Rocky planets larger than Earth could not sustain life because their thick, crushing hydrogen atmospheres would be deadly.
Research led by Dr Helmut Lammer, from the Austrian Academy of Sciences, used computer modelling to test the gaseous envelopes of different sized planets. Planets that were too small did not have enough gravity to keep in their atmospheres, whereas planets that were too large had atmospheres that were crushingly thick.
The ideal size was a planet between half and 1.5 times the mass of the Earth, with a similar density.
The findings have dashed the hope of finding habitable super Earths, planets larger than ours but smaller than Neptune.