Science writer Matt Williams has been writing a series on the question of why, despite the size of our galaxy, we see no other intelligent life forms. It could be, he suggests, that “many planets out there are just too watery!”

Williams points out that, although water covers 71% of Earth’s surface, it is only 0.02% of the planet’s mass. If the proportion were much higher, Earth would be an ocean planet because the water would surface.

It’s an open question whether an ocean planet would feature highly technologically developed intelligent life forms. Dolphins, for example, are quite intelligent but they do not seek to use any technology.

The question of whether a planet could have too much water arose, Williams tells us, in a 1983 paper by scientist and science fiction writer David Brin (pictured). But it got more attention when we discovered exoplanets that may have water orbiting other stars.

Now the question becomes, how little water is too little? How much is too much? A 2019 astrobiology paper noted,

One of the unique features associated with the Earth is that the fraction of its surface covered by land is comparable to that spanned by its oceans and other water bodies. Here, we investigate how extraterrestrial biospheres depend on the ratio of the surficial land and water fractions. We find that worlds that are overwhelmingly dominated by landmasses or oceans are likely to have sparse biospheres. Our analysis suggests that major evolutionary events such as the build-up of O2 in the atmosphere and the emergence of technological intelligence might be relatively feasible only on a small subset of worlds with surface water fractions ranging approximately between 30% and 90%. We also discuss how our predictions can be evaluated by future observations and the implications for the prevalence of microbial and technological species in the universe.

Manasvi Lingam and Abraham Loeb, Dependence of Biological Activity on the Surface Water Fraction of Planet, The Astronomical Journal, 157:25 (12pp), 2019 January

Williams draws out the implications for us:

The possibility that a great many exoplanets are Waterworlds could be very bad when it comes to the search for life as we know it. Planets that are up to 50% water in mass, for example, would have oceans that are several kilometers deep. Under these conditions, these planets would consist of liquid oceans above layers of high-pressure ice surrounding a rocky core…

Could it be that life as we know it is rare not because of the absence of a key biosignature, but an overabundance of it? This would represent a total inversion of what many astronomers expected to find, but it does present a possible resolution to the Fermi Paradox.

Matt Williams, “Beyond “Fermi’s Paradox” XII: What is the Waterworlds Hypothesis?” at Universe Today

The Fermi Paradox is the question Italian physicist Enrico Fermi (1901–1954) asked in 1950: If they’re out there, where are they?

Williams is not entirely satisfied with the claim that intelligent life could not develop on ocean planets. He would find support from Douglas Adams (1952–2001), the author of Hitchhiker’s Guide to the Galaxy. Recall that, in the fourth book, dolphins survive the catastrophe that engulfs Earth and their signature line is… “So long, and thanks for all the fish.”

Seriously, there is considerable evidence that the universe is fine-tuned for life. Maybe we should look at a simpler explanation than many we have considered:

Life has probably existed on Earth for four billion years but it is only in the last century and a half that humans have been serious about space travel. If other planets in our galaxy that could possibly host intelligent life are affected by the same physical constraints as Earth, their intelligent life forms could be on a schedule roughly similar to ours. They don’t visit us for the same reasons as we don’t visit them: They may suspect we’re out here. But they don’t have the technology to find out.

It’s nothing new. Around 600 AD, the Maya civilization in what is now Guatemala was highly developed. So was the Chinese civilization. They never interacted. They couldn’t. No one had the technology to cross the Pacific reliably so there was no way they could know each other existed. Today, Chinese tourists can visit Maya cultural treasures.

Most likely, the question of whether other planets in our galaxy feature intelligent life can only be resolved in the same way, by further advances in technology. It is, however, fun and useful to continue to develop hypotheses in the meantime.

Biochemist Michael Denton explains how water enables life:

Some other takes on why we don’t see aliens, despite the vastness of the universe:

Maybe we aren’t looking in the right places? A new sky catalog reveals most likely sites for alien technology. “Exotica” lists phenomena for which conventional natural explanations don’t seem to work well.

A particle physicist offers 75 reasons we don’t see aliens. But Oxford’s Future of Humanity Institute gives high odds that we are the only intelligent beings in the galaxy. No matter whose theory about why we don’t see extraterrestrials is right, we are bound to go on wondering and searching.

and

The aliens exist—but evolved into virtual reality at a nanoscale. That’s the Transcension Hypothesis, the latest in our series on science fiction hypotheses as to why we don’t see extraterrestrials. On this view, after a Singularity, the ETs become virtual intelligences, exploring inner space at an undetectably small scale. (This link takes you to nine other hypotheses worth considering.)