Runaways and vagabonds: who destroyed the super-earths of the solar system and deprived it of a habitable planet

(ORDO NEWS) — The planets around our Sun are located in a completely different way than in other systems. And this has extremely unusual practical consequences: calculations show that two potentially habitable planets should revolve around our star, and not one, as it is now. One of them disappeared somewhere without a trace – and this is still at best. We will tell you why it happened and who exactly is to blame for this.

Deceptive Ordinary – And What Hides Behind It

When we look at the world around us, it often seems to us “ordinary”, ordinary. However, it is worth taking a closer look – and everything turns out to be exactly the opposite. Take at least the place where we live. At first glance, everything is normal in the solar system: planets are like planets. Everyone thought about this until astronomy made great strides in the study and modeling of planetary orbits.

It began with the fact that Jupiter’s orbit deviates too much from the circular one – it shows an eccentricity (orbital elongation) of 0.048, three times that of the Earth. Meanwhile, the greater its mass, the more difficult it is to break the orbit of a celestial body. Initially, the planets in the protoplanetary disk had “even”, circular orbits – that is, they could get its “pulling out” only with gravitational interaction with someone else. Jupiter has a mass 318 times the Earth and 2.5 times all the other planets of the solar system. To “stretch” its orbit so much, gravitational interactions of colossal scales are needed. With whom? Who is massive enough to influence him?

Uranus and Neptune are also good: according to calculations, in their current places they should form longer than a protoplanetary disk could exist. The only reasonable way out of this paradox is to assume that at first they formed much closer to the Sun and only then migrated further away.

However, the riddle of riddles in our system is Mars and the terrestrial planets in general. Logically, the more massive the planet is, the further it is from the Sun. After all, the further this or that part of the protoplanetary cloud is from the star, the smaller part of the substance of this part of the disk will be forced outward by the radiation of the star and the solar wind (a stream of charged particles from the solar surface). It seems that everything is so: Mercury is 15 times lighter than Venus, and Venus is 1.18 times lighter than Earth. But on Mars, the pattern broke: it is nine times lighter than our planet.

Of course, some problems can be removed by the fact that the gravity of Jupiter “pulled” part of the planetesimals – small bodies like asteroids, from which terrestrial planets were formed – outward, far from Mars. But all the calculations of astronomers show that a planet with 0.5-1.0 Earth mass should have appeared in place of Mars.

This is far from a purely theoretical question, as one might think. The fact is that Mars lies within the habitable zone of the solar system. If it had an atmosphere at least comparable to that of the earth, it would retain heat enough to make the climate of Mars look like an arid earth. However, in practice, in our time, the atmosphere there is 150 times more rarefied than ours – the gravity of a low-mass planet is too weak to hold a denser shell without terraforming.

Obviously, if Mars were five to nine times more massive than it is today, then the fourth planet of our system would be habitable. Either her life colonized the Earth, or earthly cosmonauts have already mastered this world – and with the aim of mass colonization, which is now impossible without terraforming for a hundred years.

There is one more “but”. Observations of other planetary systems show that there are planets the size of Earth, Venus, Mars and Mercury are rare. Solid planets usually have a mass noticeably larger than the earth, which is why they are called super-earths. That is, all our terrestrial planets are a kind of anomaly. Moreover, astronomers Konstantin Batygin and Michael Brown suggest (and today this is a hypothesis accepted by most scientists) that there is a super-earth in the solar system – only incredibly far from the Sun, hundreds of times farther from the Earth and tens of times farther than Neptune. Of course, there were no protoplanetary disks of this size in our system. That is, this super-earth got there in some exotic way after it had formed near the Sun. How did it all turn out so strange here?

Great maneuvering and Jupiter dash

Observations of other planetary systems have shown that such an event as the migration of planets through the system – either to their star, or further away from it – is, oddly enough, a common occurrence, occurring in tens of percent of systems. A whole class of planets, the so-called hot Jupiters, which the Sun does not have, exist only through migrations to their star.

By themselves, gaseous planets next to a star cannot arise; radiation simply “carries” light elements to more distant zones. But after formation, the gas giant can lose the stability of its orbit. For example, if it collides with planetesimals, and they gradually slow down its rotation around the star. With a drop in the speed of rotation, the planet begins to cut gradually narrowing circles around its star. At the same time, its gravity throws objects on the road out of the system. As a result, “hot Jupiters” often leave their star without any terrestrial planets at all, and they themselves approach it so close that they warm up to thousands of degrees.

It is clear from such a scenario: our system, too, at first could have been completely different. The giant planets did not necessarily originate where they are now. Realizing this, astronomers have built a number of fairly detailed mathematical models of the movement of bodies in the solar system – and found an unexpected thing.

It turned out that the most probable, from the point of view of celestial mechanics, the initial region of formation of Jupiter – the one that would be compatible with the elongation and location of its current orbit – is located at a distance of only 3.5 astronomical units from the Sun, that is, 3.5 times farther than the current distance between the Earth and the star. Meanwhile, Jupiter is now at an average distance of 5.2 astronomical units from the Sun, a quarter of a billion kilometers further.

The simulation also showed how he got there. Initially, collisions with planetesimals slowed down its rotation around the star, so the planet moved in its orbit more slowly and, accordingly, began to approach it. On the way “inward” to the Sun, the gravity of Jupiter took Saturn with it.

