![]() In later decades, however, Pluto’s planetary status became increasingly questioned by astronomers who noted that its tiny size, unusual orbital characteristics, and composition of ice and rock made it an anomaly among the other recognized planets. So also, at first, appeared to be the case for Pluto when, during a concerted search for a ninth planet, it was observed in 1930 as a seemingly lone object beyond the orbit of Neptune. When the giant bodies Uranus and Neptune were discovered in 17, respectively, their obvious kinship with the other known planets left little question regarding their addition to the planetary ranks. After the idea of an Earth-centred cosmos was dispelled ( see Copernican system) and more distinctions were made about the nature and movement of objects in the sky, the term planet was reserved only for those larger bodies that orbited the Sun. These included the Sun and Earth’s Moon, as well as the five planets in the modern sense-Mercury, Venus, Mars, Jupiter, and Saturn-that were readily visible as celestial wanderers before the invention of the telescope. ![]() Ancient sky gazers applied the term planet to the seven celestial bodies that were observed to move appreciably against the background of the apparently fixed stars. The idea of what exactly constitutes a planet of the solar system has been traditionally the product of historical and cultural consensus. At that time astronomers confirmed that other stars have objects that appear to be planets in orbit around them. Until the close of the 20th century, the only planets to be recognized were components of Earth’s solar system. Pluto also was listed as a planet until 2006. As the term is applied to bodies in Earth’s solar system, the International Astronomical Union (IAU), which is charged by the scientific community with classifying astronomical objects, lists eight planets orbiting the Sun in order of increasing distance, they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. In addition to the above description, some scientists impose additional constraints regarding characteristics such as size (e.g., the object should be more than about 1,000 km across, or a little larger than the largest known asteroid, Ceres), shape (it should be large enough to have been squeezed by its own gravity into a sphere-i.e., roughly 700 km across, depending on its density), or mass (it must have a mass insufficient for its core to have experienced even temporary nuclear fusion). Planet, (from Greek planētes, “wanderers”), broadly, any relatively large natural body that revolves in an orbit around the Sun or around some other star and that is not radiating energy from internal nuclear fusion reactions. SpaceNext50 Britannica presents SpaceNext50, From the race to the Moon to space stewardship, we explore a wide range of subjects that feed our curiosity about space!.Learn about the major environmental problems facing our planet and what can be done about them! Saving Earth Britannica Presents Earth’s To-Do List for the 21st Century. ![]() Britannica Beyond We’ve created a new place where questions are at the center of learning.100 Women Britannica celebrates the centennial of the Nineteenth Amendment, highlighting suffragists and history-making politicians.COVID-19 Portal While this global health crisis continues to evolve, it can be useful to look to past pandemics to better understand how to respond today.Student Portal Britannica is the ultimate student resource for key school subjects like history, government, literature, and more.Demystified Videos In Demystified, Britannica has all the answers to your burning questions.This Time in History In these videos, find out what happened this month (or any month!) in history. ![]() ![]()
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