This is one of the most interesting
questions I have ever been asked on
The short answer on the discovery of the "new"
planet is that technology drives a big part of
discovery. One needs to photograph a piece of
the sky night after night and remove all the KNOWN
objects to help find the new ones.
Because objects in the solar system are much
closer than objects further away, they will tend
to move a lot relative to the distant "fixed"
stars....the main limitation is on the
The amount of sun light reflected is a
function of the SIZE of the object (its diameter,
say) and its REFLECTIVITY or albedo... that
is what fraction of the sun light impinging on it
is actually REFLECTED back to us. So a small
object at great distance simply does not show up
using the best we have ... say the Hubble
telescope. There is a threshold below which
even the Hubble cannot "see" a small low albedo
The other factor is the patience of the
observer. One has to look at LOTS of data to
find a dim small distant object and that takes a
lot of effort and time. As the technology advances
(better detection limits, ability to remove
spurious effects, etc), then we can push the
detection limit downwards... But we will NEVER be
able to see from Earth say a 100 meter sized
object at the distance of say Pluto... its just so
dim that it can not be seen.
The detection limits ARE being pushed down
because there are dedicated teams of
astronomers and experts in optics that are
interested to work on this issue.