Our solar system is a crime scene of 4.6 billion years old. Cracked surfaces, displaced orbits of planets and interplanetary debris flows are cosmic equivalents of blood stains spilled on the wall and tire tracks of a car running away with a screech. These and other clues tell of the chaotic beginning of the existence of our planetary family. Our lost brothers are buried in these clues: including the ninth planet (no, not Pluto), which was knocked out by the gravitational shake-up that our Solar System underwent in our youth.
Today, the outer solar system was divided between four giant worlds: Jupiter, Saturn, Uranus and Neptune. Behind them lies the Kuiper belt, a field of icy wreckage, into which Pluto has been clowning.
“One should not be mistaken in believing that the outer part of the solar system has always been like this,” says David Nesvorny, a planetary scientist at the Southwestern Research Institute in Boulder, Colorado, who first spoke about the runaway planet in 2011.
Where is the ninth planet?
Nesvorny is one of those scientists who are trying to figure out how the solar system developed in its first few hundred million years. Using complex computer models, scientists came to a story in which young planets were formed relatively closely and then exchanged positions, periodically diving and jumping from one orbit to another. These simulations explain many small details about how planets, asteroids and comets revolve around the sun today.
Only here there is one problem. The story usually ended with Uranus or Neptune being knocked out of the solar system, Nesvorny wrote in September in the Annual Review of Astronomy and Astrophysics.
Since Uranus and Neptune remained in their places – the spacecraft eventually visited both planets – this narrative is not very flexible. But the fifth giant planet, many believe, may be the missing hero of this tale and an important player in the history of our solar system.
Phantom planet
To recreate the scenes of antiquity, astronomers turn to computer models to generate thousands of different solar systems assembled in thousands of different ways. In the lines of code, they set forth the laws of physics and the starting line of any arrangement of planets that you can imagine. Scientists set the scene: the planet is here, several asteroids are here – then they move away and let nature do its work. After a few weeks in the real world — millions of years in the simulation — astronomers finally decide to look in and see how the new-born solar system feels. The closer to reality, the better.
That is what Nesvorny did in 2009. He played with virtual solar systems, trying to find the best way to save virtual Uranus and virtual Neptune from a one-way tour into virtual deep space.
The problem was Jupiter. This giant planet is a thug, whose gravity can reach far and wide, pushing apart small worlds and constricting debris. In the most successful models, Jupiter and one of the two outer planets rebounded from each other and finally settled in their current orbits. But this happened only in 1% of cases. In the other 99% of cases, Jupiter knocked Uranus or Neptune so hard that they left the solar system and never returned.
“It’s actually much easier to talk about the existence of the fifth giant planet than not to talk,” said Sean Raymond, a planetary scientist at the University of Bordeaux in France. Although the evidence is largely circumstantial, “having one more such at that time would have made a lot more sense.”
“I would run these simulations to see what would happen without taking them too seriously,” says Nesvorny. “But then I realized that there might be some truth in them.” He spent about 10,000 simulations, changing the number of additional planets, their initial locations and masses in each case.
The best scenario – which reproduced the solar system, most similar to the real one – included an additional planet that was between the original orbits of Saturn and Uranus. This world was about as massive as Uranus and Neptune, or 16 times more massive than Earth. It was this planet that could get entangled with the orbit of Jupiter and be ejected from the solar system.
But the odds are still slim. Repeated simulations of such an installation succeeded only in 5% of cases. “The current Solar System is neither a typical nor an expected result,” wrote Nesvorny in the work of 2012, studying this idea, in collaboration with colleagues from Alessandro Morbidelli from the French Cote d’Azur Observatory. But it was a significant improvement over the 1% success rate of simulations, which included only four giant planets that we know and love today.
“It’s actually much easier to talk about the existence of the fifth giant planet than not to talk,” said Sean Raymond, a planetary scientist at the University of Bordeaux in France. Although the evidence is largely circumstantial, “having one more such at that time would have made a lot more sense.”
It may seem that this is far-fetched. How can astronomers know anything at all more than 4 billion years ago? How can they know something about the planets that remain at the moment, not to mention those that are no more? It turns out that the planets leave behind themselves battle scars that planetary detectives may try to read.
Interplanetary Blood Spots
“We are definitely confident that the planets did not form where they are now,” says Nathan Kaib, a planetologist at the University of Oklahoma in Norman.
This confidence appeared not so long ago. For most of its history, stellar observers assumed that the planets always moved in the orbits they had. But in the early 1990s, scientists realized that something was missing in this scenario.
Immediately beyond the orbit of Neptune lies the Kuiper belt, a scattering of icy debris surrounding the sun. “These are blood stains on the wall,” says Konstantin Batygin, a planetary scientist at the California Institute of Technology.
The location of the Kuiper belt objects led scientists to the inevitable conclusion: Neptune should have formed closer to the Sun than it does now. Many of the Kuiper belt objects are shot together in concentric orbits, vaguely resembling grooves on the plates. And these are not any random orbits – they are all closely tied to the orbit of Neptune.
