I have been reading a paper on about exoplanets, specifically measuring the temperature of exoplanets using infrared telescopes. The paper compares exoplanets with neutron stars (NS) remarking that while a typical NS has a radius of about 10 km, exoplanets of interest have radii of about 50,000 - 200,000 km. This means that the temperatures of exoplanets can be measured much further into the Galactic Center (GC) [page 2 of the aforementioned paper]
I am not trained in Astronomy, therefore I cannot completely understand the link between a (relatively) larger surface area and looking much further into the GC.
I would appreciate books, papers or any reference elucidating this.