= Solution
An Earth analogue has an iron-silicate interior and a thin secondary nitrogen-dominated atmosphere with water and carbon dioxide. Contextual coexistence of oxygen or ozone with methane and surface-water indicators would be a target biosignature. Its small scale height and extreme reflected-light contrast put a true Sun-Earth analogue beyond routine current atmospheric work, but future large direct-imaging missions and the <Extremely Large Telescope> target nearby terrestrial planets.
A temperate M-dwarf rocky planet has a similar solid interior but may retain carbon-dioxide, nitrogen, or water atmospheres under synchronous rotation. Water, carbon dioxide, methane, and contextual oxygen chemistry are observable targets. Its small host gives deeper transits, so the <James Webb Space Telescope> and large ground telescopes can test nearby systems, although stellar activity and atmospheric erosion complicate interpretation.
A <Hycean planet> or water-rich sub-Neptune has a water-rich interior or ocean below a hydrogen-rich atmosphere. Methane, carbon dioxide, ammonia, water, and proposed sulfur-bearing biosignatures must be interpreted against abiotic photochemistry. Its large radius and hydrogen scale height make transmission spectroscopy comparatively favorable for JWST and future <Ariel space telescope> surveys, but whether a clement ocean exists beneath the atmosphere remains model-dependent.
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