Eli5: Why do planets look like stars?
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Planets reflect light from the Sun. This light will have the same spectrum as sunlight with modification due to absorption by atoms on its surface or atmosphere.
This modified spectrum of light leads to the observation that Mars appears reddish and Saturn a creamy yellow. Stars emit light that covers the visible spectrum of light.
Hotter stars have a spectrum skewed towards the shorter wavelength end of the spectrum, so they appear bluish. Cooler stars have a spectrum skewed towards the red end, so they have a reddish colour
They don’t. They don’t twinkle like stars (as stars are single point sources of light, but planets are bigger). They also “wander” in the sky in different directions than the stars. That’s where the word comes from.
* Greek: Πλάνητες Planētes, “Wanderers”
Planets can look a bit like stars because stars just look like points of light too small to have any perceptual shape.
Due to geometry as things become more distant the amount of your visual field they cover reduces. Take a triangle and a circle, and if they are far enough away they will both just look like a dot.
A very distant star produces its own light while a planet reflects the light of our sun. It turns out that most planets we can see with the naked eye seem brighter than most stars, but that are all in that “pretty dim point of light” zone so they can be difficult to distinguish by the novice.
Planets reflect some light back out into space. Once you get far enough away you can no longer make out any details and the planets just look like a point of light in the darkness.
Some things to note though. Mars is about 200 million miles away from us. Venus is 33 million miles away. A lightyear, the distance light travels over one year, is 6 trillion miles. The nearest star is over 4 lightyears away and yet shines just as brightly as these planets. The Andromeda Galaxy, which is over 2.5 Million *lightyears* away, also shines brightly in the night sky, looking like just a single star. Our neighboring planets shine like stars because they are so close compared to the incredibly vast distances the light from neighboring stars or galaxies have to travel for the faint remnants of their light to reach us.
Planets reflect sunlight. To the naked eye planets look like stars but with binoculars you can see they are bigger. Stars always look like a point of light and planets have a definite disc visible with even low magnification.
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