[Pseudoscience-is-malarkey (OP)]

On the rare occasion that we can see exoplanet images all we're looking at is a few pixels at best. Obviously we have other data such as distance from its star, orbital speed, mass, and on some really rare occasions it's diameter and rotation speed. Obviously these are facts everyone on this subforum knows. What I cannot figure out is how astrochemists can sometimes say with certainty what an exoplanet is made of. For example, the scary exoplanet 16 Cygni Bb is said (not presumed) by them to be abundant in lithium.

[Bored chemist]

https://en.wikipedia.org/wiki/Spectroscopy

[Eternal Student]

Hi.

   Spectroscopy is the main method.   

   You can't find everything, not everything would be in the atmosphere and interact with light.   Some things are hidden away from spectroscopic analysis.    However, scientists can make more inferences based on the Chemistry that should apply after knowing what is in the atmosphere.
    For instance, the carbon to oxygen (C/O) ratio, inferred from the abundance of (amongst others) carbon monoxide, carbon dioxide, methane, and water, essentially indicates whether the chemistry in the atmosphere is oxygen or carbon dominated.
   [Taken from "How To Search the Chemical Makeup of Exoplanet....",  https://www.universetoday.com/151519/how-to-search-the-chemical-makeup-of-exoplanet-atmospheres-for-hints-at-their-history/  ]

   When needed, the processes that may occurr are assumed to be similar to what can be observed in the planets of our own solar system.

      Titan?s atmosphere for instance is carbon dominated, leading to a hazy world with hydrocarbon lakes.
["How To Search the Chemical Makeup of Exoplanet....",  as above].

     There are other important ratios like the Deuterium to Hydrogen ratio.   As you may know, Deuterium is just heavy Hydrogen and conversion between the two requires nuclear reactions and not chemical reactions.   The ratio is then essentially fixed but the different masses of the atoms make chemicals containing them (like water) behave differently.  In particular the rate at which they will freeze is different and Hydrogen is much more readily expelled from the atmosphere than its heavier cousin Deuterium.   So the current ratio of  Deuterium to Hydrogen can indicate quite a lot about the history and evolution of the exoplanet and hence the chemical composition of it now.

Best Wishes.

[Zer0]

https://exoplanets.nasa.gov/

Thanks/Credits/Courtesy -
National Aeronautics & Space Administration.
(NASA)

[evan_au]

On the rare occasion that we can see exoplanet images all we're looking at is a few pixels at best

In most cases, the dim reflected light from a small planet is swamped by the extremely bright light produced by its much larger star.
- To be resolved as separate pixels by a telescope, the planet must be very far from its parent star (ie extremely dim), and a device called a coronagraph must be used to block out all light from the parent star
- To obtain a spectrum and identify specific elements/compounds from the spectrum of the reflected light requires far more photons than just detecting the presence of a planet in a pixel.

From what I have heard, obtaining a spectrum of an atmosphere is best achieved if the planet passes directly between the star and Earth.
- During this partial eclipse, the star provides lots of photons which pass through the planets atmosphere
- By subtracting the spectrum of the star (no planet in view) from the spectrum when the star is slightly blocked by the planet, you can deduce the planet's spectrum, and hence its atmospheric composition.
- A slightly less accurate method is when the planet does not directly pass in front of the star, but it is possible to compare the spectrum when the planet is near the far side of its orbit (ie fully illuminated by its star), and when it is on the "near" side of its orbit (ie we only see the dark side). This can give a reflection spectrum.
- These are difficult measurements, as the star is far brighter than the planet, and the planet's reflected/transmitted light makes only a tiny change to the star's spectrum.
- This technique still works even if the planet cannot be resolved as an individual pixel on even the biggest telescope (in fact, it works better if the planet is closer to the star...)

[Eternal Student]

Hi.

   ...and it isn't just the star or stuff behind the planet you have to worry about.    The light from the exoplanet would travel through some interstellar gas on its way to earth.   Hydrogen emission/absorption lines would always be seen in the spectrum.   However, the method of subtracting what is seen when the planet is not in the line of sight from what's in the spectrum when the planet is in line of sight,  still gives some hope of isolating the spectrum coming just from the exoplanets atmosphere.

Best Wishes.

[evan_au]

Another source of information about exoplanet composition can be applied to exoplanets which cross in front of their star:
- By measuring the degree of light dip during transit, astronomers can estimate the size of the planet
- By measuring the subtle red & blue shift of the star as the planet orbits the star, astronomers can estimate the mass of the planet
- With the mass and cross-section of the planet, astronomers can calculate the planet's density, and determine if it is a rocky planet, a gas giant, or somewhere in the middle...

By looking at the orbit, astronomers can determine the intensity of light from the parent star, which will constrain the chemicals that can exist in the atmosphere.
- Also by looking at the orbit, astronomers can take an educated guess as to whether the planet is tidally locked to the star.
      - Tidally locked planets will have searing temperatures on one side, and frigid temperatures on the other.
      - If there is a deep atmosphere, there will be incredible winds carrying heat from the hot side to the cold side
      - If there is a shallow atmosphere,  it will tend to freeze out on the cold side, leaving the planet without an atmosphere, and exposed to vacuum