This month is a great time to see Jupiter in our night skies. Appearing in the south, the largest planet in our solar system is a brilliant sight, whether you’re observing with the naked eye, or following the path of its moons with binoculars! But what is this distant planet really like?
A Planetary Goliath
Jupiter is one of 2 gas giants in our solar system, the other being Saturn. This means that unlike Earth, it doesn’t have a solid surface. Instead, the planet consists of a dense atmosphere of hydrogen and helium, which condenses further towards the centre into a huge ocean. It’s the largest ocean in the solar system, covering the entire planet!
With a diameter of 142,984 km, Jupiter is 11 times wider than the Earth. Its large size allows it to have much stronger gravity than what we’re used to. In fact, its gravity is so strong that it can distort the orbits of comets as they travel to and from the Kuiper Belt and beyond. Over time, it’s collected material into its orbit. Some of it was there when the planet formed and some may have been captured by its gravity. But the vast majority of it formed into moons – 79 of them to be precise! Jupiter also has very faint rings, but these are nowhere near as spectacular as Saturn’s.
4 of these moons are known as the Galilean moons: Ganymede, Europa, Io and Callisto. They’re named after the person who discovered them: Galileo Galilei! These moons are far larger than the others, and so can be viewed with a basic telescope like the one Galileo used. He tracked their movement over time, and deduced that they were orbiting Jupiter itself. This was a major discovery, as it proved that not everything orbited Earth, contributing evidence to the heliocentric model of the solar system.
After Galileo’s discovery, it was 363 years before the first spacecraft arrived at Jupiter. Pioneer 10 was the first spacecraft to perform a flyby of Jupiter in 1973. Originally designed to last 21 months, the equipment survived for 30 years, as it continued its journey past other planets in the outer solar system. Further flybys were completed by Pioneer 11, Voyager 1 and Voyager 2.
In 1995, NASA put the first mission in orbit around Jupiter. The spacecraft, named Galileo, was launched in 1989. It collected tons of valuable data on Jupiter, including the only recorded observation of a comet colliding with a planet, when Shoemaker-Levy 9 impacted Jupiter in 1994.
Attention is now turning away from the planet itself and towards its moons. It’s thought that they are good contenders for life, especially Europa, an icy moon consisting of a vast liquid ocean beneath a thick crust of ice. There are currently 2 missions planning to explore these worlds more closely: NASA’s Europa Clipper and ESA’s JUpiter ICy Moons Explorer (JUICE). Who knows what they could find?
Jupiter in the Night Sky
The beauty of such a prominent planet is that you don’t need anything fancy to observe it. Jupiter’s massive size makes it the second brightest object in the sky after Venus, whose proximity to Earth makes it appear brighter. So as long as you know where to look, you can easily see it with the naked eye!
At the moment, Jupiter sits close to Saturn in the sky and will do for months to come. Look for it in the south this month within the constellation of Sagittarius. If you have access to binoculars or a telescope, now is the perfect time to observe the Galilean moons. If you watch them over time, you’ll be able to see how they move: just like Galileo!