We’re seeing the planets that are easy to see, not necessarily the ‘typical’ ones.
Originally shared by John Baez
A smaller solar system
Kepler-11 is a star 2000 light-years away that’s very similar to our sun. It has at least 6 planets. But this solar system is small. All the planets would fit inside the orbit of Venus – and all but one fit inside the orbit of Mercury!
We used to think gas giants like Jupiter, Saturn and Neptune could only exist far from their host star. But that’s when we just knew one solar system – our own. Now we know that there’s a huge variety. Many have hot Jupiters or hot Neptunes – gas giants close to the star. We think they formed farther away and migrated in toward their stars when they got tired of the cold winters.
But beware: the easiest planets to detect are big ones close to the star! We’re seeing the planets that are easy to see, not necessarily the ‘typical’ ones. There are probably lots of smaller planets we haven’t seen yet.
Kepler-11 got its name because it’s the 11th star where the Kepler spacecraft saw planets. Even better, they were found in 2011. Its planets have boring names: they’re called b, c, d, e, and f in order of increasing distance from their star. But they’re pretty interesting. They have masses between those of Earth and Neptune. Their densities are all lower than Earth, so they’re probably not rocky worlds. Planets d, e and f probably have a hydrogen atmosphere. Planets b and c seem to contain lots of ice.
Puzzle: how can you have a planet with lots of ice closer to a sun-like star than Venus is to the Sun?