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Knowing the object's distance from the Sun allows astronomers to calculate its surface temperature (Artist's impression: NASA/JPL-Caltech)

‘Tenth planet’ may be bigger than expected

by Jeff Hecht

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.

Heat and light

The space telescope’s infrared observations are critical for estimating the object's size. Asteroids, comets, and Kuiper belt objects vary widely in how much visible light they reflect, but internal heat makes them glow faintly in the infrared.

Knowing the object's distance from the Sun, astronomers can calculate its surface temperature, so measuring its infrared brightness reveals its size.

But Spitzer still may not spot 2003 UB313. Its spectrum resembles Pluto's, and if it reflected the same fraction of light – 60% – its known brightness would reveal it to be only 2860 kilometres in diameter, and too small to be seen. This would nevertheless make it bigger than Pluto, which is 2285 km wide.

But if it reflected the same fraction of light as Pluto's moon Charon, just 38%, 2003 UB313 would measure 3550 kilometres across, making it visible to Spitzer’s infrared instruments.

Urgent deliberations

Brown's group is also looking at 2003 UB313 at much longer wavelengths – near the boundary of the infrared and microwave bands – using the 30-metre ground-based IRAM telescope. This too can determine the object’s heat output, though the signal will be fainter than that sought by Spitzer.

The discovery of 2003 UB313 has added urgency to the deliberations of a working group that had been pondering the definition of a planet. Until that group comes to a decision, the International Astronomical Union says "the object will not be given an official name".

Approval of a name would then fall to one of two IAU panels - on minor planets or full-fledged planets - which have different criteria. The planet panel wants to continue naming planets after Greek and Roman gods. Brown says most of the good ones are taken, but "we have a couple of interesting choices in mind".

Story continues www.newscientistspace.com

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