How do astronomers find the difference between a 'Cephid Variable Star' and distinct normal stars with opaque objects blocking it's light, like stars with planets revolving around them while both create a dip in the brightness-time graph at regular intervals of time, when this is how new stars are discovered?

Cepheid variable stars and exoplanets transiting stars have very different light curves (the relation between brightness and time).

Exoplanet light curve from NASA:

Exoplanet light curve from NASA

Cepheid variable light curve from astronomynotes.com:

Cepheid variable light curve from astronomynotes.com

Also, as Rory Allsop points out, the scale of the change in brightness is very different. Cepheid variables can be seen at great distances, which allows us to use them to measure distances by their regular period-brighness relation, whereas exoplanets can only be found more locally by this method.

A Cepheid Variable is:

a type of star that pulsates radially, varying in both diameter and temperature and producing changes in brightness with a well-defined stable period and amplitude. There is a strong direct relationship between a Cepheid variable's luminosity and pulsation period.

(From Wikipedia)

A star with planets does not give a regular dip in the same way, because orbits are not going to be exactly in line with our view, and the dip due to a planet is incredibly small. From spiff.rit.edu:

micromags: one micromag is one-millionth of a magnitude, or 0.000 001 magnitudes. People who look for transits by terrestrial planets sometimes use this unit: Earth produces a dip about 100 micromags deep.

There is no direct relationship between luminosity and period.

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