The European Space Agency (ESA) has some good news and bad news for extraterrestrial enthusiasts. The good news is that a molecule thought to be a biomarker for life has been found for the first time in abundance in a comet and around a young star. The bad news is that the find indicates that the molecule isn't the clear indicator of life that it was once believed to be.

Since we can't exactly spot microscopic lifeforms from afar, astronomers have adopted other ways to measure a particular planet's likelihood of housing alien life. Traces of certain compounds left by organic processes, often called biomarkers, can be sifted out of soil or water samples by rovers, or detected in the atmosphere by telescopes and orbiters.

Methyl chloride is fairly common here on Earth, belonging to a class of molecules known as organohalogens. These organic compounds are made up of carbon bonded to at least one halogen – fluorine, chlorine, bromine or iodine – and are produced mostly through biological processes. In theory, that means that any celestial body where we detect an abundance of these is a good place to look for life.

Recently, the Atacama Large Millimeter/submillimeter Array (ALMA) telescope in Chile found methyl chloride around a young binary star known as IRAS 16293-2422, about 400 light-years away in a star-forming region of space called Rho Ophiuchi. This marks the first time any organohalogen has been spotted in space, but rather than give hope that life exists in that system, the discovery instead throws doubt on methyl chloride's reliability as a biomarker.

The presence of these organic compounds around such a young star suggests they may arise during the planet-forming phase of a system. To get a better understanding of how the molecules may form, the researchers turned their attention to a comet, which acts as time capsules from the birth of a star, preserving the chemical composition of the cloud of material stars arise from.

A diagram showing how star systems form, and what role comets play ESA

In this case, the team zeroed in on Comet 67P/Churyumov–Gerasimenko, which was visited by the ESA's Rosetta mission between 2014 and 2016. By sifting through the data collected by the spacecraft, the team found an abundance of methyl chloride in the comet, lending further weight to the idea that the compound arises during the planet-forming phase. In particular, the signals were strongest in measurements made in May 2015, when the comet was approaching the Sun and was giving off a lot of hydrogen chloride.

"We found it but it is very elusive, one of the 'chameleons' of our molecule zoo, only present during short times when we observed a lot of chlorine," says Kathrin Altwegg, principal investigator of the project.

The find may be disappointing for those hoping to find life in the cosmos, but it doesn't mean the search is off: rather, it's just a little more complicated than previously thought.

Source: ESA