Extrasolar planets, also known simply as exoplanets, are planets that exist outside of the solar system. Just like the solar system, exoplanetary-star systems consist of different planet types such as gas giants and terrestrial planets. Gas giant planets are mostly made up of Helium & Hydrogen. They are much larger in size and tend to not have any solid surfaces. Terrestrial planets are mostly composed of a solid surface made of rocks and metals and, in comparison to gas giants, are much smaller in size.

Starlight provides most of the information about an orbiting planet. Any light radiated by the planet will be too faint compared to its parent star when observed from Earth. For this reason, starlight is the primary source of information about an exoplanetary star system. As a reference the sun is about a few billion times brighter than the Earth.

The luminosity of the Sun is Lsun = 3.8×1026 W (using its full surface area).

The luminosity of the Earth can be determined by considering the flux of sunlight incident on its surface (assuming it is a disk) and then calculating how much of this is reflected back into space:

Learth = Lsun(π)(Rearth)2(reflection coefficient known as albedo)

Therefore,
3.8×1026(π)(6.4×106)(6.4×106)(0.3) = 5.3×1016 W.

Hence,
Lsun/Learth = 3.8×1026/5.3×1016 = 7.2×109