Sir

Declan Butler reports on the difficulties the French satellite Corot is facing owing to budgetary reductions (Nature 401, 202; 1999). This report is mainly correct, but I must respond to comments made by David Hughes and cited in the report.

According to the report, Hughes argues that “extrasolar eclipses are rare, even when one is looking at 6,000 stars” and that “by definition, an eclipse would also be irreproducible”.

These comments are inappropriate for the following reasons. First, Corot will observe at least 30,000 stars, not 6,000, as there will be five target fields, each observed for 150 days, and each with a required minimum of 6,000 stars. The exact number depends on the density of stars in the chosen target fields and could increase by up to 5×10,000.

Second, ground-based detection of extrasolar planets has shown that at least three to five per cent of stars have giant planet companions. The geometrical probability of having an eclipse by a planet closer than 0.3 au is 1.5 per cent. So, over five samples of 6,000 stars, and assuming a similar planet/star ratio to that applied to terrestrial planets, Corot should detect a total of 13 to 22 such planets.

Third, a planet with an orbital period of less than 50 days would, in general, make at least three eclipses in each 150-day run, making them, “by definition”, reproducible.

Finally, single-eclipse events will also be analysed, thanks to the two-colour system implemented on the exoplanet channel, which will help reduce false alarms. About ten such events could be detected in this way.