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Jan. 21 (UPI) -- The solar system features a unique dichotomy of planets -- the inner solar system features smaller, drier bodies, while the planets found in the outer solar system are larger and wetter. According to a study published Thursday in the journal Science, this divide is best explained by a two-step formation process.
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Over the last few decades, scientists have struggled to come to an agreement over the source of the solar system's planetary dichotomy.
But in recent years, meteorite surveys have made it clear that the system's small, dry planets and big, wet bodies were divided by time and space.
"Isotopic and geochemical analyses of meteorites in the last few years demonstrated that these two 'families' of planets formed over different time intervals," study lead author Tim Lichtenberg told UPI in an email.
"There is some physical mechanism in the disk around the young Sun that kept these families separated," said Lichtenberg, an astrophysicist at the University of Oxford.
Older planetary formation theories failed to account for a variety of the solar system's idiosyncrasies, including the temporal, physical and composition divide between the inner and outer solar system.
"The old theory assumed that the gravitationally bound building blocks of planets, planetesimals, form rapidly and everywhere in the protoplanetary disk," Lichtenberg said.
"What is more, models of planetesimal interactions were consistently showing that the growth of Jupiter core takes too long to allow for efficient gas accretion, as the gas disk is known to disperse on timescales of a few million years," he said.
The new theory, developed by Lichtenberg and his research partners in Germany and Switzerland, solves these problems by dividing the solar system's formation into two distinct episodes of planetary formation, each with unique geophysical conditions.
"The first planetesimal burst incorporated a substantial amount of radioactive aluminum-26, which heated the inner terrestrial protoplanets from the inside," Lichtenberg said.
"This degassed the initial water abundances and made the inner terrestrial planets dry. This did not not happen with the outer solar system, which formed over a longer time interval," Lichtenberg said.
Scientists developed their model using a combination of numerical experiments, drawing on revelations from detailed surveys of other solar systems, as well as advances in the field of meteoritics.
The field of meteoritics involves the measurement and analyses of geochemical characteristics of meteorites, like isotopic ratios and water saturation. Isotopes are diffusion versions of the same chemical, each featuring different numbers of neutrons.
When Lichtenberg and his research partners tested the model against the chronological patterns revealed by earlier meteoritic studies, they found that a two-step formation process was able explain the solar system's planetary dichotomy.
The new research suggests this two-step process is caused by unique interactions between dust grains and water near the so-called snow line in the protoplanetary disk, where water transitions from gas to ice.
According to Lichtenberg, the new two-step theory can be used to better understand how terrestrial planets like Earth acquired their atmospheres and geophysical qualities.
"Historically, the field assumed that the Earth accreted in a dry-hot part of the disk, and water was delivered later by scattering water-rich bodies toward the inside," Lichtenberg said.
But according to the new formation theory, Earth and other planets in the inner solar systems were forged by materials rich in water-ice and began drying out only as a result of their superheated cores.
If Earth had started out with some water, and then had been delivered even more water, Lichtenberg says Earth just be one big ocean.
"This obviously did not happen," he said. "Our new model explains this by a combination of astrophysics during the disk phase and the geophysics of the rocky bodies that made the unique mix of the Earth and the other terrestrial planets that we see today."
