celestial bodies in our solar system. It is essential to comprehend these logical divisions in order to have a thorough grasp of the distinctive characteristics that define every planet in our cosmic region. This gravitational interaction unfolds as a complex interstellar arrangement that governs the precise paths and interactions observed among these celestial bodies.
Let us discuss about each planet in detail.
Mercury
Discovered by Galileo Galilei in the 17th century, Mercury is the smallest planet, orbiting at an average distance of 58 million km from the Sun with a diameter of 4,880 km, just slightly larger than Earth’s Moon. Its extreme surface temperatures, ranging from -180°C to 430°C, make it a challenging celestial body to explore, and insights into its geology and composition have been obtained through NASA’s Mariner 10 and MESSENGER missions.
Venus
Named after the Roman goddess of love, Venus is about the same size as Earth with a diameter of 12,104 km. Its thick atmosphere, primarily composed of carbon dioxide, leads to scorching temperatures around 475°C. With a retrograde rotation, a day on Venus lasts longer than its year, at 243 Earth days compared to 225 days for a full orbit around the Sun.
Earth
Our home planet, Earth, positioned at an optimal distance of 150 million km from the Sun, boasts a diverse environment with a 71% water-covered surface. With a radius of 6,371 km, Earth completes a rotation on its axis in 24 hours, causing day and night cycles. The 365.25-day orbit around the Sun results in seasons, making Earth an ideal habitat for a myriad of species.
Mars
Known as the “Red Planet” for its iron-rich surface, Mars is the fourth planet from the Sun with a diameter of 6,805 km. It exhibits a day length similar to Earth’s, but a year on Mars lasts 687 Earth days. Notable features include the largest volcano in the solar system, Olympus Mons, and exploration missions like Curiosity and Perseverance continue to deepen our understanding of the Martian landscape.
Jupiter
Jupiter, the largest planet in our solar system with a diameter of 142,984 km, features the iconic Great Red Spot, a massive storm lasting for centuries. With rapid rotation completing a day in just 9.93 hours and an orbit around the Sun taking 11.86 Earth years, Jupiter’s immense mass, mainly composed of hydrogen and helium, surpasses that of all other planets combined. It hosts 79 moons, including the Galilean moons discovered by Galileo.
Saturn
Renowned for its stunning ring system, Saturn is the sixth planet with a diameter of 120,536 km. The rings, composed of ice and rock, extend over 280,000 km. Despite a day lasting only 10.7 hours, a year on Saturn spans 29 Earth years. With a low density and fast rotation contributing to its oblate shape, Saturn boasts 82 moons, including the intriguing Enceladus and Titan.
Uranus
Discovered by William Herschel in 1781, Uranus, the seventh planet, earns its nickname as the “sideways planet” due to its extreme axial tilt. Positioned 2.9 billion km from the Sun, Uranus has a diameter of 51,118 km and is classified as an “ice giant.” Despite a day lasting 17 Earth hours, a year on Uranus extends to 84 Earth years, and its unique tilt results in extreme seasonal variations.
Neptune
Completing our planetary lineup as the eighth and farthest planet from the Sun, Neptune was discovered in 1846. Situated 4.5 billion km away with a diameter of 24,764 km, Neptune is about four times larger than Earth. Classified as an ice giant, it harbors water, methane, and ammonia in its composition. Neptune’s blue color, strong winds, 16 Earth-hour day, and a 165 Earth-year orbit contribute to its mystique, hosting 14 known moons and a ring system, keeping it a subject of ongoing scientific exploration.
Formation of Planets in the Solar System
Dust’s Main Role:
- Dust around stars, which contains elements such as carbon and iron, is necessary for the formation of celestial bodies.
- These components are required to create planetary systems.
T. Tauri Phase Insights:
- During the T Tauri phase, a star’s developing disk produces hot winds including protons and helium atoms.
- Collisions among dust particles in the disk result in small clumps.
Dust Evolution into Planets:
- Dust clumps grow into pebbles through collisions.
- Gas presence helps solid particles stick together, forming planetesimals.
Gravitational Dynamics:
- Planets, potentially starting as tiny dust grains, evolve within the disk.
- Gravity and other forces drive material collisions, akin to rolling snowballs.
Gas Giants Formation:
- In colder disk regions, ice fragments mix with dust, forming large planetary cores.
