The quest for extraterrestrial planets.
Stars have planets. At least, that was current wisdom for centuries, but only out of inference. The Sun does not look much as a peculiar star to believe our planetary system is special. But then, to assume other stars must have planetary systems similar to ours is - as said - based only on inference.
The first part of this series deals with the technical hurdles to overcome in order to acquire experimental evidence for the existence of sister planetary systems, and the methods that up to now proved successful, as well as their inherent limitations.
Prove they exist
No direct observations are yet possible due to the overwhelming glare of stars around which they revolve.
Be it our eyes, photographic film, or digital cameras, imaging results from focusing light from the desired object on a surface studded with sensors.
Rod and cone cells are the light sensors in the retina, while embedded silver iodide crystals do it in photographic emulsions, and microscopic silicon semiconductor structures in solid state devices.
When a light photon strikes the sensitive element, ideally it should be totally absorbed and its energy converted to a suitable signal like a nerve impulse, a chemical transformation or an electric signal.
Most of the time it happens that way, but a small fraction scatters, impinging on adjacent sensitive elements. A photon originally directed by the imaging optics (crystalline, mirror, lens) to fall in a certain sensor, but scattered and thus exciting an unintended one, generates false information about the imaged object. The brighter the light source, the higher the amount of scattered photons smearing the resulting image.
Scattering on the detector surface is not the only source of glare, optics through which images are focused, can never do it perfectly. A small but nonzero proportion of incoming photons transverse unintended paths due to physical imperfections or naturally occurring phenomena like diffraction. The more intense the light source, the more noticeable the effect.
Furthermore, planets are intrinsically much smaller than stars. If it were otherwise, they should ignite thermonuclear reactions and become stars. They are much smaller and do not shine for themselves, but reflect light from a nearby star. Their effective brightness is therefore orders of magnitude smaller than that of stars.
To compound things, their distance to a central star is also orders of magnitude smaller than typical interstellar distances, so as to be gravitationally bound. All of which results in a situation that conspires against direct observation of planets other than the ones belonging to the Solar System. Too faint, too small, and too near to a star to escape from glare.
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