May 4, 2001 In 1993, when the Hubble Space Telescope surveyed the Orion nebula for the first time, its images provided a substantial boost for the argument that stars with planetary systems are commonplace in the galaxy. Now, however, the most recent analyses of one the youngest, closest and brightest nebulae cast doubt on that conclusion and suggest that planets may be far rarer than astronomers have thought.
The Orion nebula is the closest example of a stellar nursery. Stellar nurseries are special regions where the vast majority of new stars in the galaxy are born. Interstellar clouds of molecular gas form, produce thousands of new stars and then gradually dissipate. The nebula in Orion is 1,500 light years from Earth and six light years or 35 trillion miles across. It forms the second point of light in the hunter’s scabbard in the Orion constellation. The Trapezium Cluster at the nebula’s center contains more than 1,500 stars. Four massive young stars illuminate the nebula, making it possible to observe many objects that would normally be invisible. The starlight they produce is so intense, in fact, that it ionizes thin layers of the gas in the region, producing a rainbow of colors. So it’s not surprising that studies of Orion have provided astronomers with some of the best information about the process of star formation.
The first Hubble Space Telescope (HST) images found that up to 90 percent of the young stars in the nebula are surrounded by "protoplanetary disks"—disks of dust and gas from which planets can form. Astronomers call such star and disk systems "proplyds." Based on the assumption that similar conditions prevail in other stellar nurseries, the finding strengthened the hypothesis that planet production is a common side effect of the star formation process.
C. Robert O’Dell, lead scientist on the first HST studies of Orion and now a research professor at Vanderbilt University, has been studying the nebula since 1964. In a May 1 presentation at the annual meeting of the American Physical Society in Washington D.C., O’Dell reports that the most recent studies of Orion appear to have come up with a planet stopper. The youngest and brightest stars in the cluster are so powerful that the ultraviolet radiation they produce should blast away the dust and gas surrounding newly formed stars before they can form planets.
"According to current estimates, it takes about 10 million years for a planet to form," O’Dell says. "The massive, young stars in Orion are more than 100,000 times as luminous as the sun. Our best estimate is that these radiation levels can destroy a protoplanetary disk in a few hundred thousand years. So it appears that most of the disks will be gone long before planets can form."
A critical factor in this calculation is the length of time it takes planets to develop. If planets form considerably faster than scientists currently think, then the percentage of stars that develop planetary systems could be substantially higher, O’Dell acknowledges.
Over the last eight years, O’Dell and W. J. Henney at the National Autonomous University of Mexico at Morelia—working with graduate students from Rice University and UNAM—have used a combination of optical and radio telescope data to construct a detailed, three-dimensional map of the nebula. Using this map, he estimates that only 10 percent of the proplyds in the nebula are shielded from the erosive star-shine.
If planetary formation times are correct, and the conditions in the Orion nebula are typical of stellar nurseries, then only one star in 10 is likely to form a planetary system, O’Dell says.
Why then does Orion contain more than 300 circumsolar disks? The answer, according to O’Dell, is quite surprising. One of the stars in Trapezium turns out to be a binary. By carefully measuring the properties of this pair of stars, Francesco Palla at the Osservatio Astrofisico di Arcetri in Italy and Steven Stahler at the University of California, Berkeley have estimated that it can be no older than 100,000 years. Orion’s massive central stars must be even younger, O’Dell contends, because they have created an intense radiation environment that has essentially shut down star formation in the nebula.
"This estimate, combined with the fact that we don’t see any evidence for depletion of the protoplanetary disks, even those exposed to the highest radiation levels, suggests that the central stars are even younger, perhaps only a few tens of thousands of years old," O’Dell says. The fact that these stars may not be any older than mankind itself doesn’t sit well with the astronomer. It goes against the Copernican principle. Copernicus argued that the Earth wasn’t the center of the universe but, rather, that the Earth orbits around the Sun. Since then this has been generalized to the Copernican principle: There is nothing special in time or space about Earth’s position in the universe.
"It is unlikely that homo sapiens and the Orion nebula should be formed at just about the same time, but perhaps we are just lucky," O’Dell says.
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