Purposeful Design
The Explanation
Intelligent Design considers whether the collection of conditions that permit life on Earth is better understood as more than an accidental planetary outcome.
The argument does not depend upon any single feature. Earth's distance from the Sun, water, atmosphere, chemistry, geological activity, climate regulation, planetary mass, orbital properties, and other characteristics operate together as parts of one changing system.
The design inference therefore becomes cumulative. The question is whether the coordinated life-permitting character of the complete system is reasonably suggestive of purpose.
Supporting Evidence
Earth possesses persistent liquid water, an atmosphere compatible with its present biosphere, abundant life-related chemical elements, a long-lived stellar energy source, and geological and chemical systems that have permitted habitable environments to persist through immense periods of planetary change.
Many of these conditions are interconnected. Changing one part of the system can affect several others.
This integrated character of habitability provides the principal basis for considering a design interpretation.
Limitations
Science presently has only one known inhabited planet. That makes it difficult to determine statistically how unusual Earth's complete combination of conditions really is.
Some characteristics once described as strict requirements for habitability have also proved less restrictive when investigated with improved models.
A design argument should therefore avoid unsupported probability claims and should not depend upon asserting that Earth's exact properties are the only possible conditions under which life could exist.
Questions That Remain
How common are comparably habitable planetary systems?
How many of Earth's characteristics are necessary for complex life, and how many are merely one possible way of achieving habitability?
As exoplanet observations improve, will Earth appear increasingly ordinary or increasingly unusual?
Intelligent Design Response
The Intelligent Design interpretation does not require every terrestrial characteristic to be individually fine-tuned or scientifically inexplicable.
Instead, it asks whether the cumulative organization of a planet capable of sustaining complex life is reasonably consistent with purposeful arrangement.
For the Christian, purpose in creation is already a theological conviction. Intelligent Design approaches the question more narrowly by asking what may reasonably be inferred from the natural evidence itself.