Jupiter is the undisputed king of the planets in our solar system! Jupiter is bright and easy to spot from our vantage point on Earth, aided by its massive size and reflective cloud tops and streaks. Jupiter even has moons the size of planets: Ganymede, the largest of them, is larger than Mercury. What’s more, you can easily observe Jupiter and its moons with a humble instrument, just as Galileo did over 400 years ago.
Jupiter’s status as the largest planet in our solar system has really been earned; You could fit 11 Earths along the diameter of Jupiter, and in case you were looking to fill Jupiter with some Earth-sized marbles, you’d need over 1,300 Earths to fill it up – and that wouldn’t be enough! However, despite its massive size, Jupiter’s true supremacy over the outer solar system comes from its massive mass. If you took all the planets in our solar system and put them together, they would still be half the mass of Jupiter by itself. Jupiter’s powerful mass has shaped the orbits of countless comets and asteroids. Its gravity can fling these small bodies toward our inner solar system and also attract them to itself, as famously observed in 1994 when comet Shoemaker-Levy 9 gravitated toward Jupiter in earlier orbits, crashing into the gas giant’s atmosphere. Its multiple fragments collided with Jupiter’s cloud tops so violently that the fireballs and dark spots were seen not only by NASA’s orbiting Galileo probe, but also by observers on Earth!
Jupiter is easy to observe at night with our naked eyes, as well documented by ancient astronomers who carefully recorded its slow motions from night to night. It can be one of the brightest objects in our night sky, only outshone by the Moon, Venus, and occasionally Mars, when the Red Planet is in opposition. That’s pretty impressive for a planet that, at its closest location to Earth, is still more than 365 million miles (587 million kilometers) away. Even more impressive, the giant world remains so bright to Earth observers from its furthest distance: 600 million miles (968 million kilometers)! While the king of the planets has a coterie of about 75 known moons, only the four large moons originally observed by Galileo in 1610—Io, Europa, Ganymede, and Callisto—can be readily observed by Earth observers with very modest equipment. These are called, appropriately, the Galilean moons. Most telescopes will show the moons as faint star-like objects lined up neatly near bright Jupiter. Most telescopes show one or at least two moons orbiting the planet. Small telescopes will show all four of the Galilean moons if they are all visible, but sometimes they can pass behind or in front of Jupiter or even each other. Telescopes will also show details like Jupiter’s cloud bands and, if powerful enough, large storms like the famous Great Red Spot, and the shadows of the Galilean moons that pass between the sun and Jupiter. Charting the positions of Jupiter’s moons during the evening – night into night – can be a rewarding project! You can download an activity guide from the Astronomical Society of the Pacific at bit.ly/drawjupitermoons.
Currently orbiting Jupiter, NASA’s Juno mission is one of only nine spacecraft to have visited this wonderful world. Juno entered Jupiter’s orbit in 2016 to begin its initial mission to study the interior of this giant, mysterious world. Years have proven the Juno mission a success, with data from the probe revolutionizing our understanding of the bravery of this invasive world. Juno’s mission has since been extended to study its large moons, and since 2021 the Intrepid probe, increasingly battered by Jupiter’s powerful radiation belts, has been making close flybys of the icy moons Ganymede and Europa, along with the volcanic Io. In 2024, NASA will launch the Europa Clipper mission to study this world and its ability to host life within its deep subterranean oceans in more detail. Find the latest discoveries from the Juno and NASA missions at nasa.gov.