An unusual couple, at least according to cosmic standards. Image: NASA / JPL-Caltech
Read aloud An earth without a moon is hard to imagine for humans, but for cosmic proportions they are a rare pair. Trabants, like Earth's Moon in a massive collision, are likely to be found in only five to ten percent of planetary systems, says a research team headed by Nadja Gorlowa of the University of Florida at Gainesville. Astronomers believe that the moon was born 30 to 50 million years after the sun came into existence. A protoplanet the size of Mars, called Theia, clashed with the young Earth. From the rubble a Trabant clenched, which became quite large in relation to the earth. Most moons emerge at the same time as their parent planets or are later captured, already fully developed.

Gorlowa and her colleagues have now examined more than 400 young stars with the Spitzer Space Telescope, which are exactly the right age for a moon-forming collision. "When a moon is created by a destructive collision, dust should spread everywhere, " explains Gorlowa. "So we would expect a lot of dust from many stars if such collisions were a common occurrence." But the researchers only found telltale dust in one of the 400 solar systems.

From the lifetime of the dust and the possible time span within which a collision could take place, the researchers came to a maximum of five to ten percent of all planetary systems that could accommodate a moon like Earth's. "Since we do not know if a collision that we discovered has really created a moon, such celestial bodies could be even rarer, " co-author George Rieke of the University of Arizona at Tucson points out,

The researchers also conclude from their observations that the process of planet formation after 30 million years is almost over in almost all stars. So far, astronomers have assumed that planets take 10 to 50 million years to grow out of the dust surrounding a young star. display

In any case, the moon is a stroke of luck for life on earth: without a moon, the Earth's rotation would be much more unstable, the climate would fluctuate between much larger extremes. There would be no eclipses and the day would only be six hours long. Some researchers even believe that without the moon there would be no life on earth. In their imagination, the first cells formed in sheltered tidal pools. And without a moon, there would be virtually no tides.

Nadja Gorlowa (University of Florida, Gainesville) et al: The Astrophysical Journal, Vol. 670, p. 516 Ute Kehse

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