Biologists usually think of life in terms of the ability of a system to harness energy from the environment to grow and to reproduce. However in Stanislav Lem's book 'Solaris' there is a vivid description of an exoplanetary ocean that doesn't grow or reproduce and yet communicates with human visitors and "feels" alive. So if we adhere to the principle 'you know it when you see it' then perhaps Solaris is alive. Therefore, my question is: if life is not necessarily made of cells, DNA and proteins, then what is it? Is there a more philosophical understanding of what life is?
Helmuth Plessner in his Die Stufen des Organischen und der Mensch. Einführung in die Philosophische Anthropologie from 1928 (!) [The Levels of the Organic and Man, Introduction to Philosophical Anthropology (unpublished as english translation).] proposes a philosophy of nature where the basic category of life (in phenomenological terms) is transcending its own borders from within and outside-in, and therefore having its border.
This is thought to be the basic destinguishing feature of life: Non-living nature does not have its border as part of its own (membranes, skin, etc). For 'dead things', the border is simply the nothing between two different mediums, not belonging to either of them. Living beings, as having their border, posit themselves within an environment, they have a 'positionality'.
The notion of border is summarised in Krüger, Hans-Peter (2007): 'The Second Nature of Human Beings: An Invitation for John McDowell to discuss Helmuth Plessner’s Philosophical Anthropology', Philosophical Explorations 1:2, pp. 107–119 as follows:
What do we imagine as a living phenomenon? In contrast to stones or other inorganic materials, life makes an impression as though it could stand or move in itself and from itself. In such imaginations of a living phenomenon, we presuppose that the living being has something like its own border (membrane, skin, or medium). Two directions of activity are made possible by this border: from the inside to the outside, and vice versa. Internally, the border enables differences between organs or between a core and a periphery. Regarding the environment, the border allows a selective opening of the body. One can imagine that the border could be folded internally, and at the same time, that his internal folding permits a selection of the environment, a selection in accordance with the internal differentiation. Plessner called this phenomenon ‘maintaining a position’, or in short ‘positionality’. Positionality means: a being has a relation both to itself and to others, thanks to its border. (p. 116)
Plesssner secures this (and the other categries he establishes for each level of the organic) by appling the methods of phenomenology, hermeneutics and transcendental philosophy all together, including the contemporary state of the art biologists' empirical experiments, and by this methodologically avoiding the anthropologic circle, i.e. defining categories and insight only on the basis of the specifically human (see Foucault's episteme of the modern times, but written decades earlier).
To apply your example of Lem's: With communicating, reacting to the environment and probably even having self-consciousness, the ocean would possibly even qualify for the highest level, under the name of (but explicitely not excluding other life-forms and species) the human.
I haven't read Solaris and hardly know where to go with this question. It touches upon so many essential questions in philosophy.
You are basically asking about a definitional limit between life and non-life, biology and physics, which are often described as two different types of causality. The classic, if outdated, text on this from the physics standpoint is Schrodinger's "What Is Life?" Which suggests crystals as a borderline case. An idea which remains scientifically open.
On the other hand, the philosophical idea of a pure, inhuman "system-environment" differential is best described by Luhmann and other "systems" theorists, who are not really interested in "life" as some essential boundary.
However, your description of the "sea" in Solaris seems problematic. If it just "abides" as a self "in itself" without erasing memory and reproducing, then it does seem to be missing an essential quality of life. It is just some mythical god who endlessly exists. Rather meaningless.
I suspect that such an entity could not actually "communicate." Part of the definition of "communication" entails finite, separated selves. And "finite"means mortal. I suspect this "sea" was not thought out with philosophical consistency.
To go further into the problems here would be hard work, and I am very lazy and not terribly knowledgeable. I hope this little bit helps.
How about a clearer answer from inside biology?
Framings like the Gaia movement of James Lovelock would rather see life equally at the level of the individual cell or body and at the level of an entire species, an ecosystem, or the whole planet. If you look at biology from that direction, 'grow and reproduce' is a biased version of the normal biological criterion, colored by our notions of progress.
For those larger things, growth may be basically impossible, and reproduction is extremely rare and perhaps pointless. (How would Earth grow? Why would it bother to consider a bigger version of itself better? It surely does not need to beget another planet.)
The criterion for living becomes self-directed homeostasis, instead. You can simplify that down to the ability to heal or adapt, rather than to grow or reproduce.
The emphasis on systematically displacing entropy (directing energy) for a given goal that arises naturally and is not imposed from outside remains the central part of the definition, the goal itself is just more general.
I recently read an article which may be a good starting point for a meta answer if you are trying to make a formal argument.
Life is anything that grows and eventually dies, i.e., ceases to proliferate and be cognizant. Can we say that viruses, for example, are cognizant? Yes, insofar as they react to stimuli; but they are alive essentially because they reproduce and grow. Computers are non-living because even though they can cognize, they do not develop biologically (grow), and cannot produce offspring.
Aristotles notion of what constitutes life is that it has a soul; this shouldn't be thought of in terms of the Christian/Islamic/Judaic conception, but something that is capable of 'moving' itself, and capable of perception amongst other capabilities. He explores it in De Anima, so one can think of the soul as that which animates.
Given this notion, then the sea in Solaris would constitute life; it moves itself - look at the surface of the sea - it moves; and it obviously is capable of perception, as its communicating with humans; still, it seems to me that its like the world-soul or the world become soulful.