I have only read Solaris in translation, but in the English translation I have (Berkeley 1982; I believe it's essentially the same as the original 1970 translation) Dr. Kelvin initially hypothesizes that it's a neutrino-based structure:
"Everything looks normal, but it's a camouflage. A cover. In a way, it's a super-copy, a reproduction which is superior to the original. I'll explain what I mean: there exists, in man, an absolute limit — a term to structural divisibility — whereas here, the frontiers have been pushed back. We are dealing with a sub-atomic structure."
"Just a minute, just a minute! Kindly be more precise!" Sartorius interrupted.
"The atom is the ultimate constituent element of our bodies. My guess is that the Phi-beings are constituted of units smaller than ordinary atoms, much smaller."
"Mesons," put in Sartorius. He did not sound in the least surprised.
"No, not mesons... I would have seen them. The power of this instrument here is
between a 10th to a 20th of an angstrom, isn't it? But nothing is visible, nothing
whatsoever. So it can't be mesons. More likely neutrinos."
"How do you account for that theory? Conglomerations of neutrinos are unstable..."
"I don't know. I'm not a physicist. Perhaps a magnetic field could stabilize them. It's not my province. In any event, if my observations are correct, the structure is made up of particles at least ten thousand times smaller than atoms. Wait a minute, I haven't finished! If the albuminous molecules and the cells were directly constructed from micro-atoms, they would be proportionally even smaller. This applies to the corpuscles, the micro-organisms, everything. Now, the dimensions are those of atomic structures. Consequently, the albumen, the cell and the nucleus of the cell are nothing but camouflage. The real structure, which determines the functions of the visitor, remains concealed."
They do some research and explore the idea of a type of "neutrino field":
I pored over microfilm texts for an hour, and made myself wrestle with the unfamiliar language of neutrino physics. The undertaking seemed hopeless at first: there were no less than five current theories dealing with neutrino fields, an obvious indication that none was definitive.
Even at the end they have hypothesized a "rotating stabilization field" but do not know the way it is constructed or maintained:
"[...] Do you know the necessary conditions for stabilizing a neutrino field?"
"No, nor do you. Nor does anyone."
"Exactly. All we know is that the structure is inherently unstable, and can only be maintained by means of a continuous energy input. Sartorius told me that. This energy creates a rotating stabilization field. Now, does that energy come from outside the 'visitor,' or is it generated internally? You see the difference?"