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?
Currently, Schneider's encyclopedia lists 990 worlds, and he expects 10 more within a few weeks. But other exoplanet databases are coming up lower, including NASA's own, which lists just 906.
One reason for the difference is that Schneider adds planets when confirmation of their existence is presented at conferences, while NASA only accepts those announced in a journal. The latter takes longer. NASA also updates its list less frequently.
Data tricks
Lists aside, there's also the problem that no one can quite agree on the exact definition of a planet. This isn't the problem that befell Pluto, considered the smallest planet in our solar system until 2006, when it was demoted to a dwarf planet.
It's actually the opposite – it's not clear how to distinguish planets at the larger end of the spectrum from failed stars called brown dwarfs. As a result, some ultra-heavy planets in catalogues today may eventually be removed.
Another wrinkle is that even confirmed exoplanets, spotted multiple times, can turn out to be tricks of the data. So far Schneider has deleted around 10 of these from his catalogue – but more could emerge. For example, the discovery of Schneider's favourite exoplanet, Alpha Centauri Bb, the closest world to Earth, was recently called into question.
Astronomers are keen to find a way through all this and mark the 1000th exoplanet. Whatever happens, it's a problem Schneider didn't imagine worrying about in 1992 when the first two exoplanets were glimpsed. "At the time we could not hope to have so many planets 20 years later."
Explore our interactive graphic: How many Earths?.
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