Exoplanets are detected indirectly using the transit method and radial velocity method

Why exoplanets are hard to detect directly

• Exoplanets are extremely difficult to observe directly because:
• The light from the host star is vastly brighter than the reflected light from the planet.
• They subtend extremely small angles compared to the resolution of telescopes.
• Astronomers must therefore use indirect detection techniques. The two main methods are the transit method and the radial velocity method.

The transit method

• When a planet passes in front of its host star (as seen from Earth), it blocks some of the starlight.
• By measuring the apparent magnitude (brightness) of the star over time, a light curveA graph of the brightness or intensity of a star plotted against time. Periodic dips in a light curve can indicate the presence of an orbiting planet or an eclipsing binary companion. can be obtained.
• The light curve shows a characteristic periodic dip each time the planet transits in front of the star.
• From the transit data:
• The depth of the dip tells us about the size of the planet (a larger planet blocks more light).
• The duration of the dip can be used to determine the orbital period of the planet.

• Limitations of the transit method:
• The accuracy is reduced if the Earth, planet, and star are not aligned in the same plane.
• Only planets with a relatively short orbital period can be detected, as multiple transits need to be observed.

The radial velocity method

• As a planet orbits its host star, both the planet and the star orbit around their common centre of mass.
• During the orbit, the star moves slightly towards and then away from the Earth as the planet moves to different positions in its orbit.
• The line spectrum of the star will show blueshift when the star moves towards the Earth, then redshift when it moves away.
• This causes very small but measurable periodic shifts in the wavelength of the light received from the star.
• The key part is that the time period of the planet's orbit is equal to the time period of the Doppler shift variation.

• The main limitation of this technique:
• Low-mass planets (like Earth-sized planets) do not cause as much "wobble" as high-mass planets, since they have a smaller gravitational pull on the star. This makes small planets much harder to detect.