Objects in the Kuiper Belt 鈥� the region beyond Pluto鈥檚 orbit where many comets originate 鈥� may be much smaller than astronomers thought, according to observations made with NASA鈥檚 Spitzer infrared telescope.
The objects are so far away that even the most powerful telescopes only see them as faint specks of light. While their existence had been inferred decades earlier, the first one was only discovered in 1992. More than a thousand have been found since.
Their distances from Earth can be determined very accurately by determining their orbits, but the only way to estimate their sizes and masses has been to make educated guesses about their reflectivity and density.
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Astronomers had assumed that their reflectivity, or albedo, was similar to that of comets, whose nuclei are very dark. Halley鈥檚 comet, which was studied close up by the Giotto spacecraft in 1986, has an albedo of 4% and this had been taken as the standard value for Kuiper Belt objects (KBOs).
Hot objects
But an object鈥檚 albedo can be calculated directly by measuring the amount of heat it radiates 鈥� its 鈥渇ar-infrared thermal emission鈥�. This is because the object鈥檚 heat is directly related to the darkness of its surface, just as a dark pavement gets hotter in the sun than a lighter one.
It took the power of the Spitzer telescope, which orbits above Earth鈥檚 atmosphere, to apply this method to the extremely faint and distant KBOs.
A team led by John Stansberry at the University of Arizona is using Spitzer to conduct a survey of KBOs. The astronomers鈥� detailed calculations for an object called 2002 AW197 鈥� thought to be one of the largest KBOs 鈥� found it to be much brighter than expected, with an albedo of 18% (plus or minus 4%).
This means that instead of the assumed diameter of 1500 kilometres, or 70% the size of Pluto, it is really only 700 kilometres across.
Major downgrade
At less than half Pluto鈥檚 diameter, it would have only one-tenth the planet鈥檚 mass, since their compositions are believed to be similarly icy. A preliminary analysis of several other KBOs shows an average albedo of 12%, Stansberry says.
The entire Kuiper Belt had been thought to contain perhaps 10,000 objects more than 100 kilometres in diameter, adding up to a total mass of about one-tenth that of Earth. That estimate will now need a major downgrade.
鈥淧luto is more of an extreme than we thought,鈥� Stansberry said. That may help to protect Pluto鈥檚 status as the outermost planet, rather than just the largest known KBO as some astronomers would argue.
The new findings were presented on Friday at the American Astronomical Society鈥檚 Division of Planetary Sciences meeting in Louisville, Kentucky.