A Tale of Two Atmospheres

When on the hunt for exoplanets, there is only one thing on the minds of everyone - Aliens. The search for extrasolar planets is directly related to the pursuit of extraterrestrial life and a second home for our species.

This relatively new field of astronomy has opened up many flabbergasting doors in the realm of planetary astronomy. Many different types of planets have been discovered on our journey. One of the newer types we have come to discover is a category we call super-Earths or mini-Neptunes.

An artist's rendition of Kepler 22b - a famous super-Earth

Needless to say, this category of exoplanets includes all planets with masses in between that of Earth and Neptune. The classification of these planets as super-Earths or mini-Neptunes depends on their atmospheric characteristics. If an exoplanet has a thick atmosphere of primarily helium or hydrogen, it is classified as a mini-Neptune. Conversely, if it has an atmosphere of carbon dioxide or nitrogen, it is a super-Earth.

When searching for extraterrestrial life, we generally look for super-Earths. They are big enough to find using the known methods, and they have atmospheres similar to our host planet's. Life as we know it can only exist on planets with such atmospheres. Mini-Neptunes are simply not suited for life as we know it.

MIT graduate student Björn Benneke has proposed a method for determining the atmosphere of this category of exoplanets, though it is limited to very specific scenarios.

When an exoplanet orbits its host star in a way that allows us to see it cross the star, we are able to observe the light of the star as it goes through the planet's atmosphere. We are then able to divide it into it's component wavelengths and determine what elements are interfering with its travel. These elements interfering with the light's travel are the elements that reside in the exoplanet's atmosphere. Once these are identified, we are then able to identify the exoplanet as a super-Earth or a mini-Neptune.

A representation of a planetary transit of a star

The major problem of this type of exoplanet identification is that it is very limited. We need to be able to see the planet cross in front of its host star. This happens in a very small fraction of the exoplanets we have identified, as it is a geometric rarity.