With current experimental build 2.91 you can base this on Volume Object.
create the shape from a Mesh Object.
add Volume Object and in modifiers choose Mesh to Volume, as source Object choose created shape one
add Point Light object in trail direction (close to a sphere)
With second modifier Volume Displace you can use texture to distort. Here is a few ...
You can do it with particles:
Create your meteor, give it a particle system, in the Field Weights switch off the Gravity, in the Velocity give a bit of Normal and Object Aligned X to give the particles a direction. Create a stretched sphere that will be your particle, in the particle system Render panel choose Render As > Object and under Object > ...
Watts per square meter per steradian. See: https://devtalk.blender.org/t/cycles-unit-of-light-energy-attn-brecht/12456
Edit Below is the previous version of the answer because it is informative, even if it doesn't directly answer the question:
There are many factors that determine the values of the pixels. The units are simply luminance for ...
It can be hard to spot, if you don't know, but Blender's factory settings give the World a default color of (0.051,0.051,0.051), with an emissive strength of 1.
So you're right: there is "Somehow, another source of light" in your scene.
You've plugged some image textures into the alpha socket of your Principled BSDF nodes, in Eevee it doesn't create transparency by default as long as you haven't enabled Alpha Blend, but in Cycles it automatically creates transparency. What is your goal with this setup?
Alpha is useful for example if you want to use the alpha chanel of a picture in order to ...
This seems like a question about the performance of your specific components, which the Blender Stack Exchange is not able to help with as much.
You can reduce the viewport sample number (the default is 16) to shorten the time that it take to render (as you mentioned you did). You can also reduce the render samples and/or resolution of the camera to speed up ...
Following is the answer to the problem i had:
after conversing with @lemon , he/she as found a the answer.
Regarding the modifiers i was appliying, the subdivision surface modifier was set to 0 for the render as you can see in the image.
So, the smooth modifier was smothing a mesh that has to few vertices to be seen on screen.