The solar system can be pictured as being quite tidy. Close to the Sun, you have four rocky planets. Far from the Sun, you have four gas giants. The leading theory for this distinct formation points to Jupiter – scientists think it formed first, creating a separation. A new theory has now proposed something very different.
As reported in Science, an international team of researchers proposes that material in the inner solar system aggregated first into planetesimals. As these fragments grew due to radioactive decay, they became hot, molten mini-worlds. The heat was responsible for them losing most of their volatile content including water, which was then pushed towards the outer solar system.
There, the giant planets began to form, becoming larger and wetter. All four of these planets, their moon systems, and other bodies from the outer solar system are water-rich. This scenario was formulated using computer simulations, and it is based on the analysis of meteorites from the inner and outer solar system, as well as observation of planets orbiting other stars.
"The different formation time intervals of these planetesimal populations mean that their internal heat engine from radioactive decay differed substantially. Inner Solar System planetesimals became very hot, developed internal magma oceans, quickly formed iron cores, and degassed their initial volatile content, which eventually resulted in dry planet compositions. In comparison, outer Solar System planetesimals formed later and therefore experienced substantially less internal heating and therefore limited iron core formation, and volatile release,” lead author Dr Tim Lichtenberg from the University of Oxford said in a statement.
"The early-formed and dry inner Solar System and the later-formed and wet outer Solar System were therefore set on two different evolutionary paths very early on in their history. This opens new avenues to understand the origins of the earliest atmospheres of Earth-like planets and the place of the Solar System within the context of the exoplanetary census across the galaxy."
In the scenario, the planetesimals in the inner solar system formed roughly 500,000 years before the planetesimals in the outer solar system. This gap allowed for elements such as aluminum-26 to decay. In the inner solar system, this decay happened inside the planetesimal, making them hot. By the time the gas giant planets started forming, most of this element was gone.
The formation of the newer, cooler outer planetesimals was influenced by the motion of the snow-line, the region where the water that surrounds a star turns into ice crystals. At first, due to the Sun being much dimmer, this was much closer to the Sun. But as our star began to brighten, it moved outwards. Ice crystals are key in the formation of planetesimals so the snowline taking its time in moving created the conditions for this two-step process.
The simulations suggest that the first formation saw a lot of collision between planetesimals, before becoming big enough to attract material and moving closer to the Sun, in the orbits we find them today.
This scenario will certainly cause a stir in the planetary community, and it’s going to be interesting to see what evidence – for or against – researchers will find in the coming years.