A hole in one of the methods proposed to seek extraterrestrial life has been patched, providing astronomers with better tools for studying cool stars and “hot Jupiters”, with benefits that may extend to the search for candidates to host life.
The detection of methane (CH4) is one of the major quests of modern astronomers, but they've been using incomplete tests. Methane is produced in volcanoes on Earth (and probably on Titan), but it has a limited life in most environments, so its presence suggests constant refreshment - quite possibly from living things, which produce 90% of the methane in our own atmosphere.
Moreover, methane produced by microbes has a higher ratio of carbon-12 to carbon-13 than that from non-living sources, so in those cases where the isotopes can be measured it could provide an even more powerful indication of the presence of life.
Atoms in hot gasses release distinctive spectral lines that allow us to look at the light emitted and determine the presence of specific elements, while light passing through gasses is absorbed at the same wavelengths. However, hydrogen and carbon are so common through the universe that finding them is no surprise. Detecting molecules is harder. The combination of a carbon atom and four hydrogen atoms gives a unique spectrum of electron transitions, allowing methane to be distinguished from those atoms combined with other elements.
However, the wavelengths of these transitions vary depending on the temperature at which they occur. A paper in Proceedings of the National Academy of Sciences observes, “Previous methane data are incomplete, leading to underestimated opacities at short wavelengths and elevated temperatures.” Astronomers operating with an incomplete list of possible transitions will miss the presence of methane, or underestimate its frequency, when studying a light source.
The authors have responded by providing a calculating 9.8 billion transitions covering temperatures up to 1500K across infrared, visible and ultraviolet light. By comparison, experimental studies captured a third of a million. "Current models of methane are incomplete, leading to a severe underestimation of methane levels on planets. We anticipate our new model will have a big impact on the future study of planets and 'cool' stars external to our solar system,” says University College London (UCL)'s Professor Jonathan Tennyson, one of the authors.
"The comprehensive spectrum we have created has only been possible with the astonishing power of modern supercomputers which are needed for the billions of lines required for the modeling,” said lead author, UCL's Dr Sergei Yurchenko. “We limited the temperature threshold to 1,500K to fit the capacity available, so more research could be done to expand the model to higher temperatures still. Our calculations required about 3 million CPU (central processing unit) hours alone.”
Experimental studies had found all the most common transitions, and therefore the ones that show up most strongly when analyzing light from sources containing methane. However, the authors note, “Even though the lines below the cutoff are very weak, the very large number of them means they contribute significant total absorption.”
Most of the missing transitions occur at temperatures far too high for life – no one is expecting to find methane producing microbes the atmosphere of planets hotter than Venus. However, even at room temperature Yurchenko and his colleagues detected previously unknown weak transitions, and including these in future studies could improve the chances of detecting methane on planets at temperatures similar to Earth, albeit only slightly.
The paper will have applications closer to home. The authors note, “Studies of many topics [including] halon flame inhibitors, combustion, gas turbine energies and exhausts all rely on an understanding of the spectroscopy of hot methane."