In a flurry of often uncritical reporting, the University of Sheffield has announced it has found evidence of life beyond Earth, publishing its findings in the online Journal of Cosmology. Particles of material, recovered by a balloon from the stratosphere at an altitude of 27km, included a diatom fragment, a frustule, or "shell" if you like, of this most ubiquitous group of ocean and freshwater-dwelling micro-organisms.

These fragments, according to the study, seeded into the upper atmosphere by a comet and then collected by Earth-bound scientists, are proof that comets are a vehicle for life, a claim that has been a long-held view of one of the co-authors of the paper, Professor Chandra Wickramasinghe. Professor Milton Wainwright, from the university's department of molecular biology and biotechnology, proposes this explanation based on a supposition that there are no known mechanisms for lofting terrestrial debris, including microbial matter, into the stratosphere. Even if fragments do get there, they would soon drop out.

Astrobiology is often afflicted by optimism about alien life. People want to believe that we are not alone. But the concern with this new claim is not that it is possibly another example of over-optimism but that it is inconsistent with one of the guiding principles of constructing a scientific hypothesis. It was William of Ockham, a 13th-century theologian and philosopher, who chanced across the idea of "parsimony". In everyday language, one should seek the simplest explanation for observations; the one with a minimum number of assumptions should be accepted until that simplicity can be traded in for greater explanatory force.

So here are some facts. The authors have observed a fragment of a diatom in the stratosphere. Diatoms are one of the most common type of water-dwelling organisms in the world. They are evolutionarily quite advanced. There is no fossil evidence for them before about 185 million years ago, although they may predate this time. From this, and other molecular evidence, they do not appear to be "deep-branching" – ancient lineages of life on the Earth.

These organisms are so common that they comprise the major fossilised component of "diatomaceous earth", a very crumbly material that is known to be atmospherically transported across the planet. The most prolific source of diatom dust, the Bodélé depression in Chad, sends the fragments of these ancient diatoms, laid down in ancient lakes, to Europe – there are a lot of dead diatoms floating around in the lower atmosphere. Further ephemeral facts to consider are that scientists estimate there are approximately 100 billion billion billion microbes on the Earth, not including diatoms, based on direct observations. To date we have not detected, unequivocally, a single active micro-organism beyond the Earth's atmosphere.

So the question is not whether there are alien fragments in the stratosphere. This can be determined by further experimentation. The scientific method will, ruthlessly and objectively, do this job over the coming years. Everything comes out in the wash eventually. Rather, the matter is whether, given our current state of knowledge, the most parsimonious and economical explanation for the observations reported is that the material is of extraterrestrial origin.

The question that Ockham would have demanded is this: is it more likely that our knowledge of particle transport from the lower atmosphere into the stratosphere is incomplete (like many areas of science) or that the microbes observed are alien denizens of passing comets?

You might ask why I haven't provided a long and detailed exposition of the various mechanisms by which diatoms and other microbial matter might float into the stratosphere in refutation of the Sheffield hypothesis. The answer is that one does not have to. Suppose we knew absolutely nothing about atmospheric transport processes. I suggest that the answer to the question above, given these facts, would still be the same – the most likely explanation, at the current time, is that our understanding of atmospheric transport processes is incomplete.

Why does the group that proposes these findings ignore Ockham so completely, igniting an international media frenzy on a claim of alien life that becomes pinned to British science and astrobiology? Probably because they are not testing a scientific hypothesis, but instead attempting to make data fit a pet theory that life has come to the Earth on comets. The scientific adage that has become a cliche – "extraordinary claims require extraordinary evidence" – applies particularly to the claim of the discovery of extraterrestrial life. A fragment of diatom in the stratosphere is not extraordinary evidence for extraterrestrial life. It might be extraordinary evidence for poor science.