The scientists from the University of Georgia estimate the number of bacteria on our planet to be five million trillion trillion - that's a five with 30 zeroes after it.

There are far more bacteria on earth than there are stars in the universe.

"There simply hadn't been any estimates of the number of bacteria on Earth," said microbiologist William Whitman.

"Because they are so diverse and important, we thought it made sense to get a picture of their magnitude."

When people think of bacteria, they likely first consider the harmful ones that cause disease.

But the bacteria inside all animals combined, including humans, makes up less than one percent of the total amount.

By far the greatest numbers are in the subsurface, soil and oceans.

There are more of them in nature

Scientists call bacteria "prokaryotes," a term that describes a single-cell organism without a genetic nucleus.

Prokaryotes are extraordinarily diverse and range from plant-like cells that produce molecular oxygen in the oceans to soil-borne bacteria.

Scientists have found these cells 40 miles high in the atmosphere and miles beneath the ocean floor.

In order to estimate the total number of bacteria on earth, the group at Georgia divided the planet into several areas, including oceanic and other aquatic environments, the soil, the subsurface of soil, and other habitats such as the air, inside animals and the surface of leaves.

"By combining direct measurements of the number of prokaryotic cells in various habitats, we found the total number of cells was much larger than we expected," said Mr Whitman.

Another important part of the study was an estimate of carbon content in bacteria.

The team found that the total amount of bacterial carbon in the soil and subsurface to be yet another staggering number, the weight of the United Kingdom.

Surprisingly, the group at Georgia found that the total carbon in bacteria is nearly equal to the total carbon found in plants.

The inclusion of this carbon in global models will greatly increase estimates of the amount of carbon stored in living organisms. The new estimates could also change assumptions about the relative amount in plants of other essential nutrients such as nitrogen and phosphorus.

It had been estimated before that one-half of the living mass on Earth is microbial, but the new figures indicate that this estimate is probably much too low.

The study could open new areas of research, especially about the rate of mutations and how bacteria operate in nature. The new numbers also point out that events that are extremely rare in the laboratory could occur frequently in nature.

Because the number of bacteria is so large, events that would occur once in 10 billion years in the laboratory would occur every second somewhere on the Earth.