When Hipparcos saw the shadow of an alien planet
13 December 1999Astronomers have just realised that news of a planet orbiting a distant star came from ESA's Hipparcos satellite eight years ago, although noone noticed it until now. The first observation, on 17 April 1991, was made long before Michel Mayor and Didier Queloz of the Observatoire de Genève astounded the world in 1995 with their discovery of a planet around the star 51 Pegasi. Since then the search for alien planets has become a highly competitive theme in astronomy, and the present tally of stars known to possess planets is 28.
Nokl Robichon and Fridiric Arenou of the Observatoire Paris-Meudon have re-examined sightings of the star HD 209458, by the Hipparcos star-mapping mission during its life in orbit, 1989 to 1993. They find, among 89 observations, that the star's light was slightly dimmed on five occasions by the shadow of a large planet passing in front of it. The French astronomers looked closely at the Hipparcos data when they heard that colleagues in the USA, monitoring this star from the ground, saw it diminish in intensity for two and a half hours, on two occasions in September 1999.
"To be honest, we would never have found the planet in the data if we had not known where and when to look for it," Robichon comments. "But the prospect of using satellite observations in this way, for smarter detections of alien planets, is now very exciting."
HD 209458 is a sun-like star in the Pegasus constellation. It is also known as HIP 108859 in the Hipparcos Catalogue, and Hipparcos fixed its distance at 153 light-years. Hints that it might possess planets came from observations made from the ground over the past few years, by the usual method of looking for slight shifts in the wavelength of light. These can indicate wobbles in a star due to the gravity of the planets circling around it.
Fresh observations by the Geneva group, using the Swiss 1.2-metre telescope at the European Southern Observatory's site at La Silla, Chile, confirmed and refined the detection of wobbles in HD 209458. They correspond with a large Jupiter-like planet so close to the star that it orbits around it twice a week.
Only in about 10 per cent of such cases would astronomers expect the planet to pass directly in front of the star, as seen from the Earth. But when Michel Mayor gave out the details about HD 209458, David Charbonneau of Harvard University and Timothy Brown of Boulder, Colorado, decided to check it with a 10-centimetre telescope built by Brown especially for looking for transits of planets. Their good luck prompted Robichon and Arenou in Paris to check the Hipparcos data. Staffan Svderhjelm of Lund Observatory, Sweden, had the same idea, and obtained similar results.
"Adding the early Hipparcos data for the transits gives us observations spanning eight years," Arenou notes. "We now know the orbital period of the big planet of this star with amazing accuracy. Our figure of 3.52474 days is twenty times more precise than one could deduce from the star's wobble alone in a few weeks."
The chance now exists to check the Hipparcos data on other stars with planets, looking for dimming by transits. More dramatic results on alien planets should come from Gaia, a project under study by ESA as a successor to Hipparcos. It will detect the wobbles of stars directly, by very small changes in their motions, and should identify about 30,000 stars with large planets. In a few thousand cases, Gaia should also see the dimming effect of planets passing in front of the stars.
The January 2000 issue of the journal Astronomy and Astrophysics will carry a full report by Nokl Robichon and Fridiric Arenou on the Hipparcos results on HD 209458. Their results, and also those obtained independently by Staffan Söderhjelm of Lund Observatory, were summarized in an International Astronomical Union Circular (No. 7323, 1/12/99).
To facilitate the discovery of further planetary transits in the Hipparcos data, ESA has provided a special-purpose web page for astronomers: The Hipparcos Epoch Photometry Search facility (see related link in the right-hand menu).