Chloromethane (CH3Cl) is a common product of industrial processes that is also released by phytoplankton and some plants. However, this doesn't explain its presence in space. The discovery of its spectral fingerprint around the newly formed star IRAS 16293-2422, and in the atmosphere of comet 67P/Churyumov-Gerasimenko, indicates that chloromethane can be made without biology.
This is exciting news for astrochemists, who have never seen a member of chloromethane's class of molecules in such environments before. But it's worse news for those hoping to use methyl chloride as an indicator of the presence of life.
IRAS 16293-2422 lies about 400 light-years from Earth, part of a star-forming region spawning stars with masses similar to the Sun. It is enveloped by gas and dust expected to eventually become planets. The Atacama Large Millimeter/submillimeter Array (ALMA) has been searching this surrounding gas for spectra indicating the presence of molecules previously unseen in such locations.
Among these is the class known as organohalogens, which include chloromethane, also known as methyl chloride.
"Finding organohalogens near these young, Sun-like stars was surprising,” said Dr Edith Fayolle of the Harvard-Smithsonian Center for Astrophysics in a statement. "We simply didn't predict its formation and were surprised to find it in such significant concentrations. It's clear now that these molecules form readily in stellar nurseries, providing insights into the chemical evolution of solar systems, including our own."
The presence of chloromethane at this stage of stellar development suggests it is likely to be incorporated into planets as they form. It may have helped make up the early Earth, affecting the conditions under which the first life formed.
Further evidence for this theory arises from the detection of the molecule by the Rosetta spacecraft in the gasses blown off comet 67P. Both discoveries were announced in the same paper in Nature Astronomy. The concentrations detected by Rosetta and ALMA were quite similar, suggesting not a lot is destroyed as comets form from the material at the edges of star systems. The double finding also helps explain Curiosity’s detection of chloromethane on Mars, possibly originating from comets or asteroids.
Chloromethane both from human production for refrigeration and from other living things is so common in the Earth's atmosphere that it has been proposed as an indicator of the presence of life on other planets. That now looks to be unlikely, as we know it could have arrived in other ways.
High concentrations might still be indicative of technological sources, but that is a two-edged sword. As one of the chemicals responsible for destroying the ozone layer, stratospheric chloromethane might indicate a planet more like Earth than we'd like – one inhabited by a species that doesn't look after it.