Both versions [아/어]있다 and 아/어 있다 (with space) exist and can be used interchangeably.
Expresses the continuing state of a completed action.
Can be used when talking about standing, sitting, lying, whereas 고 있다 can not.
Expresses an action currently in progress.
Can be used with verb of wearing such as 입다, 신다, 쓰다.