Meteorite data indicate Jupiter underwent distinct growth phases

Jupiter, as seen by NASA’s Saturn-bound Cassini spacecraft as it passed by on 29 December 2000. Image: NASA/JPL/Space Science Institute

Analysis of meteorite data indicates the growth of Jupiter into the solar system’s most massive planet was held up for about two million years during a phase in which kilometre-size planetesimals crashed into it, releasing heat that prevented rapid cooling, contraction and further gas accretion.

Jupiter now has 300 times the mass of the Earth with an equatorial diameter of 143,000 kilometres (89,000 miles). The details of how it evolved have been hotly debated for decades, but Swiss researchers have shown the planet grew in distinct phases.

The planetary embryo rapidly accreted small centimetre-size particles, building a substantial core over about a million years. But the growth rate slowed over the next two million years or so as kilometre-size bodies crashed into the new world, releasing enormous heat.

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

“The standard model for giant planet formation is based on the accretion of solids by a growing planetary embryo, followed by rapid gas accretion once the planet exceeds a so-called critical mass,” the authors write in the paper’s abstract. “However, the dominant size of the accreted solids (‘pebbles’ of the order of centimetres or ‘planetesimals’ of the order of kilometres to hundreds of kilometres) is unknown.

“Recently, high-precision measurements of isotopes in meteorites have provided evidence for the existence of two reservoirs of small bodies in the early Solar System. These reservoirs remained separated from (about one million years) until (about three million years) after the Solar System started to form. This separation is interpreted as resulting from Jupiter growing and becoming a barrier for material transport.”

In this scenario, Jupiter reached a mass about 20 times that of Earth within one million years and slowly grew to 50 Earth masses in the subsequent two million years. That is “puzzling,” the researchers write, because a planet of this mass is expected to trigger fast runaway gas accretion.”

The researchers concluded Jupiter’s growth was marked by two distinct phases.

“First, rapid pebble accretion supplied the major part of Jupiter’s core mass,” they write. “Second, slow planetesimal accretion provided the energy required to hinder runaway gas accretion during the 2 Myr. Third, runaway gas accretion proceeded. Both pebbles and planetesimals therefore play an important role in Jupiter’s formation.”

While pebbles are important to quickly build the core, “the heat provided by planetesimals is crucial to delay gas accretion so that it matches the timescale given by the meteorite data,” the researchers said.

They argue the results may hold clues to long-standing questions about the formation of Uranus and Neptune and exoplanets in the same mass regime.