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Canterbury astronomers discover a massive planet

Canterbury astronomers discover a massive gas-giant planet

A team of astronomers in the Department of Physics and Astronomy at the University of Canterbury, Christchurch, New Zealand has discovered a massive planet orbiting a faint star in the southern Milky Way.

The discovery was made by Foundation for Research Science and Technology Top Achiever doctoral scholar, David Ramm working with his supervisors Dr Jovan Skuljan and Professor John Hearnshaw. The observations were made with a new Hercules fibre-fed spectrograph at Mt John University Observatory, Tekapo, in the South Island of New Zealand. Dr Ljiljana Skuljan also contributed to some of the observations. The technique relies on very precise velocity measurements that detect the wobble of the parent star due to the tug of the planet on the star as it undergoes its orbit. The planet itself is far too dim to observe directly, but its presence is inferred from the wobble in the star. The star is somewhat larger and hotter than the Sun, and is the hottest yet known to harbour a planet.

The mass of the planet is about seven and a half times greater than that of Jupiter, which is the largest planet in the solar system. Its orbit is almost circular and the size is by chance almost exactly the same as the size of the Earth's orbit around the Sun.

Although over a hundred extra-solar planets have been discovered since the first in 1995, this is one of the more massive ones so far found. The new planet takes just under a year to complete one orbit. Professor Hearnshaw said what made the discovery so exciting was that by chance they probably saw the orbit practically edge on, so that once every 324 days,the planet passed in front of the star as seen from the Earth, thereby blocking about six per cent of the star's light for a few hours.

“This crucial observation enables the size and density of the planet to be determined.”

The diameter is about 450,000 km, or three times greater than that of Jupiter, and the density is about 20 per cent of the mean density of the Sun or Jupiter, or about 300 kg per cubic metre (which is 30 per cent of the density of water).

The observations of the what is probably a transit were made over 13 years ago by the European space probe Hipparcos, but the significance of the brief dip in the brightness of the star at that time was not recognised until the Canterbury astronomers re-examined the old satellite data. This is only the third planet known to make transits in front of its parent star, and is by far the largest orbit of those that do so. Professor Hearnshaw said the discovery was also unusual in that the parent star was in a binary system, that is, two Sun-like stars orbiting each other.

“The second fainter star is more than 20 times further away from the brighter primary star than the planet is. However such binary systems have often been thought to render possible planetary orbits unstable. That this one has survived the pull of the companion star will be of great interest to many theoreticians.” Many properties of the planetary atmosphere may be deduced in future observations of the transit. The next transit is predicted to occur in late May 2004. In 2003 the transit is predicted to occur in early July, and the uncertainty in the orbit means it has probably occurred already. It is quite possible that the giant planet just discovered is not the only planet orbiting the star in question. Some evidence of at least one other planet in a smaller orbit may be present in the data. However this interpretation could change as more observations are collected. The discovery by the Canterbury team using Mt John data is important because of the high mass of the new planet, its large size and low density, the fact that it is only the third planet known to make a transit of its parent star, it is the hottest star known to have a planet, and it is one of the few planets to orbit a star in a binary system.

“All these circumstances make this a rather eventful discovery, and one that is likely to receive intense scrutiny from astronomers at Mt John and around the world in the future,” Professor Hearnshaw said. “In a sense, this was an entirely serendipitous discovery. The star was being observed explicitly to make a precise measurement of its mass. Although we were very conscious that planet detection was a possibility using observations of the type we were doing, we had not expected such an interesting discovery in this case, given the binary nature of the star.”

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