If it's a simple statement of this form, the result of the expression is returned.

so res => res.json() is just like having a callback like:

function(res) { return res.json() }

and the next part in the then() chain, takes that result as "planet".

Basically, that last part of the chain, has a signature like:

(function callback(...), function errorCallback(...))

and the callback part gets the result of the previous step as "planet" (if all went ok). If there was an error in the previous steps of the chain, the errorCallback would get called, with the exception passed in as "error".