File Photo: Few features can be seen in N 81 from ground-based telescopes, earning it the informal nick-name "The Blob." Astronomers were not sure if just one or a few hot stars were embedded in the cloud, or if it was a stellar nursery containing a large number of less massive stars. Hubble's high-resolution imaging shows the latter to be the case, revealing that numerous young, white-hot stars---easily visible in the color picture---are contained within N 81. Hubble Heritage Project Image DESKTOP AVAILABLE
The results, to be published in Astrophysical Journal Letters in December, have left astronomers seriously considering the notion that planet-forming conditions may not be uniform across galaxies.
Ron Gilliland of the Space Telescope Science Institute in Baltimore, Maryland, and Tim Brown at the US National Center for Atmospheric Research in Boulder, Colorado, used the Hubble Space Telescope to search the globular star cluster 47 Tucanae.
They looked for a tell-tale dip in brightness that can occur when a planet outside the Solar System passes in front of its star. Most of these exoplanets are "hot Jupiters"; gas giant globes that circle their stars closely, taking just a few days to complete an orbit.
Of the 50 identified planets outside our Solar System, only one is aligned to create this blinking effect when viewed from Earth. HD 209458 was first spotted winking at astronomers in November 1999. The Hubble astronomers observed this star to calibrate the effect before embarking upon their own fruitless search.
Ian Crawford at University College London believes the work could be significant. "Taken at face value, it means that planets are rarer around stars in globular clusters than they are around freely floating stars in the solar neighbourhood."
The first results were presented at an American Astronomical Society meeting (New Scientist, 17 June 2000, p 12), but astronomers are now realising that they might have predicted the findings. Crawford explains: "It is becoming statistically significant that those stars found to have planets are rich in heavy elements. They looked at a very metal-poor globular cluster and it has got fewer planets than expected. It all makes sense."
Astronomers suspect that heavy elements provide a planetary "seed" dense enough to attract the gases characteristic of hot Jupiters. Jupiter itself is thought to have a heavy-element core 15 times the mass of the Earth.
Astronomers are considering other reasons why planet-forming conditions appear to vary across the Galaxy. The density of stars in a globular cluster is much greater than in the Sun's neighbourhood. This may have ripped planet-forming discs apart before planets could condense.
But Crawford is cautious not to draw too many conclusions from the findings. "They expected to see 17 planets and they have seen none. It does not mean that there are no planets there."
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