According to the Great Maneuvering hypothesis, once upon a time Jupiter traveled through the solar system, wreaking havoc with its gravity. This hypothesis is still not fully accepted by the scientific community due to its complexity, but more recently, new evidence has appeared in its favor.
Astronomers led by René Heller from McMaster University have posted the corresponding preprint on arXiv.org, and the paper itself has already been accepted for publication in Astronomy & Astrophysics. To better understand why scientists need such a hypothesis, there are several important questions that need to be addressed first.
Unusual system
Until very recently, the structure of the solar system did not raise any questions: there was simply nothing to compare it with. True, the existing models of the formation of planets from a protoplanetary cloud did not give the picture that is observed by astronomers in practice, but this was attributed to the imperfection of the models themselves. The first discoveries of exoplanets in the 90s of the last century did not particularly affect the situation: the sample was small, there were few exoplanets.
In 2009, the Kepler telescope was put into operation, the main purpose of which was precisely the search for exoplanets. As of 2015, NASA has registered more than 4 thousand candidate planets seen by the spacecraft. And after the first thousand of them, it became clear that our stellar system is very far from typical.
Firstly, we have four planets the size of the Earth or less, and not a single super-earth - bodies with a radius of 1.25-2.00 times the Earth. At the same time, in the stellar systems examined by our telescopes, super-earths, on the contrary, are one and a half times larger than the so-called "Earth-sized planets".
Most of the 800 "terrestrial planets" (left) actually have a radius slightly larger than our planet, and in mass exceed it from 1.5 to 17 times; Earth, Venus, Mars and Mercury are significantly lighter than typical solid planets of other systems
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The quotes here are not accidental: this class includes all bodies with a radius of less than 1.25 Earth. But most of them are larger than our planet and significantly heavier than it (for example, Kepler-10c is 17 times more massive than Earth). There was an understanding that the development of the planetary system around the Sun went in some other way than in exoplanetary systems with super-earths.
Secondly, in most of the currently known systems, gas giants are much closer to the central star than our Jupiter and Saturn. Sometimes even closer to Mercury. Giants could not arise in such a place - the radiation of a star would simply prevent the planets from forming. This means, scientists concluded, that giants are formed far from the star, however, then they are slowed down by the substance remaining from the protoplanetary disk, moving to orbits closer.
In our system, however, deceleration, if there was, had completely different consequences - the giant planets are still located quite far from the Sun.
Time to migrate
And in 2010, Kevin Walsh's group put forward a hypothesis that explained both the absence of super-earths in the solar system and the relative remoteness of gas giants by the same event - the so-called Grand Tack Hypothesis.
According to Walsh, when the solar system was from 1 to 10 million years old and the terrestrial planets had not yet formed, Jupiter migrated from an orbit of 3.5 astronomical units (approximately 525 million kilometers from the Sun, one astronomical unit is equal to the average distance from the Earth to the Sun) into an orbit of 1.5 astronomical units, where Mars is now. There, the giant planet stopped, presumably due to the gravity of Saturn, which migrated after Jupiter into an orbit 2 astronomical units from the Sun. The giant then began to slowly move back until it returned to its current orbit of 5 astronomical units.
If it were not for the migration of Jupiter and Saturn, which is carried away by it, to the Sun and back, the inner region of the Solar System (above) would look like this now (below).
The Great Maneuvering hypothesis aptly explained many highly unusual features of the solar system. Jupiter, during his journey to the Sun and back, had to clear the place of formation of the terrestrial planets from the "extra" mass of gas and dust, depriving them of the opportunity to become super-earths. At the same time, the places where Mars and the asteroid belt were formed were most affected by the gravity of the giant planet, which led to their abnormally small (and it is, from the point of view of the evolution of the solar system, such) mass.
But for all the attractiveness of the hypothesis, it looks rather complicated, which is why many astronomers still doubt its correctness. In the new work, Rene Eller and co-authors decided to test what effect the Great Maneuvering could have on the moons of Jupiter. Their idea is simple: it is necessary to simulate the development of the solar system with and without maneuvering, and then compare the results. If simulation with maneuvering is more like the truth, it means that the new work will be another proof of the hypothesis. If without maneuvering, then so be it - it means that the hypothesis of a migrating Jupiter is too exotic.
Of greatest interest for such simulations are Ganymede and Callisto, two large satellites of Jupiter, half water and half solid. The fact is that if the maneuvering hypothesis is correct, then both of these bodies should have formed before the actual maneuvering itself: objects with such a proportion of water ice do not appear in places that are closer to a certain distance from the Sun. According to the authors' calculations, taking into account the influence of the youngest Jupiter and its circumplanetary disk, Callisto and Ganymede could arise no closer than 4 astronomical units from the Sun.
