Is there a long-lost fifth gas giant of our solar system?

Is there a long-lost fifth gas giant of our solar system?


Was our solar system once home to a fifth gas-giant? An astronomer from the Southwest Research Institute in Texas believes it might have been.

First, it isn’t entirely unusual for planets to be ejected from planetary systems. In fact, astronomers think one or more planets may have been kicked from a system when there are hot-Jupiter exoplanets orbiting their parent start from an unusually close distance.. Usually only a fraction of the distance that separates Mercury from the Sun. Most think these planets likely formed much further out than their rocky counterparts, but were somehow sent inwards to the inner solar system, booting out the unfortunate planets that lie in their path. These rogue (or “free-floating”) planets that are ejected out of their planetary systems wander the space between stars whilst directly orbiting the galaxy itself instead of a parent star. Some of our estimates of these planets say that they could potentially out number the amount of stars 2:1.

(You can check out an article I wrote previously about these rogue planets in the further reading section in the sources.)

David Nesvorny, the leading proponent of the theory believes there is ample evidence to suggest this was the case for our own solar system. Observations he and other astronomers have made about the population of the Kuiper Belt that lies beyond the orbit of Pluto has aided in understanding the chaotic time that occurred when our solar system was just 600 million years old. This time, known as the late heavy bombardment period — played a large role in shaping the orbits of the outer planets in our solar system. Any large perturbations in the orbits of the planets would shake up the Kuiper belt objects, sending forth large asteroids and comets to the inner solar system where they would be met with the terrestrial planets and their moon(s).

There is a problem with our models of this mysterious period of time though, which left behind a scant amount of evidence in the form of impact craters found on the Earth’s natural satellite, the Moon. Strangely enough though, our solar system seems to be the exception rather than the rule. All of the planets in our solar system are in wide orbits that are nearly circular in nature, which leaves no room for any planets or their natural satellites to collide with their neighboring planets. Other known planetary systems circling distance stars tend to have orbits that are steeply inclined to one another, whilst our planets are coplanar. This allows some of the Jupiter-sized planets to migrate inwards while the other planets may exist in highly elliptical orbits that regularly take them too close to their parent star or too far, which can freeze and then cook the planet — likely killing off any life before it had the chance to thrive. Slow changes in the orbit of Jupiter, would have undeniable affects of the orbits of the inner planets, which at that time may have included both Uranus and Neptune, as both are way too far from the sun to have been created in their current locations. Both are too massive and not enough time has transpired for the materials to coalesce into the combined 15-Earth mass worlds as we know them. . It’s possible that Earth could have collided with Mars or Venus during this time, which would have been particularly bad news for you and I.

Nesvorny’s colleagues suggested an alternative scenario around this problematic observation. Instead of Jupiter’s orbit slowly changing over a period of several million years, its orbit may have changed rather quickly, which wouldn’t have been as harmful to the terrestrial planets lurking close to the sun. There’s another problem with that theory though.. computer simulations of this scenario, which tries to input the proper data to explain the orbit of the planets we see in our solar system now — shows time and time again that Uranus or Neptune would have been kicked out of our solar system into interstellar space, which obviously isn’t the case as we know now.

So instead, Nesvorny added another large, Neptune-sized body with several dozen times the mass of Earth into the computer simulations. In it, Jupiter jumped correctly into orbit whilst the inner terrestrial bodies remained intact after the hypothetical fifth planet was ejected after a losing tussle with Jupiter.

As for the validity of this theory, it’s hard to say. One of the leading theories that deals with the formation of the moon suggests that a hypothetical planet named Theia collided into Earth some 4 billion years ago, blasting a portion of the Earth apart, leaving behind the raw materials to coalesce into the moon. Though the hypothetical Theia itself wouldn’t have been a fraction as massive as it would need to be to account for the perturbations of the orbits of the outer planets in our solar system. (I wrote an article about this scenario that you can find in the sources section just below)

ANOTHER theory suggests that an undiscovered planet may very well lurk in the outer trenches of our solar system un-noticed as of yet. The elliptical orbit of ‘Planet-x’ would take it so far out from the sun, one orbit around may take thousands of years to complete. (I ALSO wrote an article about this topic. It too can be found in the sources section below) Unfortunately, for the time being, computer simulations are only second best to hopping into a delorian space-ship and witnessing the birth and evolution of our solar system.

By Jaime Trosper


“Solar System May Have Lost Fifth Giant Planet:”

“Was a Giant Planet Ejected From the Solar System?”

Further Reading:

“Did a Planetary Collision Create the Moon?”

“Does Another Planet Lurk in the Outer Trenches of Our Solar System?”

“Rogue Planets Might Outnumber Stars 2:1:”

Image via: Southwest Research Institute
(Here’s a beautiful high-res version: