Rick Robinson

Mar 27th 2020

The Great Divide: The Origin of Our Planetary Split


Our solar system resembles two very different planetary systems combined into one. These two groups of planets are separated by a disproportionately wide gap between Mars and Jupiter, dubbed “the Great Divide” by a study published in Nature Astronomy. Astronomers have been puzzled by this divide for decades.

A Spiraling Mystery

The two planetary systems are divided as such:

  • The four major planets that are closest to the sun — Mercury, Venus, Earth and Mars — are compact, solid bodies made up largely of heavy elements such as silicon and iron.
  • The four outer planets — Jupiter, Saturn, Uranus and Neptune — are much larger and are mostly made up of light substances such as hydrogen and water ice, mostly in gaseous form.

According to The Planetary Society, a “snow line” between the inner and outer planets explains how the early solar system was divided into such different groups. Substances like water ice formed solid grains beyond the snow line, which eventually coalesced into the outer planets. Closer to the newborn sun, which shone brighter than today’s mature sun, the grains were vaporized and dispersed.

But this explanation does not fully account for the wide gap between the two groups of planets, or how different the planets are. The inner planets are not only short of hydrogen and helium, they are also short of carbon. But grains of carbon only boil away at very high temperatures, and would spiral inward unhindered past the snow line. For years, says Science Daily, astronomers put the blame on Jupiter, whose enormous mass might have trapped grains and kept them from moving inward.

Enter the “Great Divide”

Now a pair of astronomers has offered an alternate explanation for both the Great Divide and the dramatic differences between the planets on each side of it.

According to Science Alert, the researchers, Ramon Brasser and Stephen Mojzsis, started by examining infrared images of very young stars, many of which have been found to be surrounded by concentric dust rings. These are believed to be an early stage in the formation of planetary systems. And the researchers suggest that, 4 1/2 billion years ago, a similar set of dust rings formed around the newly formed sun.

The effect of these dust rings, the researchers argue, was to form regions of alternating high and low pressure in the early solar system that blocked carbon- and water-rich grains of material from the outer region from spiraling inward.

The Rocky Mountains of Space

This barrier effect led the authors to come up with the name the Great Divide, inspired by the Rocky Mountains, which divide the Midwestern watershed of the Mississippi from the watershed of west coast rivers such as the Columbia.

The solar system’s Great Divide would not have been an absolute barrier. Some grains must have slipped across — enough for Earth to end up not only with some carbon, but also enough water to form its oceans. While the Great Divide, in this theory, was necessary for Earth to become earthlike at all, its permeability was also essential for the formation of life on Earth.

As for the Great Divide itself, it vanished in the early days of the solar system, its dust ring particles being dispersed or else incorporated into planets. Its former location, between the current orbits of Jupiter and the asteroid belt, is now all but empty.

Time for Stress Testing

These events happened more than 4 billion years ago, but for planetary scientists, now is when the real fun begins. A dramatic new (possible!) solution to a long-standing puzzle does not make scientists just drop everything and hail this latest discovery. First they need to check it out.

As Live Science reports, some commentators are not entirely convinced. One astronomer notes that the Brasser-Mojzsis team has not yet offered a detailed model of the ancient solar ring system to confirm that it would indeed function as a Great Divide.

So we may need to check back in after a few years to learn the final outcome of this particular adventure in solar system research. That’s the way it goes, out on the frontier.