Two of the Solar System's most remarkable structures: one so thin it is barely there, one so vast and faint that no one has ever seen it.
From edge to edge Saturn's main rings measure about 270,000 km, more than 20 Earths side by side, yet they are mostly only around 10 metres thick. Zoom in from the whole planet down to single gaps. Every ring edge and gap is placed at its measured distance from Saturn.
Ring positions are measured distances from Saturn's centre, mostly from Voyager and Cassini. Brightness across the rings is a simplified version of how much light each region blocks; real rings have far more fine structure. Thickness is not drawn to scale: at true scale the rings would be thinner than this line.
| Ring or gap | Distance from Saturn's centre | What it is |
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Cassini watched stars pass behind the rings and measured how the light flickered. That maps ring density down to a few tens of metres, far finer than any photo.
In 2017 Cassini dived 22 times through the gap between Saturn and its rings before plunging into the planet. It weighed the rings: about 40% of the mass of Saturn's small moon Mimas.
The rings are very clean ice, which suggests they may be only a few hundred million years old. Other scientists argue they could be as old as Saturn. The debate is open.
Small "shepherd" moons such as Pan and Daphnis orbit inside gaps and keep them open. Pan clears the Encke Gap; Prometheus and Pandora confine the narrow F ring.
Far beyond Neptune, the Sun is thought to be wrapped in a huge, thin shell of icy bodies: the Oort cloud. Nobody has seen it. We infer it from comets that fall in from every direction on orbits lasting thousands to millions of years.
Logarithmic scale: each ring outward is ten times farther than the one before. On a normal scale, if Neptune's orbit were the size of a coin, the Oort cloud would be roughly 80 metres across. 1 AU is the Earth–Sun distance, about 150 million km.
| Object or region | Distance from the Sun | Note |
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In 1950 Jan Oort noticed that long-period comets arrive from all over the sky with orbits reaching very far out. He proposed a vast spherical reservoir: the cloud now named after him.
An icy body a few kilometres across, tens of thousands of AU away, reflects almost no sunlight. Even the largest telescopes cannot pick them out, so the cloud's size and population are estimates.
Stars that pass close to the Sun nudge Oort cloud comets inward. Gliese 710 is predicted to pass through the cloud in about 1.3 million years; you can follow it on the 100 ly map with Motion turned on.
Our farthest spacecraft left the Sun's bubble of solar wind in 2012. Even so, it will take around 300 years to reach the inner edge of the Oort cloud, and perhaps 30,000 years to pass through it.