The moon, which has of late emerged as an arena for Asian giants to show off their technological prowess, enjoys an uncommon place in the universe.

The sole natural satellite of Earth, it has been an enigma not only for those with an artistic bent of mind but also for scientists. Its formation is still a mystery, with several theories doing the rounds. A major objective of some of the ongoing missions, like China’s recently launched Chang’e 1 and India’s Chandrayaan-1, as well as those still in the planning stages, is to resolve this puzzle.

Among the various explanations for the moon’s formation, the most prominent is an impact theory which postulates that a body as big as Mars smashed into infant Earth some 30 million years into the Sun’s birth, breaking off a piece of its mantle. Some of the resulting debris fell into an orbit around the planet, eventually coalescing into the moon as we see today.

Nadya Gorlova, a post doctoral researcher at the University of Florida, and George Rieke, an astronomer at the University of Arizona, however, say that such massive moon-forming collisions are “uncommon” in planetary systems.

To arrive at this conclusion, Gorlova, Rieke and others looked for dusty signs that such collisions are expected to leave around nearly 400 newly formed stars. These were all about 30 million years old — roughly the age of the Sun when Earth’s moon formed. The scientists found that only one of these stars — that was in an open cluster called NGC 2547 — was immersed in such a signature dust cloud. The study appeared in the November 20 issue of the Astrophysical Journal.

“We don’t know if the collision we witnessed around that star is definitely going to produce a moon….. so moon-forming events could be much less frequent than what our calculation suggests,” Rieke observes.

According to him, the formation of a moon takes about 20 million years, nearly 30 to 50 million years into the life of a star. The dust cloud once created is expected to take about two million years to clear.

And that is not all. “Not all major collisions re-assemble into moons — a big collision is necessary, but not sufficient,” Rieke told KnowHow. This made them conclude that even if there are massive collisions, the possibility of such events leading to moons like ours being formed is considerably below 5 to 10 per cent.

The scientists used data provided by the Sptizer space telescope of the National Aeronautics and Space Administration (Nasa). The space-borne telescope observes stars using several filters and the data are transmitted back to Earth. Gorlova and others used data available from one of the filters that transmits light from 6.5 to 9.5 micron wavelength (one micron equals a millionth of a metre). It is known that the cooler the object, the more it radiates at longer wavelengths. Therefore, while stars radiate strongly in the visible range of the spectrum, dust — the particles of which are cooler than 1,500 degrees Celsius — radiates in the infrared region.

“There are billions of stars in the Milky Way (galaxy), and we surveyed only 400 in this study. However, it is estimated that our Sun was also born in a similar cluster of stars. And we have evidence that the composition of these stars is similar to that of the Sun despite the 4.5 billion years age difference,” Gorlova told KnowHow.

The larger objective of the study was to know whether there are planet systems similar to our own. And if yes, whether they have planets in the “terrestrial” zone where the solar system has Mercury, Venue, Earth and Mars, says Rieke. And also whether they have characteristics like those of the moon or can support intelligent life.

The probe for a large moon-forming event touches all of these points, particularly the last one. “The gravity of the moon raises tides on Earth, and so it is likely that the earliest advanced animals — all of which were sea creatures — first ventured on land by crawling up a beach at high tide and were then stranded there when the tide ebbed,” explains Rieke, who was Gorlova’s teacher at Arizona. “Eventually some of them must have evolved to survive on the beach, and that was a good development because there were no other animals — particularly predators to eat them — up there.”

“So, the moon is possibly the reason why we can walk around on land,” says Rieke.