But new research suggests that it's possible E.T. isn't producing any of these and is emitting a gas nearly as toxic to humans as carbon monoxide — phosphine.
The toxic gas, which is used in the semiconductor industry, is a colorless gas that smells like garlic or fish and could be seen as an interesting "biosignature," researchers believe.
"We perform a broad and thorough assessment of phosphine as a biosignature gas in anoxic exoplanet atmospheres," researchers led by Clara Sousa-Silva wrote in an abstract of a study being presented at the annual Astrobiology Science Conference. "We consider not only the spectroscopic potential of phosphine, but also its thermodynamic false positives, biochemical viability, and atmospheric survival."
Although it's poisonous to life on Earth, phosphine is present in the atmosphere, albeit a "trace component," the researchers added in the abstract. Traces of the gas can be found in sewage, marshlands and the intestinal tracts of fish and human babies, as well as several other instances.
According to the CDC, the gas can be absorbed into the body by inhalation and "direct contact with phosphine liquid may cause frostbite." It's also highly flammable, further complicating matters.
"[I]t's no one's obvious choice," Sousa-Silva, a molecular astrophysics postdoctoral associate at MIT, said in remarks given at the conference obtained by Live Science.
Sousa-Silva added it's possible that exoplanets could produce phosphine if they do not contain oxygen. Phosphine by itself is not toxic, but the combination of it with oxygen is, she noted.
Phosphine has been overlooked as a "biosignature gas" for all of the aforementioned issues with phosphine, including its intense use of energy, but Sousa-Silva and others believe it could be "a significant marker for life on anoxic worlds if produced at comparable surface fluxes to those found on anaerobic ecosystems on Earth."
"Furthermore, if detected on a temperate planet, phosphine is an extremely promising biosignature gas, as geochemical false positives for phosphine generation are extremely unlikely," the researchers added in the abstract.
If phosphine is indeed a byproduct of extraterrestrials, it could be one reason why humanity has had trouble finding any traces of life so far.
Earlier this month, a comprehensive study that spanned more than 3 years, found no evidence of extraterrestrial life among more than 1,300 stars in close proximity to Earth.