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Planetary News: Trans-Neptunian Objects (2006)Hubble Measures Size of "10th Planet"By Amir Alexander13 April 2006 More so perhaps than any other object in the sky, for 2003UB313 size matters. When the giant space rock, nicknamed "Xena," was first announced in July of 2005, it came with a startling assertion: it is larger than Pluto, the ninth and smallest solar planet. If this is the case, many argued that it should be recognized as a planet, the 10th orbiting our Sun, and the first discovered in 75 years.. It is precisely this possibility that has kept the International astronomical Union (IAU) debating the status and name of Xena for almost a year now, without reaching a definite conclusion. But if it is not as large as Pluto, then Xena would be relegated to the ranks of other giant Kuiper belt objects (KBO's), such as Sedna and Quaoar, impressive and interesting in themselves, but not large enough to upset the nine-planet status quo. But how big is 2003UB313 exactly? Estimates have varied considerably, and while all of them have concluded that Xena is indeed larger than Pluto, the latest and most direct measurement gives it only the slimmest of margins. According to a measurement by the Hubble Space Telescope, Xena's apparent diameter is 2,400 kilometers (1490 miles), plus or minus 100 kilometers (60 miles); Pluto's diameter, as measured by Hubble, is 2,288 kilometers (1422 miles). Estimating the size of an object as small and distant as Xena is extremely difficult, according to KBO hunter Michael Brown of Caltech, whose team discovered Xena and several other giant KBO's. All that an Earthbound telescope can see of the object is a single, unresolvable, point of light. The brightness of this light can serve as an important clue as to the object's size: the larger it is, the brighter it is. The brightness, however, depends on another important factor as well – the object's reflectivity, or "albedo." For a given brightness, as seen from Earth, a more reflective object is actually smaller than a less reflective one. Without knowing an object's actual albedo, a size calculation based on brightness can only be a rough estimate.
To get around this problem, scientists in Germany last summer measured not the visible light, but the heat radiation emanating from 2003UB313. Like visible brightness, heat radiation increases with size, and it is also sensitive to an object's temperature, which can be known with considerable accuracy based on its distance from the Sun. Significantly, such radiation is far less dependent on albedo than visible light. Based on their observations, the team concluded that Xena is as much as 30% larger than Pluto. None of these roundabout methods, however, can substitute for a direct measurement. And although no Earth-bound telescope is capable of measuring that point of light which is Xena, the Hubble Space Telescope can – just barely. When viewed through its lens, 2003UB313 takes up just one and a half picture elements, which is just enough to take an accurate measurement. Brown and his colleagues were concerned, nevertheless that Hubble's own optics were distorting Xena's image and size. They therefore waited until Xena passed close to a background star, and then took 28 pictures that included both KBO and star. By carefully comparing the images of the two, they concluded that Xena's diameter is just over 100 kilometers (60 miles) greater than Pluto's.
2003UB313's relatively small size has a startling implication: it makes Xena the second brightest known object in the solar system, behind only Saturn's moon Enceladus. When Brown and his group originally estimated that Xena was significantly larger than Pluto, they assumed that its albedo was similar to Pluto's. This appeared to be a safe assumption, since with a reflectivity of 60% the ninth planet itself is one of the brightest objects orbiting the Sun. It seemed highly unlikely that the new object was brighter than that, and the size estimates based on this assumption were therefore considered conservative. But as seems often to be the case in planetary science, the unlikely scenario now appears to be the true one: since Xena is relatively small, its reflectivity is extremely high – around 88%. And unlike Pluto, which has a pockmarked appearance and reddish hue, 2003UB313 appears to be mostly smooth and a dazzling white. Why Xena is so bright and white is a question that scientists can only speculate on. One possibility entertained by Brown is that since Xena is currently close to its aphelion (its furthest point from the Sun), its atmosphere likely froze and settled on the ground. This snow-like covering could give Xena its high reflectivity, which will decrease substantially by the time 2003UB313 approaches closest to the Sun in 260 years, with its atmosphere restored. Another possibility is that Xena is leaking methane from its interior, which freezes when it comes into contact with the cold surface. At this point these are just educated guesses, and scientists don't really know the answer. Studies of Xena's spectrum currently being conducted at the Gemini observatory in Hawaii may help uncover some of the mysteries of Xena, or it may be years before planetary scientists learn more about this remarkable new member of our Solar System. But if the latest measurement holds, the day may not be far when the official count of planets around the Sun will move up by one, to 10. |
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