The moons of the outer planets, research shows, have vast oceans and water is an essential ingredient of life as we know it. Natalie Elliot, a science writer with a specialty in astrobiology, explains,

The hottest spots in the search for alien life are a few frigid moons in the outer solar system, each known to harbor a liquid-water ocean beneath its icy exterior. There is Saturn’s moon Titan, which hides a thick layer of briny water beneath a frozen surface dotted with lakes of liquid hydrocarbon. Titan’s sister Saturnian moon Enceladus has revealed its subsurface sea with geyserlike plumes venting from cracks near its south pole. Plumes also emanate from a moon that is one planet closer to the sun: Jupiter’s Europa, which boasts a watery deep so vast that, by volume, it dwarfs all of Earth’s oceans combined. Each of these aquatic extraterrestrial locales might be the site of a “second genesis,” an emergence of life of the same sort that occurred on Earth billions of years ago.

Natalie Elliot, “New Approach Could Boost the Search for Life in Otherworldly Oceans” at Scientific American (July 16, 2021)

Of course, researchers are eager to explore that possibility; the critical question is how to do so. Elliot goes on to talk about the twin dilemma of looking for life off Earth: We could assume it exists when it doesn’t. Phosphine on Venus did not turn out to be ET and neither did Oumuamua.

But Elliot points out, reasonably, we could fail to detect life where it does exist. We did not know about the very existence of archaea, the third kingdom of life, until the discoveries of Carl Woese and his colleagues in 1977.

All that time and we never knew they were even here…

Some researchers are now refining their life tracking skills. At the Bulletin of Mathematical Biology, efforts to detect specific signatures of life (stoichiometric ratios) are discussed: “Classic work on Earth’s oceans has shown that life displays a striking regularity in the ratio of elements as originally characterized by Redfield (Redfield 1958; Geider and La Roche 2002; Eighty years of Redfield 2014). The body of work since the original observations has connected this ratio with basic ecological dynamics and cell physiology, while also documenting the range of elemental ratios found in a variety of environments.”

Elliot spells out their goal:

Once astrobiologists begin routinely discerning the distinctive chemical ratios associated with living ecosystems in our own planet’s quiescent waters, they can fine-tune the specifications for spaceflight-capable devices—and, just maybe, at last reveal a second genesis, written within the mathematics of a subsurface ocean’s chemistry.

Natalie Elliot, “New Approach Could Boost the Search for Life in Otherworldly Oceans” at Scientific American (July 16, 2021)

Some will respond, what if alien life operates on fundamentally different principles from life on Earth?

The people who raise that objection may not realize that they are actually asking a philosophical question. We are looking for what we understand as life. If there are things out there that are not life as we understand it, we would naturally want to know about them. But with no information, we cannot search for them. We must start with “search terms,” as the group that published in the Bulletin is doing.

You may also wish to read:

The Ocean Planets Hypothesis is that intelligent beings may flourish in the interior oceans of the moons of gas giant planets — or within exoplanets — but they are trapped there

and, a contrary view:

We won’t find ET on ocean planets, researchers say. We will see few extraterrestrials if a great many promising exoplanets are Waterworlds