Monday, June 12, 2017


Jupiter may have formed at the dawn of the Solar System, and protected Earth

Scientists have been able to pin down the age of our Solar System by measuring the decay products of naturally radioactive materials incorporated into meteorites when they first formed. The oldest such inclusions date back 4.5682 billion years ago--a point in time often used as a marker for the birth of the Solar System.

Since the planets are thought to have formed as solid materials condensed from the primordial gas and dust making up the solar nebula or protoplanetary disc, and then gradually clumped together, the planets must all be younger than 4.5682 billion years. Planet formation is thought to have stopped within 10 million years or so, because by then the young Sun's solar wind had blown away most of the gas and dust needed for planet building.

The question is: just when did planets start to form and how long did it take?

A clever new analysis suggests that Jupiter, the largest and most massive planet in the solar system, formed surprisingly early and surprisingly fast--within the Solar System's first one million years. That's important because that early formation of Jupiter may have kept any planets larger than Earth--so called super-Earths--from forming in or migrating into the inner solar system.

Jupiter as captured by the Hubble Space Telescope in April, 2014, with an  
overlay of the giant planet's brilliant aurora in UV (Hubble, July 24, 2016)
Credit: NASA

The creative new approach to dating Jupiter's birth taken by Thomas Kruijer and colleagues at the Institute for Planetology at the University of Muenster was inspired by their realization that there are two related but different populations of meteorites orbiting the Sun (and occasionally falling to Earth where they can be studied). Like identical twins raised apart, the meteorites show strong similarities, yet differ because of their histories.

The researchers realized that the most likely explanation for those two families of meteorites was that as Jupiter formed it created a gravitational barrier that divided a primordial reservoir of gas and dust into inner and outer zones, within which the two different populations evolved.

"Our data showed that these meteorite populations did not mix for several million years," says Kruijer. "The only plausible way to explain this observation is that a gas giant planet, most likely Jupiter, acted as a barrier separating the two reservoirs."

They researchers focused on molybdenum and tungsten isotopes in samples from 19 iron meteorites. The meteorites' radioactive decay profiles placed them into two families known as carbonaceous (CC) and non-carbonaceous (NC) meteorites, and pointed to their formation at different times within two spatially separated reservoirs of the same primordial material. The isotopic signatures show that the NC meteorites formed early and in the outer reaches of the solar nebula while the CC meteorites formed somewhat later and closer to the Sun.

"Only in the last few years has it become clear that meteorites show some kind of dichotomy in their genetic heritage," says Kuijer. "This in part reflects the advances in analytical techniques that have been made, most notably in the precision of isotope analyses."

Iron meteorite (from Barrington Crater, Arizona)
Credit: Taty2007

The two families of meteorites appear to have become isolated from one another about a million years after the solar nebula first started to condense. The most likely cause of the separation, the authors believe, was the birth and growth of Jupiter. The model that best fits their analysis is one in which Jupiter's rocky core grew to 20 times the mass of the Earth by the time the solar nebula was a million years old. That mass was sufficient to split the protoplanetary disc into inner and outer parts.

Protoplanetary disc around the star TW Hydrae
showing a gap caused by a forming planet
Credit: Hubble Space Telescope, NASA/ESA

"Our study is the first to show that Jupiter can actually be dated by establishing when these reservoirs formed and for how long they survived," says Kuijer. "Our results show how the solid interior core of Jupiter formed very rapidly, within only about one million years after the start of solar history, making it the oldest planet in the Solar System," says Kuijer.

Their models indicate that Jupiter's core continued to grow for another few million years, followed by the accretion of the giant planet's dense atmosphere. Jupiter currently weighs in at 317.8 Earths.

If Kruijer and his colleagues are right, Jupiter is not only the biggest and most massive planet in our solar system, it's also the first and oldest.

In addition, they think, we may owe Jove thanks for keeping conditions closer to the sun just right for a planet like Earth rather than for a much larger and probably uninhabitable super-Earth.

"If a super-Earth had formed in the inner Solar System, then the evolution of the terrestrial planet region would have looked completely differently" Kuijer says. "This would have happened long before the Earth formed. I don't think that there would be sufficient material left at Earth's current orbit to subsequently still build an Earth-like body. Possibly, ice giants like Uranus or Neptune would have made it all the way to the inner solar system. In this light, Jupiter's early formation might have been a pre-requisite for building a planet like Earth."


You can find the June 12, 2017 PNAS article, "Age of Jupiter inferred from the distinct genetics and formation times of meteorites," by Thomas Kuijer,  Christoph Burkhardt, Gerrit Budde and Thorsten Kleine, here.


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