The Juno spacecraft orbits Jupiter this season, approaching close to the surface of this giant planet. Although we are not flying on this spacecraft itself, by looking at the pictures in newspapers around the world this week we can all share in the excitement of discovery.
Of course we already know what Jupiter looks like, kind of. As early as the year 1610 Galileo observed that the four innermost moons of Jupiter, now called the Galilean moons, orbit Jupiter and not the Earth. This observation, and Galileo’s observations of the phases of Venus and the imperfections of the Moon’s surface helped finally to rule out the Ptolemaic model of the solar system in which the Earth is at the center.
We are now taught as kindergarteners that Earth and the other planets, including dear Pluto, orbit the Sun. Meanwhile the Moon orbits Earth, the Galilean moons orbit Jupiter, and now we know of many dozens of other moons all orbiting their individual planets.
The scientific notion that there are many separate and independent centers of motion in the solar system appears to have started as a result of observations of Jupiter. This way of thinking influences our understanding as we look beyond the solar system too and consider that other planets orbit their stars, our Sun orbits some nearby stars, our Sun orbits the Milky Way, and the Milky Way orbits other galaxies.
This evolution of thought has led us away from the notion that the Earth is at the center to a completely ungrounded feeling, for even on the largest scale of all, the Universe as a whole, we think that there is no center!
Our nearest gas giant planet which is so large it is one thousandth the size of the Sun, has played a significant role in broadening our horizons. With a lot of luck, and even more hard work, one aim with the Juno mission is to learn about the different layers of the atmosphere of Jupiter which has as many clouds made of ammonia (hence the yellowish colors) as it does clouds made up of water.
We aim to study its complex atmosphere to inform our understanding of how Jupiter formed, and even if Jupiter may have migrated from further out in the solar system to its current location. What a beautiful week to be doing astronomy.