How the planets were created - updates, corrections and completions

Discoveries of more and more planets outside the solar system forces researchers to update the theories about their formation and behavior

Economist

In 1995, in orbit around the star 51 Pegasi, the first planet outside the solar system was discovered. In the five years that have passed since then, with the discovery of more and more planets outside the solar system (the number today stands at 50), the accepted opinion on how they formed and behaved has been undermined. New theories, largely based on the study of those "new" planets, were born.

According to the accepted model, the planets were formed from a disk of gases and dust that surrounded the young Sun, in a process called "absorption by lava". Chunks of matter collided and coalesced until they became large enough to become the rocky cores of planets. In the case of gas giants, the rocky lavas drew in gases through gravity. The end result was the familiar picture of the solar system: small, rocky planets near the sun, planets surrounded by gases at a great distance from it, and all of them - except for Pluto, a tiny snowball that is not considered a real planet anyway - move in more or less ordered and circular orbits.

As long as the researchers relied on our own solar system as an example, the model seemed quite reasonable, and the popular explanation was that solar systems around other stars, although unknown at the time, were similar to our own. That's why the researchers were surprised when it turned out that the other solar systems are completely different. For example, the planet 51 Pegasi is a gas giant like Jupiter, but its orbit is closer to its star than the orbit of Mercury (the planet Mercury), around our Sun. Since then, other gas giants have been discovered moving in orbits close to their star, which has intensified the mystery.

And there are also the eccentric planets, like the one that revolves around the star Epsilon Eridani. These move in highly elliptical orbits, so their distance from the parent star varies greatly as they rotate. This phenomenon is distinctly different from the behavior of the planets in the Earth's solar system.

And that's not all. In the two systems where more than one planet has been discovered, a Jupiter-like gas giant and one or more eccentric planets coexist. This is a form of organization that cannot be explained by the conventional theory of planet formation. It is clear that the old recipe needs updating - some new ingredients must be added to explain the formation of the different types of solar systems. Two speakers at the International Astronomical Union conference held recently in Manchester - Alan Boss from the Carnegie Institution in Washington and Pavel Artimovich from the University of Stockholm - reported on the progress of the updates.

Dr. Boss is particularly enthusiastic about a formation mechanism called "disc instability," which he believes may be just as important as the "absorption by the core" model. Unlike accretion, which occurs slowly, instability of the disc is a sudden process, in which planetary cores are formed as a result of the instability of the protoplanetary disc's gravity (the remnants of the gas disc from which the parent star formed).

Disk instability is not a new idea. The idea was first raised in the XNUMXs, but was abandoned very quickly. The reason for this was that, according to this theory, gas giants do not form around a rocky core, while the researchers assumed with certainty that such a core existed in the center of Jupiter. Today, when the need for new ideas arose, Bose revived the old advocacy. He points out that according to recent studies, Jupiter may not have a rocky lava at all. This means, he says, that Jupiter may have formed because of disc instability.

Boss does not claim that the lava absorption theory is wrong, and that the disc instability theory is correct; In fact, within the same solar system, some stars may be formed in one process, and other stars in the other process. But the adoption of this additional formation mechanism - if it is possible to confirm the possibility of its existence in computer simulation - will significantly increase the range of possible results.

While Boss promotes innovative thinking about the formation of planets, Dr. Artimovich is interested in the evolution of solar systems that have already formed. Given how many new planets move in their orbits around a certain star (without going into the question of how they were formed), how is their final configuration determined? Clearly, the existence of hot gas giants moving in the vicinity of their parent star cannot coexist with the old idea that planets form around a star, and then remain fixed in their orbits.

This old idea is also based on the conventional theory of the formation of our solar system. However, this model was never able to properly explain the fact of the existence of the planets Uranus and Neptune, which are larger than expected given their distance from the Sun. Furthermore, the recent analysis of the chemical composition of Jupiter shows that the planet may have formed further away from the Sun, and has grown closer to it over time. It seems that a process that can be called "migration" - the movement of a planet to or from the parent star - is needed to explain the structure of our solar system.

It is possible that one of the mechanisms that cause migration is related to the interaction between the planet and the remnants of the disk from which it was formed. But this is a complex process whose details are far from being deciphered. The biggest mystery is why gas giants don't collide with their parent stars.

Interactions between a planet and the disk may also affect the shape of the planet's orbit around its parent star. Dr. Artimovich claims that there may be a critical mass of a planet, which determines the shape of the orbit. When the mass of the planet exceeds the critical mass, an elliptical orbit is formed; When it is smaller than that, a circular path is created. This may explain why the Sun's planets have more or less circular orbits, while larger planets move in eccentric orbits.

In addition to the interaction between the disk and the planet, interactions between the planets themselves also seem to help shape the structure of the solar system. Sometimes such interactions push some planets out of their original system, sometimes they change their orbits. Again, this would explain the existence of large planets, which move in eccentric orbits around their stars. These are the only survivors of a gravitational struggle between several planets.

And what about the consequences for Earth-like planets? Because existing methods are unable to detect such small planets, the theory of their formation, which is based on that of the solar system, has not yet had to deal with conflicting observations. In any case, before getting ready to deal with those small bodies, it will be necessary to decipher the secret of the formation and development of the gas giants that determine the fate of their tiny brothers.

Economist

(Originally published on 12.8)
{Appeared in Haaretz newspaper, 16/8/2000}