Titan (in the lower left corner) is not far from the Moon in size and gravity, but where it formed there were more light elements, therefore a relatively small satellite has a nitrogen atmosphere four times denser than Earth.
What kind of traces could the great Tacking leave on the satellites? It's all about the atmosphere. The authors of the work proceeded from the assumption that the atmosphere of Saturn's moon Titan, and the now atmospheric Jupiterian Callisto and Ganymede, were initially similar, as well as their masses and formation zones.
At the same time, estimates of existing models say that Titan's atmosphere, four times denser than Earth's, can be lost by gravitational means no earlier than in a septillion years. Even if for the satellites of Jupiter, this figure is reduced several times, such an atmosphere simply could not be lost by them during the lifetime of the solar system. Therefore, scientists suggested that the heating of the satellites, caused by the tidal forces of gravity of the gas giant, played a key role in the loss of the atmosphere.
At the same time, modeling without tacking showed that, despite the powerful gravitational field, Jupiter could provide heating and loss of the gas envelope only in satellites close to this planet, like Io and Europa. But Ganymede and Callisto would be behind the "snow line" of the primary near-Jupiterian disk and would not have been able to lose the atmosphere due to heating.
Apparently, Callisto is rich in light elements (like Titan), and even has an under-ice ocean, but it does not have a significant atmosphere.
When the authors of the work introduced into their modeling the effects of the Great Maneuvering, "placing" Jupiter with its disk at 1.5 AU. from the Sun, where it would receive about ten times more solar radiation, the situation has changed.
According to modern data, the Sun in the first million years of its life emitted from 100 to 10,000 times more X-rays and ultraviolet radiation than it emits now. A body with a nitrogen atmosphere, such as the present Earth or Titan, in such conditions inevitably lost its gas envelope. The fact is that the energy of the photons of such radiation is much higher than that of visible light, and, having absorbed them, the nitrogen particles had to quickly gain speed of several kilometers per second and leave the atmosphere. According to the authors' calculations, under such conditions, the primary nitrogen atmosphere of the Earth would be lost in just a few million years. And bodies like Ganymede and Callisto in an orbit of 1.5 AU. should have lost their atmosphere even faster.
This conclusion favorably distinguishes the Great Maneuvering model from the assumption that the planetary orbits remain unchanged. Within the framework of the latter, it is very difficult to imagine how exactly Jupiter's satellites could lose their atmosphere without losing water ice along the way.
Titan has its own atmosphere
To explain why, under these conditions, Titan did not lose its atmosphere, together with Saturn in 2 AU. from the Sun, the authors drew on data from modeling the primary circumplanetary disk of Saturn. According to it, Titan as a satellite could not form before the Great Maneuvering. The planets of the Solar, just as we see in exoplanetary systems, were formed at different rates, and when the most massive (Jupiter) had already completed this process, Saturn had not yet "gained" about 10 percent of its mass. This means that by the time of the Great Maneuvering, it was still actively absorbing matter from its circumplanetary disk. In such conditions, Titan, if he existed at that moment, would surely fall to Saturn. Therefore, Eller concludes, in reality, Titan could have formed only a few hundred thousand years after the completion of the maneuvering.
How did the Earth have a nitrogen atmosphere in such conditions? The authors point out that, according to a number of other works, in the primary atmosphere of the Earth with its significant gravity there was a lot of carbon dioxide, which interacts with energetic photons in a completely different way, and after absorbing them, it could effectively re-emit the received energy into space, cooling the upper layers of the then Earth's atmosphere …
Astronomers come to the conclusion that in the current configuration of the solar system, it is almost impossible to propose another scenario in which some satellites of the giant planets have an atmosphere four times denser than Earth, while others do not have it at all. But within the framework of the Great Maneuvering hypothesis, the present appearance of the satellites of Jupiter and Saturn can be explained much more successfully than if we assume that both of these planets never migrated to the Sun and back.
And at the same time, the hypothesis has many unresolved problems. The key one is still that it is extremely difficult to verify it completely. Too much has changed in our system over the past 4.5 billion years and many important factors that influenced the early period of its history can be restored only indirectly. It is not only about the speed of migration processes, which strongly depended on the not entirely clear density of the ancient circumsolar protoplanetary cloud. A number of models force us to assume that during the migrations of that time, gas giants could have ejected one or two large planets from the solar system by gravitational interaction, and in this case, the bodies we observe may not give completely exhaustive information about past events. For a more complete confirmation of the hypothesis, more complete observational data are needed for the same Ganymede and Callisto, which Eller's group hopes to receive from the European spacecraft JUpiter ICy moons Explorer (JUICE), which is to travel to the moons of Jupiter in 2022-2030.
Boris Alexandrov