How do we know that the light coming off from distant planets is as a result of red shift or the properties of the elements prevalent on the plant?

Question

My understanding is that when an object moves away from you it increases the wavelength of the light presenting with a colour that is on the red side of the colourscale. Scientists also are able to determine what elements are present on planets based on the light it emits. How can they tell the difference?

I’m sorry for possibly using the incorrect terminology! Thanks in advance folks!

Edited the post because previously I had suggested that shorter wavelengths tended to the red side when in fact longer wavelengths tended to the red side of the colour spectrum.

In: 17

Answer 1

We’re looking at the light of distant stars, not planets. Stars are bright, planets are not.

Every element releases a specific set of frequencies when it’s excited. This is actually how we discovered helium. We looked at the sun and saw peaks of light that didn’t match uo with any known element, so it was named “helium” after the Greek word for sun, Helios, because the sun is the only place we knew this element existed. Later, we were able to isolate it here on Earth.

When we look at a distant star, we can see a set of peaks, usually hydrogen, since the universe is about 73% hydrogen today, and the further back in time you go, the more hydrogen there is. We compare the peaks of hydrogen to the peaks we observe, and they’re all a little closer to the red end of the spectrum, so we say they’re red shifted, which means they’re moving away from us.

We can also look at type 1a supernovae. A type 1a supernova occurs when a white dwarf and a living star are in the same system. The white dwarf starts stealing hydrogen from the parent star until eventually it stole enough to reignite fusion. This always happens with the same amount of mass, so the resulting flash is always the same. When we see them happen close, there’s no redshift, and when they happen far away, there is.

The type 1a supernova method is applied to stars that are close enough that we can see individual stars. After that, we start relying on redshift because we can’t see individual stars, but the glow of a galaxy as a whole.