Jupiter is the oldest planet of the Solar System
Jupiter is not only the largest planet of the Solar System, but it is also the oldest, say researchers from the University of Münster. They, for the first time, determined a precise age of Jupiter, which was previously only approximately known. One problem has always been that there are no samples from Jupiter and, therefore, that no direct measurements were possible. Now, the researchers have determined Jupiter’s age using meteorites. The study is published in the online Early Edition of "Proceedings of the National Academy of Science of the United States of America".
The study found that Jupiter reached a size of about 20 Earth masses in less than one million years after Solar System formation at 4.567 billion years ago. Once Jupiter had reached 20 Earth masses, it took another roughly three million years for it to grow to its full mass. "Although Jupiter is so massive, it grew extremely fast in less than four million years. Although theoretical models predicted such a rapid growth, these predictions were very uncertain", says Dr. Thomas Kruijer, first author of the study. For comparison, it took about 100 million years to form the Earth, which is only 1/380th the mass of Jupiter.
To determine the age of Jupiter, the researchers used meteorites, fragments of asteroids that today reside in a belt between Mars and Jupiter. The scientists used isotope measurements to show that the asteroids originally derive from two distinct regions of the Solar System, one within and the other beyond the orbit of Jupiter. The fact that material from beyond Jupiter is present in meteorites is a fundamentally new and surprising finding, the researchers emphasize. They used the isotopic compositions of the meteorites as a genetic fingerprint to deduce the relationships between different meteorites.
20 Earth masses in less than one million years
Through dating the meteorites, the researchers showed that asteroids from inside and outside Jupiter’s orbit formed between one and four million years after Solar System formation. "During that period there was no material exchange between the two regions. This lack of mixing can be explained through the formation of Jupiter. Model calculations showed that once Jupiter reached about 20 Earth masses, it prevented material exchange across its orbit", explains Thorsten Kleine, professor at the University of Münster and senior author of the study. Conversely, this then means that Jupiter must have grown to 20 Earth masses within the first million year of Solar System history.
At four million years Jupiter is fully formed
The 20 Earth masses correspond to the mass of Jupiter’s solid core. Once that core had formed, the growth of Jupiter continued through accretion of gas. This process was relatively slow initially until Jupiter reached a mass of about 50 Earth masses. The researchers were able to determine that point in time, because once Jupiter reached 50 Earth masses it started to scatter material from beyond its orbit into the inner Solar System. "That process cannot have started before the meteorite parent bodies stopped forming, because otherwise we would have found isotopic evidence for mixing of outer and inner Solar System materials", explains Thomas Kruijer who works at Lawrence Livermore National Laboratory in California (USA) now. Because there is no evidence for such mixing in meteorites that formed about four million years after Solar System formation, the researchers concluded that Jupiter reached 50 Earth masses no earlier than four million years. At this point Jupiter then was sufficiently large to start a process called runaway gas accretion, allowing it to reach its final mass of 384 Earth’s masses very quickly.
Implications for the early evolution of the Solar System
The rapid growth of Jupiter has far-reaching implications for understanding the early history of the Solar System and the formation of the four inner (terrestrial) planets Mercury, Venus, Earth and Mars, the researchers say. Jupiter’s growth led to scattering of water-rich asteroids into the inner Solar System, where these bodies may have been incorporated into the Earth. As such, water-rich asteroids are likely candidates for the source of water on Earth. But the rapid growth of Jupiter also inhibited significant mass transport into the inner Solar System, potentially explaining why Mars is so small and also why our Solar System, in contrast to many extrasolar systems, has no super-Earths (that is, large terrestrial planets). "That we have material that initally formed beyond Jupiter is a completely new and suprising finding. It will dramatically change our view on early Solar System history", says Thorsten Kleine.
The work was carried out as part of the Collaborative Research Centre Transregio 170 entitled "Late accretion onto terrestrial planets" at Münster University and was supported with funding from the German Research Foundation as well as from the European Research Council.
T.S. Kruijer, C. Burkhardt, G. Budde and T. Kleine (2017): Age of Jupiter inferred from the distinct genetics and formation times of meteorites. Proceedings of the National Academy of Science of the United States of America (Early Edition); doi:10.1073/pnas.1704461114