A few weeks ago researchers at ALMA, a major new telescope array in the Atacama desert in Chile, published the most detailed image ever seen of a planetary disc around a young star, just as planets are starting to form from a disc of tiny bits and pieces, see below. Apart from the colour, doesn't the pattern of concentric rings around a sphere, immediately remind you of Saturn's ring system?
It's not a co-incidence. Gravity, the most powerful force in the universe over long distances, is at the root of these patterns. In fact gravity causes stars and planets to form too. The process is not fully understood but it is thought to work like this:
Space is full of dust and gas. All matter attracts other matter by gravity, and although small particles only have a small gravitational effect, when enough of them are close together they start clumping together, making a larger mass with more powerful gravitational attraction, and attracting even more matter and becoming even more powerful...and so on until the mass is so huge with so much gravitational pressure in its core that it reaches the extremely high temperatures needed for nuclear fusion to happen, and starts shining as a star.
Planets from spinning clumps
In the meantime, because the power of gravity decreases with distance, matter towards the edge of the mass is not dragged to the centre, but instead starts to spin around the star, forming a disc. And eventually, due to gravity again, this matter starts to clump together too whilst still spinning around the star, forming planets. As these planets start to form, they grow by collecting up all the matter in their orbits, clearing a path in the disc and creating the pattern of rings that like the one we can see in the ALMA image.
Saturn's rings are also formed by the effects of gravity on orbiting matter, this time matter orbiting a planet, not a star. The age of the rings is not known for certain but recent data collected by Cassini suggests that some of them may well be billions of years old, forming soon after Saturn itself, at the same time as its many (more than 60!) moons.
And just as for the star in the ALMA image, the pattern of concentric rings was shaped by these orbiting satellites.
In some cases the moons swept up existing debris in their path, as you can see in this amazing close up image of tiny Dahpnis, only 8 km across, ploughing a trail through Saturn's A-ring.
Other rings, such as the E-ring in the orbital path of Enceladus, appear to have be formed of material ejected by the moon itself.
But whatever the source of the ring material, it is the force of gravity that we have to thank for sculpting their elegant arcs.
http://www.almaobservatory.org/en/press-room/press-releases/771-revolutionary-alma-image-reveals-planetary-genesis
http://saturn.jpl.nasa.gov/ Official page of the Cassini mission – lots about Saturn, its rings and moons
http://hubblesite.org/hubble_discoveries/discovering_planets_beyond/how-do-planets-form
http://www.nature.com/news/dust-reveals-ancient-origin-for-saturn-s-rings-1.15743
Image Credits:
1. Cassini Imaging Team, SSI, JPL, ESA, NASA http://apod.nasa.gov/apod/ap070306.html
2. ESO http://www.eso.org/public/images/ann14083a/
3. NASA/JPL/Space Science Institute http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=4098