Big old Jupiter
Scientists use meteorites to show that Jupiter is almost as old as the solar system.
JUPITER may have been a very early bloomer, gaining significant mass within the first million years of the solar system’s birth, according to a new analysis of meteorite fragments.
The findings, described in the journal PNAS, could shed light on the early dynamics of gas giant planets in our solar system and beyond – and could help explain Earth’s development too.
Jupiter, king of the planetary pantheon, is the local heavyweight of our heavens; it tips the scale at about 318 Earth masses, dwarfing even Saturn (coming in at 95 Earths).
Jupiter actually holds about 2.5 times as much mass as all other seven planets combined – and so its powerful gravitational tug has profoundly shaped the architecture of our solar system.
Knowing exactly when Jupiter first formed, then, will help clarify exactly how Jupiter began to help carve the tumultuous disc of debris around our young star into the stable system we see today.
But getting a bead on when that happened has long eluded scientists – though NASA’s Juno spacecraft is trying to get a handle on that and other questions.
Lead author Thomas S. Kruijer of the Lawrence Livermore National Laboratory in the United States stumbled across a clue as to Jupiter’s age.
While working at the University of Muenster, the cosmochemist and his colleagues noticed that there seemed to be a strange pattern among the iron meteorites they were studying: there seemed to be two different groups of meteorites, one of which had a distinct isotopic signature.
Many of the atoms, including iron, molybdenum and tungsten, were heavier versions weighed down by extra neutrons. Those isotopes were probably created during a powerful supernova explosion, and their composition serves as a fingerprint of that particular star’s death.
That’s no great surprise; supernovas served as a forge for heavier elements that could not be made in the hearts of living stars.
Those heavy elements went on to seed other stars and fold into their planets and asteroids, including the chunks that fell to Earth as meteorites.
As scientist Carl Sagan wrote, “We are made of star-stuff.”
In the solar system, much of the debris from many different nearby stellar explosions should have mixed together, resulting in a more or less homogeneous soup.
And they had – except for this one particular group of meteorites that seemed to have held on to its original isotopic fingerprint. It was almost as if the two populations had been separated for a long time and not allowed to mix.
“The only mechanism or way to do this is to have a gas giant in between them,” Kruijer said. “Because only such a body is large enough to separate such large reservoirs.”
The scientists think Jupiter’s massive body acted like a blockade, keeping the new supernova material from interacting with the well mixed debris farther in.
And by analysing the tungsten and molybdenum isotopes within the meteorites, they could work back to when that separation between the two populations may have happened surprisingly soon after the birth of the solar system around 4.6 billion years ago.
If they’re right – and much more work needs to be done to confirm this – Jupiter may have grown to about 20 Earth masses in those first one million years of the solar system’s formation, Kruijer said.
The gas giant would have hogged much of the incoming material that came within its gravitational influence, starving the rocky inner planets (Mercury, Venus, Earth, Mars) and stunting their growth.
Jupiter’s early growth might also explain why there are no super-Earths in our system, even though they seem to be plentiful around other stars.
For us, that’s probably a good thing, Kruijer pointed out.
After all, had Earth been allowed to accumulate more mass, it may have developed into a super Earth, with higher gravity and a thicker atmosphere, and would probably have been a far less inviting home for life to emerge.
“In a way we should be thankful to Jupiter,” he said.