Jupiter

The fifth planet from the Sun, Jupiter is the largest planet in the solar system by a considerable margin: its mass of 317.8 Earths is more than twice as great as the rest of the planets combined, with an equatorial diameter of 142,984 kilometers. It is the nearest to the sun of the gas giants—planets composed primarily of gas rather than solids. The third-brightest object in the night sky after the moon and Venus for most of the year, Jupiter has an apparent magnitude of –2.94 from the Earth’s surface.

Jupiter’s orbit takes 11.86 years, and its average distance from the sun is 5.2 astronomical units (778 million kilometers). It rotates very rapidly, completing a rotation at the equator in about nine and a half hours, compared to the almost 24-hour rotation of Earth. Because the planet is so vast, this means an equatorial rotation velocity of about 12.6 kilometers per second.

The internal structure of the planet is not yet known. The core is most likely dense and solid, consisting of heavier elements that have precipitated out of the atmosphere, but there is not yet any direct evidence of this. Because it is mostly gas, it is much less dense than the Earth: Its 317.8 Earths worth of mass take up 1,321 Earths of volume. If Jupiter had more mass, its volume would actually shrink—the increase in the planet’s gravity would draw the vast gas giant’s atmosphere in, compressing it.

Atmosphere and Climate

By mass, Jupiter’s atmosphere is about 75 percent hydrogen and 24 percent helium; the remainder is made up of methane, water vapor, ammonia, carbon, ethane, neon, hydrogen sulfide, hydrocarbons (for example, benzene), oxygen, sulfur, and phosphine. Because hydrogen is significantly lighter than helium, it accounts for an even larger share of the volume—about 90 percent—with helium making up about 9 percent. In many respects, Jupiter’s composition is closer to that of the sun, which has a somewhat higher proportion of helium, ten times as much neon, and a lower concentration of inert gases than do the inner planets like Earth.

Clouds of ammonia, water, and ammonium hydrosulfide contribute to Jupiter’s agate appearance. Upper ammonia clouds lead to colored bands called belts (dark) and zones (light). Jovian weather results in the Oval BA, a storm in the southern hemisphere that first began in 2000 and began to turn red in 2005, and the Great Red Spot, a southern storm that has persisted for at least 300 years. Lightning storms are common throughout the atmosphere and have been observed at close range by probes.

Because of helium rains and primordial heat remaining from Jupiter’s formation, it radiates more heat than it receives from the sun.

Planetary Rings and Moons

Jupiter has at least 79 moons, the most famous of which are the four Galilean moons discovered by Galileo Galilei in 1610: Io, Europa, Callisto, and the largest, Ganymede. These four moons are the largest objects in the solar system apart from the sun and the planets. Europa is famous as one of the solar system bodies that could conceivably hold life. The surface of Europa is a smooth layer of ice, below which is believed to be a liquid ocean kept warm by the tides, which could harbor the same kinds of microbial life found in Earth’s deep oceans. Evidence of water vapor plumes on Europa have also been detected. Despite their distance from the sun, the tides of Io, Europa, and Ganymede warm their surfaces; the moons’ orbital resonance with one another leads to stronger tidal acceleration.

Beyond the Galilean moons are many other natural satellites of Jupiter, most of which are fairly small and therefore were not detected until much later. Indeed, scientists continue to discover new moons of Jupiter as technology and detection methods improve. For example, in 2018 scientists announced the discovery of twelve previously unknown moons of Jupiter, detected during the search for a possible ninth planet. The group included one unusual moon, given the name Valetudo, that orbited in the opposite direction of the other objects nearby.

Jupiter’s planetary ring system is small and faint, visible from Earth only with the most powerful telescopes. It appears to consist mainly of dust particles ejected from the moons, and includes four concentric layers, the outermost of which (the gossamer rings) are connected with the orbits of the moons Amalthea and Thebe. The age of the rings is unknown: particles are gradually removed from the ring by solar radiation and electromagnetism, and replenished when high-velocity particles strike the moons, so there are no remaining "original" components of the rings that could indicate age.

History of Observation and Exploration

Because Jupiter is visible with the naked eye, there is a long history of observation, and many ancient cultures had records of the planet. In ancient Greece, Ptolemy proposed a geocentric model in which Jupiter took about 11.86 years to orbit the Earth. Galileo’s discovery of the Galilean moons in 1610 struck a blow against geocentrism by proving the existence of celestial objects that orbited something other than the Earth. The Great Red Spot was discovered by telescope around 1665 by both Giovanni Domenico Cassini and Robert Hooke. As telescopes became more sophisticated, more and more moons were discovered.

Jupiter has been the subject of several exploratory missions by robotic spacecraft. The first Jupiter mission probe, Pioneer 10, was launched March 3, 1972, and began its exploration of Jupiter in November 1973. It sent back data about the outermost moons and the magnetosphere, as well as a series of photographs. Pioneer 11 arrived the following year, producing detailed images of the Great Red Spot and other Jovian features. In 1979, Voyager 1 and Voyager 2 discovered Jupiter’s rings and greatly increased scientific understanding of the moons and the climate of the Great Red Spot, as well as discovering the first extraterrestrial volcano on Io.

Other probes provided images or measurements while en route to other destinations. The Galileo orbiter and entry probe orbited Jupiter from 1995 to 2003, but was unable to provide as much information as planned because of an antenna failure. In 2011, NASA launched Juno, which arrived at Jupiter in 2016 and entered a pole-to-pole orbit, studying the planet's composition and climate. In 2021, a Juno flyby mission measured the depth of Jupiter's Great Red Spot, measuring it at around 300 to 500 kilometers (190 to 310 miles). Various other missions to Jupiter or its moons have been proposed, especially in light of mounting evidence of water and the potential for extraterrestrial life on some of the moons. In 2023, the European Space Agency launched the Jupiter Icy Moon Explorer.

Bibliography

Amos, Jonathan. "European Space Agency: Blast Off for Jupiter Icy Moons Mission." BBC, 14 Apr. 2023, www.bbc.com/news/science-environment-65273857. Accessed 1 Oct. 2024.

Carnegie Institution for Science. “Jupiter’s Moon Count Reaches 79, Including Tiny ‘Oddball.’” Phys.org, 17 July 2018, phys.org/news/2018-07-jupiter-moon-tiny-oddball.html. Accessed 1 Oct. 2024.

Davies, Ashley Gerard. Volcanism on Io: A Comparison with Earth. Cambridge Planetary Science, 2014.

Esposito, Larry. Planetary Rings: A Post-Equinox View. Cambridge Planetary Science, 2014.

Impey, Chris, and Holly Henry. Dreams of Other Worlds: The Amazing Story of Unmanned Space Exploration. Princeton UP, 2013.

“Jupiter.” Solar System Exploration, NASA Science, 2018, solarsystem.nasa.gov/planets/jupiter/overview/. Accessed 1 Oct. 2024.

Plait, Phil. “Young Astronomer Captures a Shadow Cast by Jupiter.” Bad Astronomy, Discover Magazine, 18 Nov. 2011, blogs.discovermagazine.com/badastronomy/2011/11/18/young-astronomer-captures-a-shadow-cast-by-jupiter. Accessed 1 Oct. 2024.

Pont, Frederic J. Alien Skies: Planetary Atmospheres from Earth to Exoplanets. Springer, 2014.

Showalter, Mark R., Imke de Pater, Giuli Verbanac, Douglas P. Hamilton, and Joseph A. Burns. “Properties and Dynamics of Jupiter’s Gossamer Rings from Galileo, Voyager, Hubble, and Keck Images.” Icarus, vol. 195, no. 1, 2008, pp. 361–377.