Explain the radial velocity method of exoplanet/exomoon detection and its significance in expanding our understanding of planetary systems.

The radial velocity (RV) or Doppler method detects unseen companions indirectly — from the tiny to-and-fro motion a companion's gravity induces in its host body. It is one of the five principal detection techniques, alongside transit, direct imaging, microlensing and astrometry [1].

How the method works

• A planet and its host orbit a common centre of mass; the host therefore traces a small wobble along our line of sight [1].
• When the host recedes, its light is red-shifted; when it approaches, blue-shifted. High-resolution spectrographs measure this shift in spectral lines [1].
• The periodicity of the shift gives the orbital period; its amplitude yields the companion's mass and orbital distance [1].

Companion's pull → host wobbles
 host recedes  → spectral lines RED-shifted
 host approaches → spectral lines BLUE-shifted
 cycle repeats → orbital period + minimum mass

Doppler wobble underlying the radial velocity method

Significance for planetary science

• It produced the first planet found around a Sun-like star and seeded a catalogue that crossed 6,000 confirmed exoplanets in 2025 [2][3].
• It gives mass, which transit photometry cannot; combining both yields density and hence whether a world is rocky or gaseous [1].
• Extended to sub-stellar hosts, it detected in 2026 the first plausible exosatellite — a companion of minimum mass ~0.9 Jupiter around the brown dwarf CD-35 2722 B, using CRIRES+ on ESO's Very Large Telescope [3][4].
• It thus revealed a three-tier hierarchy (star → brown dwarf → satellite) and blurred the moon–planet boundary, reopening classification debates much as Pluto once did [3].
• Its limits are honest ones: it yields only a minimum mass and favours massive, close-in companions [1] — which is why the CD-35 2722 result is termed "plausible", not confirmed [4].

A century-old physical principle, sharpened by modern spectrographs, has moved astronomy from counting planets to mapping whole hierarchical systems. Continued investment in large ground-based facilities and international collaborations — the kind India joins through global observatory partnerships — will let follow-up observations confirm such candidates and refine how we define worlds themselves.