Fortunately for us, terrestrial planets form more slowly than gas giants and complete their formation after them. If Earth and Mars already existed, they would have had a hard time. Jupiter stopped just 1.5 astronomical units from the Sun – that is, it was a little closer to Mars. Formed planets would receive an overly strong impulse from its gravity, and their orbits would be too unstable. With a high probability, they would simply fly out of the system.

Yet the migration of Jupiter has brought serious trouble to the unborn solid planets. Its gravity has dramatically impoverished the orbital region of modern Mars. Gas and dust from this part of the protoplanetary disk were literally gobbled up by Jupiter, part of the planetesimals he had to throw out of the system. The region where the Red Planet was formed was so depleted in material that Mars received much less of it than Earth.

The question arises: why did Jupiter not stay 1.5 astronomical units from the Sun, but turned around and went back? Judging by the models, Saturn, moving after him, at a certain stage entered into orbital resonance with Jupiter. This means that the period of two revolutions of Saturn around the Sun became equal to three periods of rotation of Jupiter (resonance 3: 2 – however, there is a possibility that it was equal to 2: 1).

The presence of orbital resonance means that the bodies regularly “stand” opposite each other. The gravitational interaction between them increases dramatically, and their orbits receive serious adjustments. Interaction with Saturn pulled Jupiter back just in time. What if the giant planet hadn’t stopped at 1.5 AU, but had reached 1.0 AU – that is, to the Earth’s orbit? Or even further? Perhaps in this case there would be no one to ask questions. The Earth would be a small planet like Mars, fortunately, Jupiter’s “walk” would not leave her much material to form. If there was life on it, it would hardly be as developed as today.

Jupiter’s journey back and forth could not but affect Uranus and Neptune. Initially, they were no further than 17 astronomical units from the Sun. The migrations of the two “inner” giant planets eventually threw the outer ones into more distant orbits. They remain there today: Uranus, on average, is 19 astronomical units from the star, and Neptune is 30.

Finally, the migration was not in vain for the satellites of Jupiter itself. These are fairly large objects with a diameter of thousands of kilometers. If they calmly “stood still” away from the Sun, their primary atmosphere would be lost by gravitational path longer than our system exists. But the “walk” to one and a half astronomical units strongly warmed up these atmospheres, because of which the largest Jupiter satellites lost it.

Why we don’t have super-lands and the story of the lost giant

As you can see in the illustration above, super-Earths are the most massive class of planets, but we don’t have a single one. Such planets form faster than small bodies of the terrestrial group, therefore, by the beginning of the migration of Jupiter, they could exist in our system. In this case, an unenviable fate awaited them: they had to be destroyed by falling on the Sun.

The remaining material of the protoplanetary disk in the regions of existence of the solid planets of the solar system was too small to form the second generation of super-earths. All that was enough for him was the formation of four terrestrial planets, of which about half of the mass falls on the Earth.

But that’s not all. Almost any modeling of early migrations in the solar system shows that after them its current configuration is unlikely – in the region of a few percent, or even less. According to the most probable scenario, the planets should be in different orbits, and the elongation of their orbits should be somewhat different. Astronomers puzzled over this problem for a long time until they tried to introduce additional planets into the simulation.

It turned out that in the presence of not four – as today – but five giant planets in the early solar system, the likelihood of the formation of a system in its current appearance is greatly increasing. Almost certainly about 4.6 billion years ago, there were actually five such planets, and the fifth was an ice giant – like Uranus or Neptune.

Moreover, astronomers Batygin and Brown, who put forward a hypothesis about a ninth planet rotating hundreds of astronomical units from the Sun, note that this is most likely a large object (or one of them), once thrown out by Jupiter’s great journey back and forth.

Billiards on a cosmic scale

So, almost the entire architecture of the solar system was determined by the events of the time when the earth did not yet exist. In the place where Mercury, Venus, our planet and Mars are now, there could be a super-earth – or even more than one. There were five giant planets then, not four. The chaotic migration of Jupiter and Saturn destroyed all the ancient super-earths, throwing them into the Sun, and at the same time threw the current ninth planet of the solar system “into the cold”. At the same time, Jupiter and Saturn themselves moved away from the Sun, and Uranus and Neptune moved away from the star even more.

This shows that the migration of giant planets is the key mechanism that controls the scenario of the evolution of the planetary system. Those of them that have “hot Jupiters” may generally be deprived of terrestrial planets and chances of habitability. Their giant planets are so hot because their migration ended close to the star – they did not have their own Saturn, which could turn them around. The constant presence of giant planets in orbits close to the star makes the presence in the habitable zone of an even smaller solid planet less likely.

Those systems, where there was no migration of giant planets at all, get several super-lands – densely “packed” in close orbits. There are many similar systems in the vicinity of the Sun: for example, in the TRAPPIST-1 system, 40 light years away. While it is difficult to say how inhabited they are, but if they are really suitable for life, then there can be more than one inhabited planets in such systems, and sometimes more than two. Whether it is so or not, astronomers can find out in the coming decades.

Before the start of the mass discoveries of exoplanets, the point of view was widespread among scientists, according to which the solar system is a favorable place for the formation of terrestrial planets and life. In light of Jupiter’s Great Maneuvering, this hypothesis does not seem entirely justified. The first set of solid planets in our system were simply destroyed. And the second set, the planets like Earth and Mars, assembled according to the principle of “scraped along the bottom of the barrel, along the barn with a broom”, miraculously avoided the destruction or throwing out “into the cold” of interstellar space. It seems that the birth of a home for life in our system was extremely difficult, and the number of planets suitable for habitation is much lower than what could be expected in a more peaceful place.

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