Washington, May 26 (Reuters): Recipe for an “instant” Earth-like planet: scrape up cosmic dust swirling around a newborn star and wait a mere three million years.

Even the building blocks for giant gas planets like Jupiter might form just as quickly, about three times faster than many scientists believe, a team of astronomers reported today.

Three million years may sound like a long time when set against the human life span, but it is a relative blink of the eye in cosmic time. Earth is considered a middle-aged planet at about 4.5 billion years or so, and compared to Earth, these theoretical 3-million-year-old planets would be formed when the star they orbit is the equivalent of a one-week-old baby.

Astronomers Elizabeth Lada of the University of Florida in Gainesville and Karl Haisch of the University of Michigan in Ann Arbor concluded that the beginnings of planets might form about three million years after stars are born by studying the dusty disks that form around the infant stars.

These disks are made of cosmic dust and gas that can either be absorbed into the still-forming star or spun out into clumps of material that can become planets. But without a disk, it is unlikely that planets will form around a star.

The team found that while disks surrounded many star babies as they clustered together in stellar nurseries at about 1 million years of age, there were relatively few by the time the stars were 3 million years old and none by the time they were 6 million years old.

“For the very youngest (star) clusters, 80 (per cent) to 90 per cent of stars in the cluster have a disk,” Lada said in a telephone interview. “But when we looked toward older clusters, the number of stars that had an indicator of a disk decreased with age until we got out to five or six million years, when the dust that we’re sensitive to is gone.”

The astronomers looked at four prime star-forming regions located in the constellations Orion and Perseus, located some 1,000 light-years from Earth. A light-year is about 10 trillion km, the distance light travels in a year.

To detect potentially planet-forming disks around the young clustered stars, the scientists monitored infra-red light. They found the dusty disks took infra-red light from the central star and gave off infra-red light of their own, so when Lada and her colleagues found excess infra-red emissions, they presumed the presence of a dust disk.

Because they found that in most cases, the dust disk dissipates in three million years or less, they figured that terrestrial, rocky planets like Earth — which are made from such dust — must at least start to form in that time.