Sara Imari Walker, of Arizona State University, puts her finger on a key issue:

The discovery of life on another planet should be a momentous event for humanity, but any announcement of a biosignature detection made right now will not be a milestone but a mess, because scientists will have no consensus that we’ve even made a discovery. Here on Earth, we don’t recognize life by its atmospheric byproducts. In fact, none of our current biosignatures address the central question: What about us makes us alive? Our biosignatures are not definitive signs of life because we don’t have a coherent theory of what life is…

Carl Sagan famously showed that adopting a definition that includes the ability to eat, metabolize, excrete, breathe, move, and be responsive to external stimuli—seemingly straightforward criteria—might lead any aliens encountering the Earth to assume automobiles are the dominant life form. A popular definition that “life is a self-sustaining chemical system capable of Darwinian evolution” excludes any organisms that can’t reproduce because they are not capable of evolution—therefore mules and many senior citizens are excluded if you read the definition too strictly.

Sara Imari Walker, “We Need to Change How We Search for Alien Life” at Slate

Well, now that she mentions it, there are over 100 definitions of life. Many don’t particularly make sense, as she points out. Also, there are many ways evolution can happen other than Darwinian evolution (horizontal gene transfer, for example). Why be exclusive?

There’s a longstanding controversy in the pursuit of extraterrestrial life as to whether life forms in other parts of, say, our galaxy, must be carbon-based, for example.

The famous astronomer Fred Hoyle (1915–2001) wrote a novel, The Black Cloud, in which the ETs were gaseous. Would that work? The jury is still out:

The Black Cloud is a vast, intelligent cloud of interstellar hydrogen, about 150 million kilometers in diameter. At its center is a complex neurological system made up of massive molecular chains.

“Black Clouds travel through space, making occasional stops in the vicinity of a star in order to use the energy to produce food and the chemicals that make up their bodies. When the Cloud reaches the vicinity of a sun, it assumes a disk-like shape that enables it to absorb energy more efficiently. By condensing hydrogen in a small area of the cloud, producing a fusion reaction, the Cloud creates an explosive jet of gasses that acts like a rocket, enabling the Cloud to accelerate, decelerate, and change direction.” – Ian Summers, writing in Barlowe’s Guide to Extraterrestrials p1 Fascinating. This sounds fairly plausible, right? It’s certainly very clever. The cloud’s fusion engines would technically be plasma, so its not 100% gas, but lets not dwell on that.

But how does this thing keep itself from drifting apart? And where does its intelligence reside?

Mark Ball, “The black cloud – a plausible gaseous lifeform?” at Sci Fi Ideas

Maybe it’s best to see the matter as a spectrum: ETs needn’t be exactly what we might expect but if they are too far off the scale (existing, for example, only as abstract mathematical entities), we might not recognize them as life.

But it’s Sci-fi Saturday and speculation is fun.

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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 (Lots more linked.)