They are gargantuan planets, many times bigger than the Earth — but they were once the size of pebble.

Gas giants like Jupiter and Saturn are the biggest planets in our Solar System, and they could easily swallow our Blue Planet and most of the others without it making much of a mark, but a new study published in the journal Nature found that their initial sizes were just a centimeter, according to a UPI report.

It all goes back to the early days of the universe, as tiny clumps of dust and ice began to form into our solar system, orbiting our infant sun. And now, new research points to the “pebble accretion theory that was first proposed by Swedish scientists three years ago — which actually was a big shift from the original idea that ice and dust turned into large balls quite slowly, and then basically formed the nuclei of Jupiter and Saturn.

But there was a problem with the latter idea: there wasn’t enough time before the sun’s circumstellar disk would have run out of planet-forming materials for these gas giants to form in this fashion.

Instead, the pebble-accretion model suggests that there was a sped-up process — but that model also had a problem, in that when it was input into a computer, hundreds of Earth-sized planets were suddenly in our Solar System. Obviously, something was off about that idea.

So scientists tweaked the model, allowing pebbles to interact and influence each other, accumulating and then pushing the smaller protoplanets out of the way, letting them snowball in size.

They put this concept into the computer models, and it was a success: the computer spit out anywhere from one and four young gas giants, which certainly works with reality.

In fact, it appears to mesh well with the Nice model, which helps scientists understand how maturing gas giants behaved and interacted when they drifted to the outer Solar System.

But what about smaller terrestrial planets, like Mars, and our own? Researchers will use the model to see if the pebble accretion model can work in these cases as well. But the early results are good, and scientists may have solved a very big problem in figuring out how these planets formed.

Jupiter is the largest gas giant, and the largest planet in the Solar System as a result. It is the fifth planet from the sun and has a mass of one-thousandth of the sun. Compared to the other planets, however, it is huge: if all the other planets were combined, Jupiter would still be two and a half time as big. It has the mass of more than 300 Earths.

Saturn is the other gas giant, and despite being much smaller than Jupiter, the sixth planet from our sun still has nearly 100 times the mass of Earth.

The inner four planets are all terrestrial planets: Mercury, Venus, Earth, and Mars. The outer two planets, Uranus and Neptune, are both ice giants. Pluto, which was once classified as our ninth planet, is now seen as a protoplanet.