Giant planets outside the solar system hold many mysteries

In science fiction works, aliens with tentacles driving spacecraft to visit the earth are very exciting. And the search for extraterrestrial life is one of the greatest temptations that attracts people to explore the universe.

The evolutionary history of the Milky Way is very long. If there are other intelligent and technological civilizations on planets similar to the Earth, how will these civilizations differ from Earth's civilization?

As we all know, most stars in the Milky Way are billions of years older than the sun. So most alien life should have evolved longer than humans on Earth. Observations from space telescopes have shown that the oldest stars have existed for 13.7 billion years, while the Earth was formed "only" 4.6 billion years ago. Figuratively speaking, humans are probably just "children in the tribe." Unless all more advanced civilizations eventually self-destruct, alien civilizations have enough time to develop to a more advanced stage than humans on Earth.

In order to find extraterrestrial planets with life, scientists often assume that they have an environment similar to that of Earth. Although this seems a bit imaginative, it is the only reliable "starting point" for Earthlings. It was not until the 1990s that scientists confirmed the existence of other planets outside the solar system. As of March this year, with the help of sufficiently powerful astronomical telescopes, the total number of confirmed extrasolar planets has exceeded 5,000.

What surprised astronomers was that the Jovian planets (similar to Jupiter) outside the solar system were not in orbits far from their stars. Compared with the planets in the solar system, the orbits of those planets are generally closer to their respective stars, and many are even closer than Mercury. Researchers also found that the orbits of the planets outside the solar system are generally very flat ellipses, which is very different from the nearly circular orbits of the planets in the solar system.

Why is this so? Some people believe that it is impossible for giant planets to form so close to a star like the Sun. When the Sun was first formed, lighter gases (such as helium and hydrogen) were the main components of the giant planets. They would be "blown away" from the core of the solar system by the strong solar wind and would not have had time to form a "big guy" like Jupiter.

In other star systems, giant planets should have been formed at distances similar to those from the Sun to Jupiter, but those giant planets either gradually moved toward the stars or were "thrown" back into the star systems by external forces.

As for the reason why giant planets move toward stars, it may be that the high-density primordial nebula hinders the giant planets from revolving, forcing them to "fall inward" and their orbital radius becomes smaller and smaller. It is also possible that multiple giant planets of similar mass are formed at the same time, but no one giant planet plays a dominant role in gravity, so the entire system is in an unstable state for a long time. As the gravitational forces increase and decrease, two or more giant planets collide with each other, knocking the giant planets to an area closer to the star, or being thrown out of that star system and becoming free planets "wandering" between the stars.

Through calculations, it was found that a planet like the Earth, which is at a suitable distance from the star to ensure the existence of liquid water, is impossible to appear in the above-mentioned group of giant planets. In the area very close to the star, the giant planets will not have a stable orbit like the Earth's. If it is far away from the star, the orbit of the giant planet tends to be elliptical, which will cause strong gravitational disturbances, so a "small guy" like the Earth cannot stay in a stable orbit for a long time. The giant planet "neighbor" will force it to deviate from the orbit and be "thrown" into the abyss of the universe.

So far, it is difficult for Earthlings to see exoplanets with their own eyes. They are mostly discovered indirectly. Although we know some of their characteristics, it is far from proving that there is life on these planets. Therefore, more mysteries of exoplanets need to be further explored.