Exoplanets are difficult to detect because they are:

Explanation:

Exoplanets are very hard to see directly with telescopes. They are hidden by the bright glare of the stars they orbit. So, astronomers use other ways to detect and study these distant planets.

Exoplanets are difficult to detect because they are a star 'wobbles' as it orbits the center of mass, changing the wavelength of light it emits.

i think the answer would be reserve.