The presence of large quantities of oxygen ions may be able to distinguish habitable exoplanets with life from barren exoplanets in the habitable zone (resembling Venus or Mars):
Like Earth, Venus and Mars are small rocky planets; they have permanent atmospheres like Earth, and their atmospheres are exposed to the same solar radiation as Earth's. Data from the Pioneer Venus Orbiter and the Viking descent probe on Mars show that they have very similar ionospheres to each other—which don't contain a lot of atomic O+ ions. Know what else Venus and Mars are missing? Photosynthesis.
[Astronomy PhD candidate Paul] Dalba's contention is that photosynthesis on a planet's surface, which generates a surfeit of molecular oxygen, is the only thing that can account for these atomic O+ ions in a planet's ionosphere. The mere existence of life throws a planet's atmosphere out of chemical balance. O+ would be a neat biomarker because there isn't a numerical cutoff required—just the dominance of O+ among the ionic species in the upper atmosphere would indicate "thriving global biological activity" on the planet below.
Dalba claims that Venus and Mars act as negative controls, demonstrating that planets like Earth but lacking life don't have this O+ layer. Some may think that continuous volcanic activity on the surface could also generate enough oxygen, but Dalba doesn't. Chemistry involving water and UV light [open, DOI: 10.1038/srep13977] [DX] can also release oxygen. But the amount of water on Earth is insufficient to account for the requisite oxygen content, so he thinks that the presence of water on other planets wouldn't make enough oxygen there either.
Atomic oxygen ions as ionospheric biomarkers on exoplanets (DOI: 10.1038/s41550-017-0375-y) (DX)
Related: Nitrogen in Ancient Rocks a Sign of Early Life
Oxygen Ions From Earth Escape to the Moon
Researchers Suffocate Hopes of Life Support in Red Dwarf "Habitable Zones"
Seven Earth-Sized Exoplanets, Including Three Potentially Habitable, Identified Around TRAPPIST-1
Cosmic Methyl Chloride Detection Complicates the Search for Life on Exoplanets
Mars Colonists Could Produce Oxygen by Making a Plasma Out of Atmospheric Carbon Dioxide
Analysis of Microfossils Finds that Microbial Life Existed at Least 3.5 Billion Years Ago
To Detect Life on Other Planets, Look for Methane, Carbon Dioxide, and an Absence of Carbon Monoxide
(Score: 2, Insightful) by Anonymous Coward on Monday February 19 2018, @04:19PM
Because it's the only form of life we know. We cannot search for forms of life we don't know since we don't know what to search for. Earth-like life is the only form of life we have a realistic chance to detect.
It's like asking why do you look for a certain information in English-language texts, not in Chinese-language texts (assuming you speak English, but not Chinese)? The answer is not that you'd know in advance that the information cannot be found in Chinese language texts; it may very well be there, and it is entirely possible that it only is found in Chinese-language texts. But searching in Chinese-language texts would be pointless for you, because you couldn't identify the information in those texts anyway, even if it were right under your nose.