E-Space

Exo-Space, usually referred to as E-Space for short, was a smaller universe existing alongside the prime universe, N-Space.

Characteristics
It was not a parallel universe as that term is generally understood, having its own astronomy and unique inhabitants. However it wasn't quite a perpendicular universe either, since it had Humanoids and other sentient life forms that where similar to ones of N-Space; thus it was more of an oblique universe. The Doctor's companion, Adric, originated from E-Space and travelled with the Doctor back to N-Space where he ultimately died (DW: Earthshock). The Doctor's longtime companion, Romana, chose to stay in E-Space along with K-9 Mark II rather than return to N-Space with the Doctor and Adric. (DW: Warriors' Gate)

E-Space was visually distinct from N-Space in that it possesses an unusual greenish tint. It was also smaller than N-Space. (DW: Full Circle, State of Decay)

Locations in E-Space apparently had negative co-ordinates, where N-Space was positive. When the Doctor's TARDIS materialized on the planet Alzarius after it had travelled into E-space, the TARDIS scanner continued to show images of Gallifrey, which had the positive equivalent of Alzarius' special co-ordinates. Rather than being a simple optical relay, the system apparently depicts whatever exists at the ship's current co-ordinates. The Doctor suggested that the system operated off the absolute value of the co-ordinates and thus could not process negative co-ordinates. The scanner system was made functional again simply by replacing its original N-Space image translator with one native to E-Space. (DW: Full Circle)

Transit between the universes
The Doctor stated that the negative co-ordinates would make it very difficult for his TARDIS to travel there deliberately and not accidentally. (DW: Earthshock) By his seventh incarnation the Doctor was able to pilot his TARDIS between E- and N-space, and was able to retrieve Romana from E-space. (NA: Blood Harvest) K-9 was also later retrieved. (NA: Lungbarrow)

CVEs
Travel via CVE was a hazardous and apparently random process, even using a TARDIS. (DW: Full Circle)

The Logopolitans revealed they had created the CVEs which linked together the two universes as part of their plan to forestall the overdue entropic collapse of their universe (N-Space), by opening up what had previously been a closed system and thus dissipating the accumulated entropy. Following the disastrous consequences of the Master's visit to Logopolis only one CVE remained open, in the constellation of Cassiopeia (from Earth's perspective). Unless further CVEs were later created it is highly likely this is the route through which the Hydrax would enter E-Space (DW: State of Decay) in the 22nd century. (NA: Lucifer Rising)

The Gateway
On his return journey to N-Space the Doctor and his companions passed through a strange white void known to its most frequent inhabitants, the Tharils, as the Gateway.


 * (The Gateway zone between N-Space and E-Space had zero co-ordinates, thus adding credence to the 'positive and negative universes' theory.) It is not known whether it is possible to travel from E-Space to N-Space without physically visiting the Gateway.

Planets of E-Space
Known planets in E-Space include Alzarius and possibly Terradon, the origin world of the starliner which crashed on that world. (DW: Full Circle). We also know of the un-named planet which was the hiding place of the last Great Vampire. (DW: State of Decay) The Tharils presumably had a homeworld of some kind as well, as did the unidentified humanoid races which they variously enslaved and were enslaved by. (DW: Warriors' Gate)