Planets are celestial bodies that revolve around the sun. The constellation you are talking about is generally a "group" planned by human beings to know the distant stars outside the solar system. Turn several seemingly close stars into a constellation.
The basic definition of a planet is that it does not emit light or heat itself, but reflects the light of a star. Such as Earth, Mars, Mercury, Venus and so on.
This star will glow and heat. For example, our sun, the Big Dipper and so on.