Glossary term: Planet

Also known as major planet

Description: A planet is defined by the International Astronomical Union as a celestial body in orbit around a star or the remnant of a star, that is large enough to be nearly round in shape by its own gravitational force, but not massive enough for thermonuclear fusion to ever occur in their cores. It must also be large enough for its gravity to remove any objects of the same size that pass close to its orbit around the star. Therefore, they are cold bodies that shine only by the light reflected from their stars. In our Solar System, eight planets orbit around the Sun. Planets may be basically rocky objects, such as the inner planets – Mercury, Venus, Earth, and Mars – or primarily liquid and gas with a small solid core like the outer planets – Jupiter, Saturn, Uranus, and Neptune. Planets outside the Solar System are called extrasolar planets or exoplanets for short.

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Term and definition status: This term and its definition is still awaiting approval

The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".

Related Media

The planet Jupiter with the two of the four Galilean moons (visible as bright dots) orbiting it.

Jupiter's Rotation, by Vishal Sharma, India

Caption: Third place in the 2021 IAU OAE Astrophotography Contest, category Galilean moons: Jupiter’s Rotation, by Vishal Sharma, India. This time-lapse beautifully shows the rotation of Jupiter and the passage of two Galilean moons on the right side of the frame. Jupiter completes one rotation in just under 10 hours and we see as the Great Red Spot makes its way from left to right. The two moons travel a noticeable fraction of their orbit in this short time. This image was taken in 2020 in the North of India.
Credit: Vishal Sharma/IAU OAE
License: CC-BY-4.0 Creative Comments Attribution 4.0 International (CC BY 4.0) icons
The planet Jupiter, seen here as a bright disk, is orbited by the four Galilean moons, seen here as bright dots

Jupiter Moon's Movie2, by Nicolas Hurez, Paul-Antoine Matrangolo, and Carl Pennypacker, United States of America

Caption: Second place in the 2021 IAU OAE Astrophotography Contest, category Galilean moons. This sequence shows the orbit of the four Galilean moons around the planet Jupiter. Almost two entire orbits of the innermost moon, Io, can be seen, with the other moons (Europa and Ganymede, but in particular Callisto) being further away, orbiting noticeably slower. The images were obtained in 2018 with the Las Cumbres Global Observatory at different locations on Earth, allowing a continuous sequence of images over approximately half a week without gaps during the day. With clear skies and over the course of several nights, the motion of the Galilean moons can also be observed with binoculars (ideally steady your elbows on a surface).
Credit: Nicolas Hurez, Paul-Antoine Matrangolo and Carl Pennypacker/IAU OAE
License: CC-BY-4.0 Creative Comments Attribution 4.0 International (CC BY 4.0) icons
Jupiter with coloured horizontal bands of clouds. The shadow of the moon Io is seen as a dark circle in the top left

Jupiter, Io and its shadow, by Ralf Burkart, Germany

Caption: First place in the 2021 IAU OAE Astrophotography Contest, category Galilean moons. This time-lapse of Jupiter taken in 2017 from Germany beautifully illustrates the transit of one of the Galilean moons, Io, in front of Jupiter. As this is simply a moon casting a shadow on a planet it is equivalent to a lunar eclipse on Earth observed from further away. While the shadow of the moon is clearly visible from the beginning, it might be difficult to spot the moon itself against the background of the beautiful atmospheric bands of Jupiter the first time the video is seen. Watching it repeatedly allows appreciating the rapid motion and rotation in this fantastic observation.
Credit: Ralf Burkart/IAU OAE
License: CC-BY-4.0 Creative Comments Attribution 4.0 International (CC BY 4.0) icons
The planet Mercury covered by many craters


Caption: This image is a composite of a picture mosaic of the planet Mercury's surface obtained by the MESSENGER (Mercury Surface, Space Environment, Geochemistry and Ranging) space probe. MESSENGER was launched by NASA in 2004 and explored Mercury from 2011 to 2015.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington credit link
License: PD Public Domain icons
The planet Venus showing white clouds enshrouding the planet

Venus in visible light

Caption: This picture taken by NASA's Mariner 10 probe shows what the planet Venus looks like when looking at it with naked eyes. Venus is enshrouded inside a thick cloudy atmosphere dominated by carbon dioxide, never revealing its hot surface.
Credit: NASA/JPL-Caltech credit link
License: PD Public Domain icons
The planet Venus' surface with ridges and valleys

Venus' surface

Caption: This image is a computer-aided rendering of the surface of the planet Venus. Since visual light cannot penetrate the thick clouds in Venus' atmosphere, the image was obtained with radio waves. NASA's space probe Megallan, launched in 1989 mapped Venus' surface between 1990 and 1994.
Credit: NASA/JPL credit link
License: PD Public Domain icons
The Earth from space showing oceans and continents

Earth as observed from Apollo 17

Caption: Full disk view of the Earth taken on 7 December 1972, by the crew of the Apollo 17 spacecraft en route to the Moon at a distance of about 29,000 kilometres (18,000 mi). It shows Africa, Antarctica, and the Arabian Peninsula.
Credit: NASA/Apollo 17 crew/Project Apollo Archive
License: PD Public Domain icons
The planet Mars with a rusty red surface, volcanoes, valleys, craters, ice clouds and a white polar cap


Caption: This image of the planet Mars taken by NASA's Mars Global Surveyor Orbiter in 1999 shows its dry surface. The picture features the most spectacular geological regions on Mars. Besides the deep Valles Marineris valley we see four volcanoes. While three of them form the Tharsis ridge, the Olympus Mons is largest volcano we have so far discovered in the Solar System. Ice clouds cover parts of the Martian surface.
Credit: NASA/JPL/MSSS credit link
License: PD Public Domain icons