Planets

A planet or star (from Ancient Greek (astēr planētēs), meaning “wandering star”) is an astronomical object orbiting a star or stellar remnant that is large enough to have its own gravity, not too large to create thermonuclear fusion, and has “clears” the area around its orbit filled with planetesimals

The word planet has been around for a long time and has historical, scientific, mythological, and religious connections. By ancient civilizations, planets were seen as immortal or representative of gods. As science advances, humanity’s view of the planet is changing.

In 2006, the International Astronomical Union (IAU) passed an official resolution defining the planets in the Solar System. This definition has been praised but also criticized and debated by some scientists because it does not include objects of planetary mass which are determined by their place or object’s orbit. While eight planetary bodies discovered before 1950 are still considered “planets” by modern definitions, a number of celestial bodies such as Ceres, Pallas, Juno, Vesta (each object in the Sun’s asteroid belt), and Pluto (the first trans-Neptunian object discovered) what was once considered a planet by the scientific community is no longer a problem.

Ptolemy thought the planets revolved around the Earth in deferential and epicyclical motions. Although the idea of planets revolving around the Sun has been around for a long time, it wasn’t until the 17th century that this idea was proven by observations of Galileo Galilei’s telescope. With careful analysis of observational data, Johannes Kepler found that the planet’s orbit is not circular, but elliptical. As observation equipment developed, astronomers observed that the planets rotated on oblique axes and some of them had ice shelves and seasons like Earth. Since the dawn of the Space Age, close observations by space probes have proven that Earth and other planets bear signs of volcanism, storms, tectonics, and even hydrology.

In general, planets are divided into two main types: large low-density gas giants and small rocky land giants. As per the IAU definition, there are eight planets in the Solar System. According to their distance from the Sun (near to far), there are four terrestrial planets, Mercury, Venus, Earth, and Mars, then the four gas giants, Jupiter, Saturn, Uranus, and Neptune. Six of the planets are surrounded by one or more natural satellites. In addition, the IAU recognizes five dwarf planets and hundreds of thousands of small Solar System bodies. They are also still considering other objects to be classified as planets.

Since 1992, hundreds of planets revolving around other stars (“exoplanets” or “exoplanets”) in the Milky Way have been discovered. As of September 1, 2021, 4,834 known exoplanets (in 3,572 planetary systems and 795 multiplanet systems) are listed in the Extrasolar Planets Encyclopaedia. They range in size from land planets similar to Earth to gas giants larger than Jupiter. On December 20, 2011, the Kepler Space Telescope team discovered two Earth-sized exoplanets, Kepler-20e[6] and Kepler-20f, orbiting the Sun-like star, Kepler-20.[8][9][10] A 2012 study that analyzed gravitational microlens data estimated that each star in the Milky Way is surrounded on average by at least 1.6 planets. Astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA) reported in January 2013 that at least 17 billion Earth-sized exoplanets (0.8–1.25 Earth masses to be exact) with orbital periods of 85 days or less reside in the Milky Way galaxy.

Planet formation

It is not known exactly how the planets formed. The prevailing theory is that planets formed when a nebula turned into a thin disk of gas and dust. A protostar forms at its core and is surrounded by a rotating protoplanetary disk. Through accretion (a process of patch collision), the dust particles on the disc slowly collect mass to form much larger objects. The concentration of mass in one place is called the planetesimal form and it accelerates the accretion process by attracting additional material using its gravitational attraction. The concentration is getting denser until it finally collapses inward and forms a protoplanet. Having a larger diameter than Earth’s Moon, the planet forms an additional atmosphere, increasing the planetesimal’s attraction to atmospheric drag.

When a protostar grows so large that it can “ignite” into a star, the remaining disk is obliterated from the inside out by photoevaporation, the solar wind, the Poynting–Robertson drag, and other influences. There are still many protoplanets circling the star or one another, but over time most of them will collide to form one larger planet or release material for the protoplanets or larger planets to absorb. Objects that are large enough will capture some of the material in their orbital environment and become planets. Meanwhile, the protoplanets that manage to avoid collisions will become natural satellites of the planet through the gravitational capture process or remain in the belt of other objects and become dwarf planets or small objects.

The impact of small planetesimal energy (as well as radioactive decay) will warm the growing planet, so that the planet is at least half-melted. The interior of the planet began to vary in mass and create a denser core. The smaller terrestrial planets lose most of their atmospheres due to this accretion, but the gas lost can be replaced by gas escaping from the mantle and comet impacts (small planets will lose the atmosphere gained through various types of ejection mechanisms).

Through the discovery and observation of planetary systems around stars other than the Solar System, scientists have been able to elaborate, revise, or even replace this theory. The degree of metallicity, an astronomical term that describes the abundance of chemical elements with atomic numbers greater than 2 (helium), is now believed to be a determinant of the probability of a star being surrounded by planets. Therefore, some researchers suspect that metal-rich population I stars are more likely to have clearer planetary systems than metal-poor population II stars.

The order of the big planets to the small planets

Humans may think that Earth is a very large planet, but if you look further, there are giant planets that are much larger than Earth.

Reporting from NASA, October 24, 2003, the planet with the largest size is Jupiter and the planet with the smallest size is Mercury.

Jupiter is 11 times the size of Earth, while Mercury is only one-third the size of Earth.

Meanwhile, the Sun as the center of the solar system has a diameter 10 times larger than the diameter of Jupiter.

In detail, here is the order of the planets in order of size from the largest to the smallest.

Jupiter: 43,441 miles (69.911 km)

Reporting from Universe Today, Jupiter is a giant planet that is believed to be responsible for the paths of small objects that float because of its massive mass.

Sometimes, Jupiter sends comets or asteroids into the inner solar system or diverts them.

Jupiter has a large red spot that is a centuries-old storm larger than Earth.

Saturnus: 36,184 miles (58,232 km)

Saturn, which is the second largest planet, is famous for its rings. Other giant planets also have rings, but none quite as special as Saturn’s rings.

In addition, Saturn has dozens of moons. One of Saturn’s moons, Titan, has its own atmosphere.

Uranus: 15,759 miles (25,362 km)

The third and seventh planet from the Sun, Uranus, has a unique rotation.

Uranus rotates at an angle of nearly 90 degrees from its orbital plane which makes it appear to be spinning sideways.

Neptunus: 15,299 miles (24,622 km)

After Uranus, there is Neptune, the eighth planet from the Sun and the fourth largest planet.

Its great distance from the Sun makes Neptune appear dark, cold, and hit by supersonic winds.

Bumi: 3,959 miles (6,371 km)

Earth, the fifth largest planet, is the only place so far inhabited by living things.

Not only that, Earth is also the only planet in the solar system that has liquid water on the surface.

Venus: 3,760 miles (6,052 km)

The sixth largest planet, Venus, is the hottest planet in the solar system. The thick, heat-trapping atmosphere in an uncontrollable greenhouse effect makes Venus extremely hot.