From a grand to a millennium, 1000 is an important milestone no matter what you are counting. But as astronomers close in on the discovery of 1000 exoplanets, deciding when to award the title is proving troublesome.
In recent years, the hunt for alien worlds has been a runaway success, with NASA's now defunct Kepler space telescope taking centre-stage as the king of planet hunters. It alone has bagged 152 now-confirmed planets and thousands more potential candidates.
But astronomers are facing a problem, says Jean Schneider of the Paris Observatory in France, who runs the Extrasolar Planets Encyclopaedia. How do you know when you have reached 1000 planets when there are several exoplanet catalogues and each uses different criteria to decide when a planet has been spotted?
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