Astronomers have discovered that terrestrial planetsmight form around many, if not most, of the nearby sun-like stars inour galaxy. These new results suggest that worlds with potential forlife might be more common than we thought.
University of Arizona, Tucson, astronomer Michael Meyer and hiscolleagues used NASA's Spitzer Space Telescope to determine whetherplanetary systems like ours are common or rare in our Milky Way galaxy.They found that at least 20 percent, and possibly as many as 60percent, of stars similar to the sun are candidates for forming rockyplanets.
Meyer is presenting the findings at the annual meeting of theAmerican Association for the Advancement of Science in Boston. Theresults appear in the Feb. 1 issue of Astrophysical Journal Letters.
The astronomers used Spitzer to survey six sets of stars,grouped depending on their age, with masses comparable to our sun. Thesun is about 4.6 billion years old. "We wanted to study the evolutionof the gas and dust around stars similar to the sun and compare theresults with what we think the solar system looked like at earlierstages during its evolution," Meyer said.
The Spitzer telescope does not detect planets directly. Instead itdetects dust – the rubble left over from collisions as planets form –at a range of infrared wavelengths. The hottest dust is detected at theshortest wavelengths, between 3.6 microns and 8 microns. Cool dust isdetected at the longest wavelengths, between 70 microns and 160microns. Warm dust can be traced at 24-micron wavelengths. Because dustcloser to the star is hotter than dust farther from the star, the"warm" dust likely traces material orbiting the star at distancescomparable to the distance between Earth and Jupiter.
"We found that about 10 to 20 percent of the stars in each ofthe four youngest age groups shows 24-micron emission due to dust,"Meyer said. "But we don't often see warm dust around stars older than300 million years. The frequency just drops off.
"That's comparable to the time scales thought to span the formation anddynamical evolution of our own solar system," he added. "Theoreticalmodels and meteoritic data suggest that Earth formed over 10 to 50million years from collisions between smaller bodies."
In a separate study, Thayne Currie and Scott Kenyon of theSmithsonian Astrophysical Observatory, Cambridge, Mass., and theircolleagues also found evidence of dust from terrestrial planetformation around stars from 10 to 30 million years old. "Theseobservations suggest that whatever led to the formation of Earth couldbe occurring around many stars between three million and 300 millionyears old," Meyer said.
Kenyon and Ben Bromley of the University of Utah, Salt LakeCity, have developed planet formation models that provide a plausiblescenario. Their models predict warm dust would be detected at 24-micronwavelengths as small rocky bodies collide and merge. "Our work suggeststhat the warm dust Meyer and colleagues detect is a natural outcome ofrocky planet formation. We predict a higher frequency of dust emissionfor the younger stars, just as Spitzer observes," said Kenyon.
The numbers on how many stars form planets are ambiguousbecause there's more than one way to interpret the Spitzer data, Meyersaid. The warm-dust emission that Spitzer observed around 20 percent ofthe youngest cohort of stars could persist as the stars age. That is,the warm dust generated by collisions around stars three to 10 millionyears old could carry over and show up as warm dust emission seenaround stars in the 10- to 30- million-year-old range and so on.Interpreting the data this way, about one out of five sun-like stars ispotentially planet-forming, Meyer said.
There's another way to interpret the data. "An optimisticscenario would suggest that the biggest, most massive disks wouldundergo the runaway collision process first and assemble their planetsquickly. That's what we could be seeing in the youngest stars. Theirdisks live hard and die young, shining brightly early on, then fading,"Meyer said."However, smaller, less massive disks will light up later. Planetformation in this case is delayed because there are fewer particles tocollide with each other."
If this is correct and the most massive disks form their planets firstand the wimpiest disks take 10 to 100 times longer, then up to 62percent of the surveyed stars have formed, or may be forming, planets."The correct answer probably lies somewhere between the pessimisticcase of less than 20 percent and optimistic case of more than 60percent," Meyer said.
The next critical test of the assertion that terrestrialplanets like Earth could be common around stars like the sun will comenext year with the launch of NASA's Kepler mission.
Meyer's 13 co-authors include John Carpenter of theCalifornia Institute of Technology in Pasadena. NASA's Jet PropulsionLaboratory in Pasadena manages the Spitzer Space Telescope mission forNASA's Science Mission Directorate, Washington. Science operations areconducted at the Spitzer Science Center at Caltech. Caltech manages JPLfor NASA.
本文摘自jpl.nasa
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