Researchers at the University College London (UCL) used a supercomputer to compute 10-billion “transition lines” of the spectral signature of methane, 200 times more comprehensive than previous best efforts. As methane is a biosignature, the development is an advancement toward the detection of life in planets outside our solar system.
Every molecule absorbs and emits light in a characteristic pattern called the absorption and emission spectrum. In order to determine the atmospheric composition of the exoplanets, astronomers break down the full atmospheric spectrum into known patterns to identify the component molecules.
Detecting methane in the study of astrobiology is important because it is an unstable molecule in the atmosphere that only lasts 300-600 years as it is broken down by solar ultraviolet radiation. Since it is unstable, one explanation for its presence on an exoplanet’s surface is continual production by a carbon-based life. The caveat is that geological processes also replenish methane so detection is suggestive of but does not guarantee life.
The previous methane spectra are incomplete in that they contain far fewer transition lines than the new effort so do not properly reflect methane in high temperature atmospheres (i.e. hotter than Earth). At high temperatures there are more transitions because the methane molecule is excited to higher energy states. As a result the methane levels of hot exoplanets and cool stars are detected only partially or incorrectly.
Moreover, the computational effort is difficult to duplicate via experiments alone as the difficulty scales with the number of lines to be characterized. This is why previous efforts such as HITRAN in 2012 that involve a combination of experimentation and computation yielded only about 650,000 spectral lines.
The development of the novel spectrum was led by Dr. Sergei Yurchenko and principal investigator Professor Giovanna Tinettia.
The scientists tested their new spectral list which they call 10to10 by successfully predicting the absorption spectrum in failed stars known as brown dwarfs. Moreover, Dr. Yurchenko and colleagues showed that a hot Jupiter called HD 189733 b has 20 times more methane than previously believed. HD 189733 b is an exoplanet resembling Jupiter located 63 light years from Earth with average surface temperature estimated at 1,000 degrees Celsius.
The novel spectrum was created using advanced supercomputers provided by the Distributed Research utilizing Advanced Computing (DiRAC) project run by the University of Cambridge. The calculations is said to have required about 3 million computing hours.
The findings of this study were published in the Proceedings of the National Academy of Sciences (PNAS).
(Photo Credits: NASA Goddard / Creative Commons)