Before astronomers began to find planets — many, many planets — orbiting Suns other than ours, the scientific consensus was that if other solar systems were ever found they would probably look much like ours. That would mean small, rocky planets closest to the Sun and large gaseous planets further out.
That assumption crash and burned with the discovery of the first discovery of an exoplanet orbiting a star — 51 Peg. It was a hot, Jupiter-sized planet that circled its Sun in four days.
That planetary rude awakening was followed by many others, including the discovery of many rocky planets much larger than those in our system which came to be called super-Earths. And equally common are gaseous planets quite a bit smaller than any near us, given the name sub-Neptunes.
Many papers have been written theorizing why there are no super-Earth or sub-Neptunes in our solar system. And now astrophysicist Stephen Kane of the University of California, Riverside has taken the debate another direction by asking this question: What would happen to our solar system planets if a super-Earth or sub-Neptune was present?
The results of his dynamic computer simulations are not pretty: the orbits of many of our planets would change substantially and that would ultimately result in some being kicked out of the solar system forever. The forces of orbit-transforming gravity set loose by the addition of a super-Earth are strong indeed.
Let’s go back to our actual solar system for some context.
The gap in size between the size of our terrestrial planets and giant gas planets is great. The largest terrestrial planet is Earth, and the smallest gas giant is Neptune, which is four times wider and 17 times more massive than Earth. There is nothing in between.
Jupiter, by the way, is 11 times wider than Earth. If Earth were the size of a nickel, Jupiter would be about as big as a basketball. And Jupiter is twice as massive as all the other solar system planets combined.
Because of the influences of that size and mass, there is a huge distance between Jupiter and Mars, its nearest terrestrial planet. That distance is as great as the distance from Mars to the Sun, and is filled by the asteroid belt.
Early in the history of the solar system, the gravity of newly formed Jupiter brought an end to the formation of planetary bodies in this region and caused the small bodies to collide with one another, fragmenting them into the asteroids we observe today.
This would have been during the theorized time when very early Jupiter and very early Saturn migrated inward as a result of interactions in the disk they were formed. They later migrated out due to interaction with each other, Kane said.
The belt contains between one and two million objects asteroids larger than 1 kilometer (0.6 miles) in diameter and millions of smaller ones. The total mass of all those asteroids is estimated to be only 3 percent of the mass of our moon.
Kane said that in other solar systems there are many planets in a gap like that. “Planetary scientists often wish there was something in between those two planets — (Mars and Jupiter) It seems like wasted real estate,” he said in a release.
So what would happen if a super-Earth showed up?
Kane, who ran thousands of simulations that added super-Earths of various masses to our present-day system and let them run for the equivalent of results 10 million years, witnessed orbital chaos.
Because Jupiter is so much larger than all the other planets combined — its mass is 318 times that of Earth — its gravitational influence is profound. If a super-Earth in our solar system disturbed Jupiter even slightly (or a passing star or any other celestial object) all other planets would be dramatically affected.
Depending on the mass and exact location of a super-Earth, its presence could ultimately eject Mercury and Venus as well as Earth from the solar system. It could also destabilize the orbits of Uranus and Neptune, tossing them into outer space as well.
The super-Earth would change the shape of our Earth’s orbit, making it far less habitable than it is today, if not ending life entirely.
“This fictional planet gives a nudge to Jupiter that is just enough to destabilize everything else,” he said. “Despite many astronomers having wished for this extra planet, it’s a good thing we don’t have it…We should be careful what we wish for.”
Kane found one exception to the planetary mayhem, of sorts. When he made the mass of the digital super-Earth smaller and put the planet directly in between Mars and Jupiter, he saw that the planet could remain stable for a long period of time.
However, if there were small moves in any direction, he said, “things would go poorly.”
The study has implications for the ability of planets in other solar systems to host life. Though Jupiter-like planets, gas giants far from their stars, are only found in about 10 percent of the time, their presence could decide whether neighboring Earths or super-Earths have stable orbits.
These results gave Kane a renewed respect for the delicate balances that hold the planets together around the Sun.
“Our solar system is more finely tuned than I appreciated before,” he said. ” It all works like intricate clock gears. Throw more gears into the mix and it all breaks.”
Many Worlds 
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