Which method detects exoplanets by measuring Doppler shifts in starlight caused by orbiting planets?

Which method detects exoplanets by measuring Doppler shifts in starlight caused by orbiting planets?

• A. Transit photometry
• B. Direct imaging
• C. Radial-velocity method
• D. Gravitational microlensing

Answer: (C)

The main idea is using the star’s motion caused by an orbiting planet to detect the planet itself. As a planet orbits, its gravity makes the star wobble slightly, moving toward and away from us. That back-and-forth motion shifts the star’s spectral lines in wavelength through the Doppler effect: the lines blueshift when the star moves toward us and redshift when it moves away. By measuring these periodic shifts with high-precision spectroscopy, we can infer the presence of a planet, its orbital period, and a minimum mass (the true mass depends on the inclination). This approach is the radial-velocity method, also known as Doppler spectroscopy, and it specifically relies on detecting Doppler shifts in the star’s light rather than viewing the planet directly or watching for brightness dips. Other techniques—transit photometry detects dips in brightness when a planet passes in front of the star, direct imaging tries to see the planet itself, and gravitational microlensing uses light bending by gravity—don’t rely on Doppler shifts in the star’s spectrum.

The main idea is using the star’s motion caused by an orbiting planet to detect the planet itself. As a planet orbits, its gravity makes the star wobble slightly, moving toward and away from us. That back-and-forth motion shifts the star’s spectral lines in wavelength through the Doppler effect: the lines blueshift when the star moves toward us and redshift when it moves away. By measuring these periodic shifts with high-precision spectroscopy, we can infer the presence of a planet, its orbital period, and a minimum mass (the true mass depends on the inclination).

This approach is the radial-velocity method, also known as Doppler spectroscopy, and it specifically relies on detecting Doppler shifts in the star’s light rather than viewing the planet directly or watching for brightness dips. Other techniques—transit photometry detects dips in brightness when a planet passes in front of the star, direct imaging tries to see the planet itself, and gravitational microlensing uses light bending by gravity—don’t rely on Doppler shifts in the star’s spectrum.