If we find this gas on other planets, this is a good sign that there is life there

This is an idea that most space enthusiasts may never have thought of – the gas emitted by broccoli (and other plants) is one of the most telltale signs of life on a planet. At least, according to a new study by researchers at the University of California, Riverside.

This gas, methyl bromide, has long been associated with life on Earth. It occurs naturally from the process of plants defending themselves. Methyl, as the defense process is known, allows plants to expel foreign pollutants, such as bromide, by attaching a chain of carbon and hydrogen atoms to them, thus gasifying them and allowing them to escape into the air.

Methyl bromide, in particular, is interesting from an astrobiological perspective. It was used as a pesticide until the early 2000s and has several significant advantages over other potential biosignatures if it appears in the atmosphere of an exoplanet.

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First, it has a relatively short lifespan in the planet’s atmosphere. This is particularly important for searches for exoplanets, as it means that any process that produces gas is likely still active. Its existence is not merely the result of a geological event that occurred eons ago.

The second feature is one that all astrobiologists love to see – there are very few non-biological processes that produce the gas, and even those processes aren’t usually natural. Although now considered a dangerous chemical, methyl bromide was mass-produced for use as a pesticide before it was regulated due to its adverse health effects.

The third feature is the spectral wavelength it shares with the “cousin” gas that is also a bio-signature – methyl chloride, which also results from the methylation process. Their common signature will make them easier to detect from afar, and both are indicative of a biological process, although methyl chloride can be distinguished from methyl bromide, as methyl chloride has already been seen around some stars, which is likely the result of an inorganic process .

UT interview discusses atmospheres of exoplanets

Not quite an advantage, but an interesting feature about being able to detect methyl bromide is that it would be relatively difficult to detect it in Earth’s atmosphere from afar. Its concentration levels are high enough, but ultraviolet light from the sun causes water molecules in the atmosphere to split into compounds that destroy methyl bromide, so it is not present for a long time in the Earth’s atmosphere.

However, ultraviolet rays are only a problem for Sun-like stars. Around stars like M-dwarfs, which are 10 times more common in the galaxy than sun-like stars, there will be fewer ultraviolet rays likely to break the methyl bromide molecule. Since these M dwarfs will be among the first places astrobiologists look at, it could be a chance to see methyl bromide build up in their atmosphere.

However, any such discovery may have to wait a bit. Even JWST is not set up to detect trace elements in the atmosphere of an exoplanet. However, in the next few years, some ground-based telescopes will be up to the task. Hopeful astrobiologists will have to wait until after these hit the internet before they can really hunt down this very exciting bio-fingerprint.

Learn more:
UCR – Broccoli Gas: A Better Way to Find Life in Space
Leung et al. – Substituted bio-methylated sputum. I. Methyl bromide, capstone bio signature
UT – Researchers develop new framework to search for biosignatures
Utah – If Earth were an exoplanet, it would still be difficult to know if there was life here

main picture:
An example of a dwarf planetary system M.
Credit – NASA/JPL/Caltech

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