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Glossary term: Outer Planets

Description: In our Solar System, the outer planets are Jupiter, Saturn, Uranus, and Neptune. Their orbits are outside the asteroid belt, and all of those planets are so-called giant planets, with an extremely thick atmosphere made up mostly of hydrogen. This distinguishes them from the inner planets, each of which also has a comparatively thin atmosphere. The fact that inner planets are different in that regard from outer planets makes it convenient to separate the two groups, but there is no compelling physical reason why this same division should be present in any other planetary system. On the contrary, we now know that a number of other stars are orbited by at least one close-in gas giant, a "hot Jupiter".

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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 Jupiter with horizontal cloud ribbons and the great red spot

Jupiter

Caption: This full disk view of Jupiter was obtained on 21 April 2014 with Hubble's Wide Field Camera 3 (WFC3). It shows the prominent great red spot, a gigantic cyclone. Cloud ribbons cover the surface, whose colours stem from gases like ammonia and other chemical compounds.
Credit: NASA, ESA, and A. Simon (Goddard Space Flight Center) credit link
License: CC-BY-4.0 Creative Comments Attribution 4.0 International (CC BY 4.0) icons
The planet Saturn with pale brownish cloud ribbons and its thin and extended greyish rings

Saturn

Caption: The NASA/ESA Hubble Space Telescope’s Wide Field Camera 3 observed Saturn on 20 June 2019 as the planet made its closest approach to Earth this year, at approximately 1.36 billion kilometres away. The image shows coloured bands of gas on the planet's surface as well as its prominent rings made of ice and rocky material.
Credit: NASA, ESA, A. Simon (Goddard Space Flight Center), and M.H. Wong (University of California, Berkeley) credit link
License: CC-BY-4.0 Creative Comments Attribution 4.0 International (CC BY 4.0) icons
Uranus showing a uniformly greenish-blue coloured appearance

Uranus in natural colours

Caption: This is an image of the planet Uranus taken by the spacecraft Voyager 2 in 1986. Its appearance is close to what the naked eye would see. The greenish-blue colour indicates an atmosphere containing methane.
Credit: NASA/JPL-Caltech credit link
License: PD Public Domain icons
Uranus appears as a light blue disk with and a pale polar region. Thin white rings surround the planet

Uranus with rings

Caption: The NASA/ESA Hubble Space Telescope’s ACS/HRC camera observed Uranus in August 2005. The surface depicts white clouds and a bright polar region. The rings around Uranus are narrow and contain rocky material from tiny dust particles up to metre-sized boulders.
Credit: NASA, ESA, and M. Showalter (SETI Institute) credit link
License: PD Public Domain icons
Neptune is spherical and blue with thin bands of white cloud and a slightly darker spot just below its equator

Neptune

Caption: Voyager 2 Narrow Angle Camera image of Neptune taken in August 1989. The Great Dark Spot, flanked by cirrus clouds, is at center. A smaller dark storm, Dark Spot Jr., is rotating into view at bottom left. Additionally, a patch of white cirrus clouds to its north, named "Scooter" for its rapid motion relative to other features, is visible.
Credit: NASA / JPL / Voyager-ISS / Justin Cowart credit link
License: PD Public Domain icons