Spitzer Space Telescope (2003–2020)

Spitzer discovered seven Earth-sized planets orbiting the TRAPPIST-1 star in 2017, a feat that required 500 hours of infrared monitoring impossible for Hubble in optical light. Its 16-year mission mapped the Milky Way's spiral arms using 1.2 million infrared sources and peered through interstellar dust clouds to reveal star-forming regions 10 times fainter than any optical survey. While #4's resolution is lower than JWST's, its 85-centimeter mirror captured mid-infrared wavelengths (3–180 microns) that neither Hubble nor Chandra can access, providing the only complete map of the galaxy's obscured core.