Invisible alien

As described by Rose Tyler, "invisible aliens", or more specifically, the emergency program of their spaceship, had killed Sebastian Marten and Paul Steiffers on 1920 Earth.

Biology
The aliens had tiny, ovoid bodies with vestigial limbs. They were telepathically linked to their spaceship. (PROSE: Doctor vs Doctor)

History
Due to engine trouble with the spaceship, the aliens' spaceship had been trapped between real space and the time vortex. Due to their telepathic link to the spaceship's computer, once the engines failed, the crew died as well. According to Rose, while the aliens, or the program, had tried to "get out", it generated "an energy field that kills people", including Sebastian Marten, Paul Steiffers and Bart Faversham. When the Ninth Doctor and Rose signalled the ship with the standard galactic code, the computer teleported them up, and they found three bodies on board and realised the emergency program had been trying to escape on its own. (PROSE: Doctor vs Doctor)