An observational error may have understated the size of the tenth planet - if "planet" is in fact what astronomers finally decide to call it.
The upper limit on the size of the object, temporarily called 2003 UB313, was earlier set at 3000 kilometres following the Spitzer Space Telescope's failure to spot any infrared source at its location. But as its discoverer Mike Brown notes on his website, the telescope was actually pointed at the wrong place, so the object could actually be bigger than that.
The Spitzer observations were made before the object was entered in public data bases, so Brown's group at Caltech, US, had to specify the positions of it and two other giant Kuiper Belt objects.
Spitzer was aimed correctly at two of them - but somewhere along the line an error crept in with the location of 2003 UB313. "The mistake was caught by one of the many extremely careful members of the Spitzer Science Center," Brown writes. Spitzer will check the right spot to see 2003 UB313 later in August.
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