Particles from comet 81P/Wild 2 brought to Earth in 2006 by NASA's Stardust spacecraft seem to indicate that Jupiter formed more than three million years after the formation of the first solids in our Solar System.
The new finding helps test Solar System formation theories, which are currently at odds. However, one fact is certain: the formation of the giant planet affected how materials moved, collided, and coalesced during the complex planet-forming process.
Comets formed in the frigid Kuiper belt out beyond Neptune, but interestingly enough, analyses of the Wild 2 samples show that comets are composed of low-temperature and high-temperature materials which seem to have originated from completely different environments.
A research team - led by Dr. Ryan Ogliore of the University of Hawaii - analyzed a chondrule fragment known from previous research to have formed by high-temperature processes in the inner solar nebula - the cloud of gas and dust surrounding the infant Sun from which the planets formed.
What could be more contradictory than high-temperature objects from the innermost regions near the Sun becoming the predominant dust components of an icy comet in the outer solar nebula?
Using the University of Hawaii Cameca ims 1280 ion microprobe, the team measured magnesium isotopes (26Mg is the decay product of the short-lived radioactive isotope 26Al) and elemental aluminum (27Al) in their sample to bracket its formation age. Aluminum-26, with a half-life of 730,000 years, is the extremely useful clock that cosmochemists use to date ancient events since the 26Al/27Al ratio varies in objects that formed at different times.
Ogliore and team found no evidence for extinct 26Al, meaning the cometary fragment formed after nearly all the 26Al had decayed. Assuming homogenous distribution of 26Al in at least the inner solar nebula, their results suggest the fragment formed at least three million years after the first solids formed. And this would have happened before Jupiter could have interfered, for current theory hypothesizes that a growing Jupiter would have accreted material so efficiently as to open a gap in the solar nebula.
This gap would have posed a barrier to the outward migration of any material formed nearer the Sun than Jupiter's orbit, such as the fragment analyzed by Ogliore and team. The timing implies Jupiter formed more than three million years after the formation of the first solids in our Solar System.
"We were surprised to find such a late-forming, high-temperature little rock in these cometary samples," said Ogliore. “That we are able to test theories about the formation time of Jupiter and consequently, the origins of our Solar System is really a testament to the importance of sample-return missions like Stardust."