Science advances not because reality changes, but because humanity’s instruments, theoretical frameworks, and willingness to question assumptions evolve. Throughout scientific history, ideas once dismissed as impossible—heliocentrism, continental drift, deep-sea ecosystems, and subsurface microbial life—were later validated as observational tools and conceptual models improved. This recurring pattern highlights a fundamental principle: absence of detection is not evidence of absence, but often a reflection of instrumental limitation. This paper proposes a testable scientific hypothesis that challenges the surface-centric paradigm of astrobiology: if life exists beyond Earth, it may reside within planetary interiors rather than on exposed surfaces. Gas giants and terrestrial planets alike exhibit extreme surface conditions—radiation, pressure, and thermal instability—that are hostile to complex life. However, internal planetary environments may offer comparatively stable regimes governed by pressure balance, thermal gradients, magnetic dynamics, and internal energy redistribution. The hypothesis does not assert proof, but invites scientific scrutiny. Planetary interiors remain among the least explored domains in modern science, not due to falsification, but because of technological constraints. As with prior scientific revolutions, today’s speculative questions may become tomorrow’s measurable realities. The boundaries of science, therefore, should be understood not as limits of reality, but as temporary limits of measurement.