This cosmic gatekeeper divides our solar system in two

The rocky planets that are nearest to the sun consist of various materials that are very different from the ones present in the gas giants of the outer solar system because billions of years ago, our (then) young solar system was divided into two parts by a so-called cosmic gatekeeper which limited access of materials in the inner and outer regions, thus preventing them from mixing.

It turned out that the gatekeeper was actually a ring made out of gas and dust, a new study says. The fence, also known as “Great Divide” is a term that usually refers to what is now mostly empty space inside Jupiter’s orbit. 

The base of the study

Approximately two decades ago, chemists found out that the building blocks of planets, which are just asteroid-sized planetesimals or significantly smaller “pebbles” had compositions that were very different in concordance with their distance from the sun. The pebbles that piled up the outer, or “jovian,” planets packed with higher concentrations of organic molecules like volatiles and carbon, or even ices than those that cumulated on Earth-like planets that are closer to the sun, like Earth and Mars. 


This discovery was puzzling at first, mainly because the theory predicted that pebbles from the outer solar system should have drifted towards the central part of the solar system because of a phenomenon known as “gas drag” that mainly consists in the gravitational pull of the gas surrounding the young sun.

Before those findings were made public, scientists believed that the “gravitational wall that prevented mixing between the inner and outer disk of our nascent solar system was Jupiter,” according to senior author Stephen Mojzsis, a geochemistry professor at the University of Colorado Boulder. The reasoning was that Jupiter was so big and its gravitational field so strong that it simply gobbled up little pebbles before they had a chance of reaching the inner solar system.

Testing the theory

In order to test this theory, professor Mojzsis and lead author Ramon Brasser, a researcher at the Earth-Life Institute from the Tokyo Institute of Technology in Japan ran computer simulations that re-created the growth of the newborn solar system and the planets inside it.

The simulation proved that Jupiter could not have possibly grown so fast to keep out all the carbon-rich pebbles from entering the inner solar system. It turned out that a big part of the pebbles originating from the outer solar system passed straight on by the growing Jupiter.

Moszsis thinks that Jupiter is a very inefficient gatekeeper: “It’s like a porous border [through which] immigrants from the outer solar system would have flooded the inner solar system.”

The planet by itself would have allowed the passing of many pebbles, which means that planets from the outer and inner solar system would have ended up to have a similar composition, he added. 

A new theory

The two scientists proposed another theory: At an early time into the development of our solar system, there might have existed multiple rings of variable bands of high and low-pressure gas and dust roaming around the sun. The scientists believe that those rings are the reason why pebbles didn’t manage to move inward.

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