Which observational technique uses Doppler shifts to detect exoplanets around stars?

Which observational technique uses Doppler shifts to detect exoplanets around stars?

• A. Transit photometry
• B. Direct imaging
• C. Doppler spectroscopy
• D. Gravitational microlensing

Answer: (C)

The technique relies on watching the star’s spectrum for periodic Doppler shifts as the star wobbles in response to an orbiting planet. When the star moves toward us, its spectral lines shift toward shorter wavelengths; when it moves away, they shift toward longer wavelengths. Measuring these shifts over time reveals a radial velocity curve: the orbital period and the velocity amplitude, which give the planet’s minimum mass (the true mass depends on how the orbit is tilted relative to us). This method works even if the planet doesn’t emit light or transit the star, making it a powerful way to detect exoplanets. Transit photometry detects planets by dips in brightness when a planet passes in front of the star, direct imaging looks for the planet’s own light, and gravitational microlensing uses lensing events caused by gravity—none of these rely on Doppler shifts in the star’s spectrum.

The technique relies on watching the star’s spectrum for periodic Doppler shifts as the star wobbles in response to an orbiting planet. When the star moves toward us, its spectral lines shift toward shorter wavelengths; when it moves away, they shift toward longer wavelengths. Measuring these shifts over time reveals a radial velocity curve: the orbital period and the velocity amplitude, which give the planet’s minimum mass (the true mass depends on how the orbit is tilted relative to us). This method works even if the planet doesn’t emit light or transit the star, making it a powerful way to detect exoplanets.

Transit photometry detects planets by dips in brightness when a planet passes in front of the star, direct imaging looks for the planet’s own light, and gravitational microlensing uses lensing events caused by gravity—none of these rely on Doppler shifts in the star’s spectrum.