Meteorites reveal Jupiter's troubled childhood
Jupiter may be the largest planet in the solar system, but it experienced growing pains. That's the conclusion of a team of scientists from the Universities of Zurich and Bern, and ETH Zurich, who say that the giant planet grew in three stages with a two-million-year gap, during which it grew very slowly and may even have affected the development of the rest of the solar system.
If they held a local big planet competition, Jupiter would win hands down. It's width of 88,000 mi (143,000 km) is the equivalent to 11 Earths and it outweighs our planet by a factor of 300. In the upper reach of its atmosphere is a storm that has raged for centuries and is so large that it could swallow the Earth with room to spare.
The question is, how did Jupiter form? The standard model is that the solar system started out as a great cloud of gas and dust that was slowly drawn together by gravity until it formed a massive, spinning core that became the Sun and a disc revolving around it that eventually coalesced into the planets.
The problem is that researchers from the Swiss National Centre of Competence in Research (NCCR) PlanetS of the Universities of Zurich and Bern and ETH Zurich noticed after studying meteorites that there wasn't something quite right when it came to Jupiter. Instead of a smooth progression as the planet drew more and more rocky material until it became massive enough to draw in hydrogen and helium in enormous quantities, the history of the planet was divided into three distinct phases that included a two-million-year pause, which was reflected in meteorites as the growing Jupiter altered the structure of the solar system.
What they found was that the very early Jupiter formed from tiny pebbles not much more than a centimeter in diameter. These collected quickly and the new planet grew rapidly over a million years until it weighed 20 times more than the Earth. Then, over the next two million years, the pebbles were replaced by kilometer-sized rocks called planetesimals. These brought in more mass, but their impacts also brought energy that heated up the new world, which ended up weighing about 50 times as much as the Earth. It was at this point that the gravitational pull of Jupiter grew strong enough to start a runaway gas accretion that brought it to its present size.
"How could it have taken two million years for Jupiter to grow from 20 to 50 Earth masses?" asked Julia Venturini, postdoc at the University of Zurich. "That seemed much too long. That was the triggering question that motivated our study."
Previous models said that Jupiter should have grown much faster than was the case. But what the meteorite data showed was that during that two-million-year slowdown, the composition of the solar nebula changed dramatically as it divided into an inner and an outer region where material from the two no longer mixed together.
According to the calculations of the researchers, the answer was with the young Jupiter. As it grew and became hotter, it disrupted the dust disk around the Sun, causing the separation. Meanwhile, the energy brought by the giant rocks striking it heated the planet up and prevented rapid cooling, contraction, and further gas accretion, which further slowed growth. Jupiter became a barrier that cut it off from most of the material it needed to grow until it finally tipped over the 50 Earth masses mark. At this point, it started to suck in gases until it became big enough to disrupt things again, causing rocky materials to shoot back into the inner region.
"Pebbles are important in the first stages to build a core quickly, but the heat provided by planetesimals is crucial to delay gas accretion so that it matches the timescale given by the meteorite data," say the team.
They also believe that this model will have more general use in explaining the formation of the other gas giants in the solar system as well as similar sized exoplanets.
The research was published in Nature Astronomy.
Source: University of Zurich