- Slowing gas molecules in cold areas contribute
Pluto: A Dwarf Planet
Discovered in 1930 by Clyde Tombaugh, Pluto initially held the prestigious position as the ninth planet in our solar system. Its existence was suggested by Percival Lowell in 1905, and it was named by 11-year-old Venetia Burney, drawing inspiration from the Roman god of the underworld.
Pluto resides in the Kuiper Belt, a region beyond Neptune housing over 100,000 sizable bodies and 1 trillion or more comets. Originally thought to be unique, Pluto’s characteristics became common among Kuiper Belt objects, triggering a reconsideration of its planetary status.
- Controversial Demotion in 2006: In 2006, the International Astronomical Union (IAU) reclassified Pluto as the largest known dwarf planet. This decision, based on evolving understanding of the Kuiper Belt, sparked heated debates within the scientific community and among the public. The IAU established three criteria for classifying a celestial body as a planet: orbiting the sun, having a roughly spherical shape, and clearing its orbit of comparable mass objects. Pluto satisfied the first two criteria but failed the third, as it shares its orbit with objects like its moon Charon.
- Pluto’s New Classification: Despite losing its planetary status, Pluto now holds the distinguished position as the leading member of the dwarf planet category. It remains a crucial object in the Kuiper Belt, raising questions about the nature of celestial bodies in our solar system.
- Insights from New Horizons Mission (2015): NASA’s New Horizons mission in 2015 provided invaluable information about Pluto. With a diameter of 1,473 miles, less than one-fifth of Earth’s, Pluto boasts diverse surface features, including mountains comparable in height to Earth’s Rocky Mountains.
- Surface Characteristics and Atmosphere: Pluto’s surface exhibits varied ices, including nitrogen and methane, contributing to its unique terrain. The heart-shaped Tombaugh Regio and the smooth region “Sputnik Planum” are prominent features. Its atmosphere, composed of nitrogen, methane, and carbon monoxide, creates a reddish-gray “snow” on the surface.
- Complex Organic Molecules and Mysteries: Evidence from the Hubble Telescope hints at the presence of complex organic molecules in Pluto’s crust, adding to the mysteries surrounding this captivating member of the Kuiper Belt. Formerly the ninth planet, Pluto continues to intrigue scientists and space enthusiasts alike.
Exoplanets
- Exoplanets are planets found outside our Solar System, and more than 5000 have been discovered.
- They come in various sizes and types, with some being massive and others having icy or rocky compositions.
- Some exoplanets orbit closer to their stars than Mercury does to our Sun.
- Exoplanetary systems can be diverse, with configurations including multiple planets, binary stars, and potential conditions for stable water.
- Scientists categorize exoplanets using terms like super-Earths, hot Jupiters, or mini-Neptunes, indicating size and mass relative to our Solar System planets.
- The study of exoplanets is rapidly growing since the first discovery in 1995.
- Projects like Cheops (Characterizing Exoplanet Satellite) focus on understanding these distant worlds.
- Studying exoplanets helps scientists learn about their formation, evolution, and whether conditions for life might exist.
- Exploration of exoplanets also provides insights into the formation of our Solar System and Earth.
Exoplanets, which are planets orbiting stars outside our solar system, were first confirmed in 1988, and since then, astronomers have identified over 5,000 such planets. Approximately 20 percent of sun-like stars have Earth-sized planets in the habitable zone. Based on current knowledge, the Milky Way galaxy alone is estimated to have around 100 billion planets, with over 10 billion potentially habitable Earth-like ones. Considering other galaxies, the total number of planets in the universe is immense, roughly estimated to be around one septillion.
Video on Planets in the Solar System
References
- https://www.ferrovial.com/en/stem/planets-of-the-solar-system/
- https://sciencenotes.org/how-many-planets-are-there-in-the-solar-system/
- https://www.britannica.com/science/solar-system/Planets-and-their-moons
- https://www.britannica.com/science/mercury-chemical-element
- https://www.nhm.ac.uk/discover/planet-mercury.html
- https://byjus.com/physics/planet/
- https://theplanets.org/planets/
- https://exoplanets.nasa.gov/faq/43/how-do-planets-form/
- https://science.nasa.gov/resource/our-solar-system-2/
