The largest scientific study of its kind estimates that Earth could play host to more than 1 trillion different species, which means we've probably only identified a vanishingly small proportion of them – only about one-thousandth of 1 percent.

To figure this out, biologists in the US combined more than 35,000 separate analyses of microscopic and non-microscopic species. This massive compilation of documented life forms covered 5.6 million species sampled from locations across all the world's oceans and land masses (excluding Antarctica), and if the scientists are correct in their estimates, we've got a long way to go before we'll have seen all that Earth has to offer.

"Estimating the number of species on Earth is among the great challenges in biology," said one of the team, Jay T. Lennon from Indiana University. "Our study combines the largest available datasets with ecological models and new ecological rules for how biodiversity relates to abundance. This gave us a new and rigorous estimate for the number of microbial species on Earth."

While this isn't the first attempt by scientists to gauge the number of living creatures on the planet, new advances in genetic analysis mean it's much more likely to provide an accurate estimate than outdated methods used in the past – especially when it comes to microscopic life forms.

"Older estimates were based on efforts that dramatically under-sampled the diversity of microorganisms," said Lennon. "Until recently, we've lacked the tools to truly estimate the number of microbial species in the natural environment. The advent of new genetic sequencing technology provides an unprecedentedly large pool of new information."

Those advanced tools provide a whole new perspective on the incredible amount of microorganisms that can exist – even in a space as tiny as a small clump of dirt. "Before high-throughput sequencing [rapid sequencing of whole genomes], scientists would characterise diversity based on 100 individuals, when we know that a gram of soil contains up to a billion organisms, and the total number on Earth is over 20 orders of magnitude greater," explains Lennon.

With the benefit of recent data sources using the newer identification techniques – including large-scale studies such as the US National Institutes of Health's Human Microbiome Project, the Tara Oceans Expedition, and the Earth Microbiome Project – the researchers were able to develop a new scaling estimate to predict how many types of still unseen life forms could also exist, undiscovered, in the same environments.

"We suspected that aspects of biodiversity, like the number of species on Earth, would scale with the abundance of individual organisms," said biologist Kenneth J. Locey, who compiled the inventory. "After analysing a massive amount of data, we observed simple but powerful trends in how biodiversity changes across scales of abundance. One of these trends is among the most expansive patterns in biology, holding across all magnitudes of abundance in nature."

With this scaling in hand, the team estimates up to 1 trillion species are living on the planet, and with 99.999 percent still left for us to find and classify, it seems pretty unlikely that we'll ever be able to catalogue them all.

"Our analysis suggests there are more microbial species on Earth than there are stars in our galaxy," Lennon told John Ross at The Australian. "These types of discoveries are leading to major changes in how we view the tree of life. [They] are almost certainly going to require rewriting of biology textbooks."

The findings are reported in Proceedings of the National Academy of Sciences.