Monday, February 19, 2007

The hectometer-sized Trans-Neptunian Objects, are they there or not?

In August of 2006, we have published in the Astronomical Journal the first detection of Trans-Neptunian Objects (TNOs, or Kuiper Belt Objects) with hundreds of meters of diameter: Roques et al., 2006, Exploration of the Kuiper Belt by High-Precision Photometric Stellar Occultations: First Results, AJ, 132, 819-822. This discovery implies the existence of a large population of small objects in the Solar System between 50 and 150 Astronomical Units, region that all previous astronomical observations indicated as empty. The Observatory of Paris emitted a press-release on the discovery.

Using stellar occultation techniques, i.e. the luminosity decrease of a star due to a passing object, it was possible to detect two objects beyond Neptune: one at 140 Astronomical Units (1 AU = average Earth-Sun distance, i.e. 150 million kilometers) from the Sun with 320 meters of diameter, and another one, the most distant, at 210 AU with 300 meters. A third object with 110 meters was detected at 15 AU. These objects were, also, the most distant Solar System objects detected until now.

There are many Solar System objects whose orbit exceeds 100-200 AU, however they have always been detected when they were to the Sun. Additionally, all the previously detected TNOs were detected directly, i.e. physical image of the object. Due to their very large distances and the the current limits of the world's largest telescopes all directly detected TNOs possess tens of kilometers of diameter or more.

The three announced objects were detected from the analysis of about 2 million images obtained in two nights of observations with the 4.2 meters William Herschell Telescope (WHT, La Palma, Canary Islands). Each of them with an exposure of 2 hundredth's seconds with time intervals of less than one thousandth of a second.

In that same month of August, it was published in Nature, independently, the detention of 58 Trans-Neptunian Objects with diameters under 100 m: Chang et al, 2006, Occultation of X-rays from Scorpius X-1 by small trans-neptunian objects, Nature, 442, 660-663. Detected with data from the X-ray satellite RXTE (NASA), using a similar method.

With these two works the existence of hectometer-sized TNOs seemed secure. However, it has become available in astro-ph the article Jones et al., 2007, Millisecond Dips in Sco X-1 are Likely the Result of High-Energy Particle Events (not accepted for publication, yet) stating that Chang et al.'s (2006) detections are not, in fact, small TNOs but rather "cosmic-rays" that hit the RXTE detectors.

This does not invalidate the work of Roques et al. (2006) but diminishes, for the time being, its force by invalidating the independent detections of Chang et al. (2006). Despite the possible non-detection of hectometer-sized TNOs with RXTE, I am still convinced that the detentions with WHT are real. They do exist, and others will be detected.

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