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Proper names of planetary systems often follow common themes – for example, the planets of the star Copernicus are named after European astronomers. Proper names for planets outside of the Solar System – known as exoplanets – are chosen by the International Astronomical Union (IAU) through public naming contests known as NameExoWorlds.
Size (left) and distance (right) of a few well-known galaxies put to scale. The following is a list of notable galaxies.. There are about 51 galaxies in the Local Group (see list of nearest galaxies for a complete list), on the order of 100,000 in the Local Supercluster, and an estimated 100 billion in all of the observable universe.
Astronomical naming conventions. In ancient times, only the Sun and Moon, a few stars, and the most easily visible planets had names. Over the last few hundred years, the number of identified astronomical objects has risen from hundreds to over a billion, and more are discovered every year. Astronomers need to be able to assign systematic ...
The stars with the most confirmed planets are the Sun (the Solar System's star) and Kepler-90, with 8 confirmed planets each, followed by TRAPPIST-1 with 7 planets. The 1007 multiplanetary systems are listed below according to the star's distance from Earth. Proxima Centauri, the closest star to the Solar System, has three planets (b, c and d).
Several other planets, such as Gliese 180 b, also appear to be examples of planets once considered potentially habitable but later found to be interior to the habitable zone. [ 1 ] Similarly, Tau Ceti e and f were initially both considered potentially habitable, [ 65 ] but with improved models of the circumstellar habitable zone, as of 2022 PHL ...
Extragalactic planet. An extragalactic planet, also known as an extragalactic exoplanet or an extroplanet,[1][2][3] is a star -bound planet or rogue planet located outside of the Milky Way Galaxy. Due to the immense distances to such worlds, they would be very hard to detect directly.
Almost all planets detected so far are within the Milky Way. However, there is evidence that extragalactic planets, exoplanets located in other galaxies, may exist. [15] [16] The nearest exoplanets are located 4.2 light-years (1.3 parsecs) from Earth and orbit Proxima Centauri, the closest star to the Sun. [17]
The first planet discovered in a system is given the designation "b" (the parent star is considered to be "a") and later planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size.