Jupiter is the largest planet in our Solar System, and the competition really isn’t even close. It’s an absolutely massive planet and it helped define what a gas giant planet truly is. Because of its size, it might be hard to imagine a time when Jupiter had a problem growing, but new research suggests that the planet spent millions of years as a relatively modest world.

In a new paper published in Nature Astronomy, a team of researchers presents a detailed timeline of Jupiter’s first few million years. The scientists used meteorite data that has been amassed over time which hinted at the fact that Jupiter has long played a “gatekeeper” role for the planets that orbit closer to the Sun.

When the planet reached between 20 and 50 times the size of Earth it began divide the massive disk of dust and debris that was circling the Sun at that time. It effectively split the Solar System into to sections, and by studying meteorites researchers can determine when that split happened.

By building a timeline of meteorite activity, researchers can make some impressively advanced models that show the birth and earliest years of our system and the planets. Using a wealth of meteorite information collected over many years the team calculated when Jupiter began growing rapidly, and how long it took for it to reach its current size.

The model shows that Jupiter was born a baby. That’s true of all planets, so it’s not particularly surprising, but what the data also shows is that the planet grew incredibly slowly for its first few million years of existence.

“During the first stage the pebbles brought the mass,” Yann Alibert, lead author of the paper, explains. “In the second phase, the planetesimals also added a bit of mass, but what is more important, they brought energy.”

That first phase of growth, which lasted around a million years, allowed a solid core to form. The two million years that followed was slower in terms of growth, but the high-energy impacts created an incredible amount of heat to the would-be gas giant. At the three million year mark the planet was only about 50 times the size of Earth, but that’s when things began to speed up. The millions of years that followed were a bit of a growth spurt for Jupiter as it began to draw in more and more gas and balloon up to its current mass which is over 300 times that of Earth.