What's that smell? Broccoli emits gas that could signal presence of alien life

This artist's concept illustrates a young, red dwarf star surrounded by three planets.
This artist's concept illustrates a young, red dwarf star surrounded by three planets. (Image credit: NASA/JPL-Caltech)

If there is a planet or moon crawling with extraterrestrial life-forms that are anything like life as we know it, they might act like broccoli.

Alien broccoli? Not exactly. There is now another potential biosignature that could reveal signs of life on far-off worlds. Methylation is a process used by broccoli, algae and many other plants and microbes on Earth to purge toxins by morphing them into gases. These same gases, if present in the atmospheres of exoplanets, could potentially be detected by instruments such as those aboard the James Webb Space Telescope. Planetary scientist Michaela Leung of UC Riverside recently led a study that determined it is highly unlikely these gases could be emitted by anything that is not alive. 

"Methylation is so widespread on Earth, we expect life anywhere else to perform it," Leung said in a statement. "Most cells have mechanisms for expelling harmful substances. [...] There are limited ways to create this gas through non-biological means, so it is more indicative of life if you find it." 

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There is a slight chance methylated gases could arise from volcanic eruptions, but living organisms are more prolific producers. Toxic heavy metals and other substances are transformed with three hydrogen atoms and one carbon atom before being released as gas. 

Methylated gases show up in the mid-infrared, so the James Webb Space Telescope and others could possibly detect atmospheric gas particles floating around a planet when it transits its star. Webb's NIRSpec instrument has the best chance of picking up on them once it separates positive signals from background noise. 

Some of these gases have a better chance of giving away strange organisms than others. Methane often comes from biological sources, even if they are decaying, but it is also more likely to be a by-product of abiotic reactions than some other methylated gases. Methyl bromide (CH3Br) will be especially sought after because it doesn't hang around in the atmosphere for long — meaning, if it shows up, it has to have been released from something recently. That something might still be alive. 

Another methylated gas that astronomers will be looking for in the near future is methyl chloride (CH3Cl). However, methyl bromide is more easily detectable than methyl chloride, and where it would be most obvious is in the atmosphere of a planet orbiting an M-dwarf star. The downside of these gases is that UV rays disintegrate water molecules, and the gases are broken up by what remains. M-dwarfs do not emit as much UV radiation as the sun.

It could take as much as a hundred transits of just one planet before Webb or another telescope sees any traces of methylated gases expelled by living organisms. However, if any are in a distant atmosphere, they could potentially mean Earth is not alone in hosting life.

The research is described in a paper published in The Astrophysical Journal (opens in new tab).

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