|Much quicker than thought, |
planets may only take 3 million
years to form
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 on Monday.
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 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, by studying the dusty disks that form around the infant stars, that the beginnings of planets might form about three million years after stars are born.
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 clusters, 80 to 90% 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
1000 light-years from Earth.
A light-year is about 6 trillion miles (10 trillion km), the distance light travels in a year.
To detect potentially planet-forming disks around the young
clustered stars, the scientists monitored infrared light.
They found the dusty disks took infrared light from the central star and gave off infrared light of their own. So when Lada and her colleagues found excess infrared 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.
They figured the gas in the disks must dissipate just as quickly, which would mean that gassy giants like Jupiter would begin forming at about the same point in time.
These findings were reported at a meeting of the American
Astronomical Society in Nashville, Tennessee.
Astronomers had previously figured that planets could form
in as little as 10 million years or so after star birth.
However, supercomputer simulations reported last year in
the journal Science suggested an even quicker path to planetary formation, with calculations indicating that big Jupiter-type planets might form in as little as hundreds of years, instead of millions.