Why? Okay, here is why.
First, lets remind everyone that Neptune is a much bigger planet than the earth. It has 17 times more mass than the earth. I know y'all already know this, but perhaps somebody has forgotten.
Second, the mass of Jupiter is 318 times the mass of the earth.
Third, the science and techniques in this area are advancing rapidly. Finding planets in other solar systems is a new field. These techniques were only perfected recently. Since then, the number of planets being found in other solar systems is going up rapidly as these techniques are applied. However, these techniques in their infancy. Currently they are still only capable of finding giant planets.
The first planets to be found were Jupiter sized. As the techniques have been improved, smaller and smaller planets have been found.
Eventually we will develop techniques that are sensitive enough to find earth sized planets. See the article below on one of the latest developments.
So it really is only a matter of time. The universe is full of earth sized planets.
http://cfa-www.harvard.edu/afoe/espd.html
http://planetquest.jpl.nasa.gov/index.cfm
http://planetquest.jpl.nasa.gov/TPF/detectionMilestone.cfm
"TPF engineers achieve planet-detection milestone
(PLANETQUEST) -- For the first time ever, engineers working on JPL's Planet Detection Testbed have demonstrated that they can separate light as faint as that from a distant Earth-like planet from the blinding glare of its parent star. This breakthrough represents a major step toward the ultimate goal of the Terrestrial Planet Finder missions: observing habitable planets around nearby stars.
"The challenge in detecting an Earth-like planet is that the planet is a million times fainter than the host star when viewed in infrared light," said Dan Coulter, project manager for the Terrestrial Planet Finder Interferometer (TPF-I). "By simulating these conditions on the testbed, we've shown that the flight instrument has the sensitivity needed to suppress the light from the central star, and to observe a planet in the habitable zone." The habitable zone is the region around the system's star where the temperature is right to sustain liquid water, considered an essential ingredient for life.
The Planet Detection Testbed works by suppressing the light from the star, while allowing the planet's light to pass through and reach the detector, said Stefan Martin, the testbed lead engineer.
TPF-I will work in concert with the Terrestrial Planet Coronagraph to provide comprehensive portraits of neighboring planetary systems. Both missions are envisioned to launch within the next 10-20 years. "
I agree. What I don't agree with or haven't seen any evidence of (which may or may not come in the future as technology advances) is the existence of earth-like planets (with ability to support life as we know it). The question is always answered mathematically and that does not satisfy me, but I guess that is all we have at this time.
So "faith" in probability enters into this question on a scientific level.