Earth as One Successful Outcome
The Explanation
One possible explanation for Earth's remarkable habitability is simply that planetary systems have many different possible outcomes and Earth happens to be one that worked.
Stars and planets are extremely numerous. They form with different masses, compositions, orbital arrangements, atmospheres, geological histories, and environments. If nature produces a sufficiently large variety of worlds, some may develop combinations of conditions capable of supporting life.
On this explanation, Earth need not represent a specially arranged planet. It may instead be one successful result within an enormous population of planetary possibilities.
Supporting Evidence
Thousands of planets have been discovered outside our solar system, and observations reveal substantial diversity among planetary systems.
Modern astrobiology also recognizes that habitability can arise from different combinations of planetary properties. A world does not necessarily need to reproduce every feature of Earth in order to provide some form of habitable environment.
The large number and diversity of planetary systems therefore provide a reasonable basis for considering whether habitable planets can arise naturally somewhere among the many possible outcomes of planet formation and evolution.
Limitations
We do not yet know how frequently planets develop and maintain the complete collection of conditions required for life, especially complex life.
The existence of many planets by itself does not establish how common habitable planets are. Nor does it tell us how often life begins when suitable environments exist.
Because Earth remains the only known inhabited planet, the statistical frequency of Earth-like or comparably habitable outcomes remains uncertain.
Questions That Remain
How frequently do planetary systems produce long-lived habitable environments?
How many different planetary arrangements could support complex life?
How frequently does life actually arise when apparently suitable conditions exist?
Intelligent Design Response
Intelligent Design does not require Earth to be the only habitable planet. Discovering many habitable or inhabited worlds would therefore not by itself rule out design.
The design question concerns whether the underlying system that produces habitable environments is reasonably understood as the result of undirected processes alone or whether its life-permitting character provides evidence of purpose.
The number of planets is therefore relevant, but it does not by itself settle the question.