The reason for this? Most of the spotted exoplanets outside of our system are twice the size of earth, and so fall in to that range of probability. Though there maybe smaller planets, they reflect much less light an so have evaded discovery by our telescopes thus far.
“We have looked at how mass relates to the radius and developed a model which might explain the relationship”, said Li Zeng.
“The model indicates that those exoplanets which have a radius of around x1.5 Earth radius tend to be rocky planets (of typically x5 the mass of the Earth), while those with a radius of x2.5 Earth radius (with a mass around x10 that of the Earth) are probably water worlds,” he added
As far as water in our own local solar system is concerned the Italian Space Agency recently revealed evidence of liquid water below the surface of Mars south pole. Dr. Vladimir Airapetian has mentioned that it May take 3 years to find life on mars if it does indeed exist, given these new findings.
On the moon scientists are now certain there exists water ice at both the north and south pole, and possibly even at mid-latitude.
On Saturnian Moon Europa and Jovian Moon Enceladus we already knew there were expansive planet wide oceans just below a cold surface of solid ice. What we didn’t discover until relatively recently was organic chemistry on enceladus – spewing from plumes of water that erupt from the moons icey surface we found organic compounds, that is – carbon molecules that aggregate to form life here on Earth.
Furthermore, new research from Europa suggests that life may exist only 1cm below the icey surface.
EDIT: Even more recent surveys of Jupiters red spot have revealed the existence of water vapor on the solar systems largest gas giant. Over the past month evidence of water in space has exploded to a point were its now entirely transformed our perspective of extraterrestrial life and the feasibility of space colonization.
Mention Enceladus organic compounds
Europa 1cm below ice