Take Pluto, the most famous resident of the Kuiper belt. He and several hundred of his famous companions bypass the Sun exactly two times for every three rounds made by Neptune. Other streams of Kuiper objects bypass once every two turns of Neptune, or four times for every seven.
The Kuiper belt could not be in such a movement on its own. If, however, Neptune formed closer to the Sun, and then went out, its gravity should have worked like a network, capturing nearby interplanetary debris in these special orbits and launching them in a certain way.
This corresponds to some simulations that were conducted ten years ago. The formation of the planets was a terrible bloodshed, as a result of which garbage was scattered throughout the solar system. Any fragments that came too close to Neptune should have been pulled by the gravity of the planet. Since every action has an equal and opposite reaction, every time Neptune attracted a fragment, the planet repelled in the opposite direction of the fragment. Over time, Neptune slowly crawled away from the sun.
Neptune’s migration has affected other giant planets. In the end, Jupiter, Saturn and Uranus plowed the same field of debris and dealt with similar gravitational interactions. If Neptune moved, all these giant planets would move.
And it would obviously not be a breeze ride.
Continuous grinding of all these debris should have molded the orbits of the giant planets into perfect aligned circles, just as the clay on the potter’s wheel is smoothed by the strong hand of the potter. But the attack did not happen. Giant planets instead move in orbits that are slightly elongated and distorted. As if someone had crashed into a circle, spoiling the rounded pots.
Prancing Jupiter
In 2005, scientists have found the culprit. New models have shown that at a certain point the giant planets should have undergone the so-called “dynamic instability”. In other words, everything turned upside down for a million years. The most likely source of this confusion was to be a series of close passages between Saturn and either Uranus or Neptune — that is, ice giants — which sent one of these worlds in the direction of Jupiter. As it approached the giant planet, the ice giant gravitationally influenced Jupiter, slowing it down and pushing it into low orbit. But Jupiter also strongly pulled the approaching planet. The ice giant, being much lighter, accelerated as Jupiter slowed, moving away from the Sun.
This quarrel was a gravity mixer for the solar system. Jupiter jumped inside, and the rest of the outer planets popped out. This blow distorted the orbits of the giant planets and made them as they are now. He also saved the inner solar system — Mercury, Venus, Earth, Mars, and the asteroid belt — from mixing due to the prolonged gravitational effects of Jupiter and Saturn. Such a problem also occurred in earlier simulations.
This brings us to the removal of Uranus or Neptune. It is at this point in the simulations that Jupiter most often throws the ice giant from the galaxy.
This is the dilemma that Nesvorny tried to solve, without disturbing everything else in the simulations that really worked. An additional ice giant would take over the bulk of the head of Jupiter, allowing the other narrative events to unfold gradually and unhindered.
“This is perfectly believable,” says Batygin. “If you ask if there is any reason why we should have two, rather than three ice giants, the answer will be: of course not.” In fact, according to him, some calculations show that initially no more than five neptun-like worlds were created.
Batygin and his colleagues studied this question simultaneously with Nesvorny, although their motives were different. “I wanted to demonstrate that there can be no additional giant planet,” he says.
He reasoned that this alleged planet, moving from the solar system, would have broken that part of the Kuiper belt, known as the cold classic belt. If the Kuiper belt was a donut, says Batygin, the cold classical belt would be its chocolate filling – this is a family of objects whose orbits lie practically on the same plane in the Kuiper belt. A passing planet would have angered these orbits, Batygin and his colleagues believe.
Their computer simulation showed that this did not happen. Moreover, to their surprise, a rejected planet would not have destroyed the cold classical belt. This does not prove the existence of the planet – it only says that the solar planet works the way it works, regardless of whether it was there or not. Can this planet leave a more meaningful signature? Or, going back to the crime scene analogy, any tire tracks? Nesvorny believes that she could.
Trail taken
There is another part of the Kuiper belt, called the core, a narrow stream of ice fragments, the orbits of which are currently not synchronized with the Neptunian one. The origin of the nucleus is in a sense a mystery. In 2015, Nesvorny said that a jump in the external migration of Neptune – caused by the release of the planet, could be implicated in this.
When Neptune went into its final orbit and brushed debris from orbits synchronized with its own, a “strike” at the right time could launch some of these debris like an independent stream, the “core”. Simulations show that the gravitational push that caused Jupiter to jump and push out an extra planet could happen at just the right time to dislodge Neptune.
The truth is that we may never know for sure what happened during the formation of the solar system. “We cannot write the Bible of the Solar System,” says Batygin. “Only vaguely sketch her story.”
If the solar system has really thrown out one of its own, it’s in good company. In recent years, astronomers have discovered several wandering planets that have also been thrown out of their homes. Moreover, calculations show that in the galaxy there are more floating planets like Jupiter, than stars.
These are billions of refugees. Our fugitive was probably about the size of Neptune, and we don’t know how much he likes wandering around the galaxy. But we know that the Universe is full of small things, and there are more of them than big ones.
