Looking Up: The Spectral Light Knows

Jan 27, 2020

This image from NASA's MESSENGER spacecraft uses spectral data to highlight various minerals, features and properties on Mercury.
Credit NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington / nasa.gov

This week on Looking Up Bruce Bookout takes on a listener question. 

One of our listeners, Scott K., asked a great question that is the basis of how we know, what we know about the universe. His question is: “How do astronomers determine things such as the existence of water and/or the estimated temperature on a planet that is light years away?”

An exoplanet is any planetary body in orbit around another star that is not our sun, Sol.  Currently, we have detected and confirmed over 4000 exoplanets around other stars.  There are multiple methods of detecting these planets, yet one technique allows us the insight of details of the exoplanet itself.

When a given exoplanet transits across its star’s face, we can directly measure the changes to the spectra of that star. Any new spectral lines, whether they are emission or absorption lines, are the fingerprints of the elements and compounds in that exoplanet’s atmosphere.  So far, we have detected hydrogen, helium, sodium, carbon dioxide, methane, and water in the atmospheres of some exoplanets.

As for the temperature, this is also provided by the spectral light. How bright a particular color appears is a direct relation to the temperature of the object. 

This image shows examples of spectra for various elements.
Credit nasa.gov

Thanks Scott K for a great question; I hope this has shed a little light on that subject.

If you’d like to take a closer look at exoplanet spectroscopy, or any of the other wonderful and amazing things in the sky, please visit csastro.org for a link to information on our monthly meetings and our free public star parties.