It also turns out that atmospheres can be used to understand the history of an exoplanet, not just its current state. An international team of scientists analyzed the atmosphere of HD 209458b using high-resolution spectra gathered by the Telescopio Nazionale Galileo in La Palma, Spain. 

As an exoplanet transits in front of its host star, the light from the star passes through the planet’s atmosphere, exciting the molecules such that they give off energy as they return to their original states, and the light given off by that energy means we can figure out what the molecules are. In this case, the astronomers detected hydrogen cyanide, methane, acetylene, carbon monoxide, ammonia, and a bit of water vapor. 

The first four of those are all carbon-based molecules, and that is incredibly unexpected for a planet as close in to its star as HD 209458b is – a mere seven million kilometers away – which leads the team to propose that this exoplanet actually formed much farther out, far beyond the point where water changes from liquid to gas. Coauthor Dr. Siddharth Gandhi notes: There is no way that a planet would form with an atmosphere so rich in carbon if it is within the condensation line of water vapor. At the very hot temperature of this planet (1,500K), if the atmosphere contains all the elements in the same proportion as in the parent star, oxygen should be twice more abundant than carbon and mostly bonded with hydrogen to form water or to carbon to form carbon monoxide.

Remember, this is a giant planet, not a rocky body, so it should have the same composition as its star, and it does not. Every new paper teaches us something more about planetary formation, and we keep inching closer and closer to finding that first truly habitable exoplanet.

More Information

University of Warwick press release

“Five carbon- and nitrogen-bearing species in a hot giant planet’s atmosphere,” Paolo Giacobbe et al., 2021 April 7, Nature