Scientists led by Thomas S. Kruijer of the University of Münster, in Germany, measured concentrations of molybdenum and tungsten isotopes derived from iron meteorites to model the age of the system’s largest planet.
There are two distinct groups of iron meteorites, the researchers suggest, which arose separately within the nebula cloud from which the solar system eventually coalesced. They represent, they write, “two genetically distinct nebular reservoirs that coexisted and remained spatially separated” during the first few million years of the solar system’s formation.
The most plausible explanation for their separation, Kruijer and colleagues suggest, is the formation of Jupiter in between them.
Jupiter is a type of planet known as a gas giant. Its likely formation involved first the accretion of a solid core, followed by the accumulation of thick layers of gases surrounding it.
Kruijer’s team calculates that the process began very soon after the birth of the solar system, which occurred when part of a giant molecular cloud condensed under the force of its own gravity around 4.6 billion years ago.
According to the modeling, Jupiter’s inner core grew to the equivalent of about 20 times the mass of the Earth within the first million years. The Sun was still a protostar at this stage, not having become dense enough for hydrogen fusion to begin.
The growth rate then slowed down, but continued, reaching about 50 times the mass of earth three million years later.
“Thus, Jupiter is the oldest planet of the solar system, and its solid core formed well before the solar nebula gas dissipated,” the team writes.
Fixing a date for the formation of the largest planet, the scientists conclude, will allow for better analysis of how its presence affected the dynamics of the young solar system.