Since at least the 15th Century, there have been reports of what is generally termed star jelly, either falling from the sky or appearing mysteriously overnight. Descriptions vary, but witnesses usually talk about seeing a sticky, slimy goo, somewhat akin to Jell-O, and usually concentrated in puddle or patch form.
Folklore often attributes the jelly to meteors, but there is no scientific evidence for a connection. And even a shooting star that appears to be directly overhead is likely thousands of miles to the left or right and would leave any deposit far from the viewer. Star jelly has a much shorter shelf life than even the most perishable vegetables and usually evaporates or disintegrates before it can be analyzed.
Many guesses have been made as to what it is, from the scientific to the pseudoscientific to the just plain bizarre. Skeptoid’s Brian Dunning looked at some of these and concludes that star jelly is likely not a single phenomenon but multiple ones that have come to reside under the same mysterious umbrella.
Among the rational, terrestrial explanations are that it star jelly are a form of slime mold, which are neither fungus nor bacteria, and which use spores to reproduce. They prefer dead plant matter, which enables them to feeding on its microorganisms. Slime molds begin as a single cell, can reproduce quickly, and move noticeably. When growing, slime molds are wet and slimy, and appear suddenly, with a gelatinous appearance that morphs into a dusty form. Much wind or rain at all will cause it to evaporate or disintegrate. These distinctions are consistent with many star jelly reports, though not all.
Another possible answer are a cyanobacteria called Nostoc. Nostocs exist everywhere on the planet as minuscule colonies of bacteria. They are so tiny that only a botanist looking for them would be likely to make a sighting. But when wet, Nostocs swell to a much larger size and transform into gooey lumps or puddles. This would create an illusion of sudden appearance, when it was actually a change in appearance.
Another candidate is bryozoan, a phylum which exists in colonies of interdependent individuals. Most of these colonies are about a half a millimeter long and secrete exoskeletons. In some species, these skeletons are somewhat solid, making the colony look like a plant or coral. In other species, this exoskeleton is gelatinous, which turns the colony into a wet, sticky blob. Both of these eventualities could be taken to be star jelly.
Other natural substances that could explain star jelly include unfertilized frog spawn or deer sperm. And in The Book of British Amphibians and Reptiles, authors speculate that star jelly may form from the glands in frog and toad oviducts. Birds and mammals will eat the animals but not the oviducts which, when they come into contact with moisture, swell and distort leaving a vast pile of jellylike substance.
There has been speculation that star jelly being a more revolting substance, specifically chemical waste dumped from the airline toilets. However airplanes have never dumped toilet from the sky. It is possible for airline toilet systems to leak, forming blue ice can fall off when the plane descends. But the results are not gelatinous and have a color inconsistent with star jelly descriptions.
A far less reasonable airliner-related speculation is that star jelly is chemtrail residue. This is an instance of Tooth Fairy Science, where someone attempts to explain something by means of something not yet proven. Since chemtrails remain in the realm evidence-free paranoia, they make for a poor explanation as to what causes star jelly.
Jelly star sightings are sometimes accompanied by reports of widespread sickness enveloping the area. This leads to the most (literally) out there answer, that star jelly houses an alien virus. But there would be no reason to suspect that the jelly is causing a mass sickness. That would be a correlation/causation error. There’s never been a diagnosis of a pathogen tied to star jelly and any town is going to have a virus going around to some degree at any time. You could tie that virus to anything you wanted, be it rutabaga sales, tech stock prices, or the percentage of men wearing fedoras.