A planet is a celestial body that orbits a star, is massive enough for its gravity to have made it spherical, and has cleared its orbital path of other debris. In our solar system, there are eight recognized planets, each displaying unique characteristics. These planets are categorized into two main types: the rocky, terrestrial planets (Mercury, Venus, Earth, and Mars) and the gas giants (Jupiter and Saturn) followed by the ice giants (Uranus and Neptune). The defining attributes of planets, as laid out by the International Astronomical Union (IAU) in 2006, emphasize the importance of planets having cleared their orbits, which distinguishes them from dwarf planets like Pluto.
The formation of planets occurs within the circumstellar disks of dust and gas surrounding new stars. Through a process known as accretion, particles in the disk gradually collide and stick together, forming larger bodies called planetesimals, which, through further collisions and mergers, become protoplanets. This process can take anywhere from a few million to tens of millions of years, with the resulting planets varying widely in size, composition, and atmospheric conditions. The study of this process not only helps astronomers understand the formation of our own solar system but also the potential for other planetary systems throughout the galaxy.
Each planet in our solar system offers a unique environment. For instance, Venus, often called Earth's sister planet due to its similar size and proximity, has a thick atmosphere composed mostly of carbon dioxide, with clouds of sulfuric acid, making it the hottest planet in our solar system despite not being the closest to the Sun. Mars, on the other hand, presents a cold desert world with a thin atmosphere, where water ice and permafrost can be found at the poles. Jupiter, the largest planet, is known for its Great Red Spot, a giant storm larger than Earth that has been raging for hundreds of years.
Beyond our solar system, exoplanets or extrasolar planets, have been discovered in orbit around other stars. The methods used to detect these distant worlds include the transit method, where a planet's passage in front of its host star dims the star's light temporarily, and the radial velocity method, which detects variations in the star’s velocity due to gravitational tugs from orbiting planets. These discoveries have vastly expanded our understanding of planetary systems and have challenged our definitions with unique cases like rogue planets, which orbit no star, and super-Earths, which are larger than Earth but smaller than ice giants like Neptune. As technology advances, so too does our capacity to explore these distant worlds, continually pushing the boundaries of our knowledge about the universe.