Read aloud The history of the exotics. Nature takes many paths to planet formation. But most of these celestial bodies have bizarre properties, rush to death, or are unfit for earth-like life. Is our solar system the big exception in space? Nearly three dozen planets have been detected by other stars in recent years. Almost all of them are gas giants that often exceed the mass of Jupiter many times over. These "hot Jupiter", as the astronomers call inflated gas balls, pose many puzzles: how did they come about? Why did not they fall into their stars long ago? And are they perhaps the rule in nature - and the properties of the solar system is the exception in space? The problems are even more fundamental: where and how do planetary systems form and develop? And how fast and how often does that happen?

In recent years, astronomers have discovered numerous flattened, rotating clouds of gas and dust around young stars. These protoplanetary accretion disks are the birthplaces of planets. From dynamic instabilities and under the influence of electromagnetic forces and above all gravitation, some of the matter in the disk condenses into lumps. These so-called planetesimals, which grow from one millimeter to kilometer size, are the building blocks of the planets.

Fiery young stars emit a violent stream of fast electrons, protons, and atomic nuclei, and the light elements in the protoplanetary disk are rapidly propelled outward by this stellar wind and heat. Therefore, the small, dense planets Mercury, Venus, Earth and Mars with their solid surface have formed in the inner regions of our solar system, whereas the gas giants Jupiter, Saturn, Uranus and Neptune are further outward. To generalize this idea, the Hot Jupiter must have emerged from other stars further out in their birth cloud and have drifted inward over time. But why did not the hot Jupiter have long been drawn into their star - like moths into the light? What stopped her?

Model calculations show that veritable cosmic billiard games can move planets close to their home star. If several gas giants have been created, their gravitational interaction often results in one planet being catapulted out of the system and another being forced into a narrower orbit. A cosmic whiplash would also be a plausible explanation for the highly elliptical orbit, on which many of the newly discovered planets are buzzing through space. display

After all, all these findings and hypotheses allow one conclusion to be drawn: our solar system must by no means be the norm. Maybe it's even a cosmic stroke of luck. If a giant planet moves on a highly eccentric orbit or very close to its star, then stable orbits for an Earth-like planet at a life-friendly distance from that star are impossible. "The big chunks can sweep all Earth-like planets out of a solar system, " says Marcy. This has consequences: "Our existence is based on the fact that both Jupiter and Earth are running in stable, almost circular orbits."

Extraterrestrial life forms, above all intelligent, could be so much rarer, than optimists assume. But we still do not know enough about the architecture of other planetary systems. Only the exotic giant planets can be reliably detected by our current observation means. More sensitive instruments must show whether our solar system is a cosmic exception or rather the rule in nature. "Earth is still the only known world that bears life, " says Christopher F. Chyba of the University of Arizona at Tucson. "When we explore the conditions necessary for life-friendly planets, we also learn to value the value of our own world better."

=== Rüdiger Vaas

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