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| - Biases in cometary catalogues and Planet X
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| - Abstract. Two sets of investigators - Murray and Matese et al. - have recently claimed evidence for an undiscovered Solar System planet from possible great circle alignments in the aphelia directions of the long-period comets. However, comet discoveries are bedevilled by selection effects. These include anomalies caused by the excess of observers in the Northern as against the Southern hemisphere, seasonal and diurnal biases, directional effects which make it harder to discover comets in certain regions of the sky, as well as sociological biases. A simple mathematical model is developed to illustrate the geometrical selection effects controlling comet discoveries. The stream proposed by Murray is shown on an equal-area Hammer-Aitoff projection. The addition of newer data weakens the case for the alignment. There is also evidence that the subsample in the stream is affected by seasonal and north-south biases. The stream proposed by Matese et al. is most obvious in the sample of dynamically new comets, and especially in those whose orbits are best known. The most recent data continue to maintain the overpopulation in the great circle. This pattern in the data occurs with a probability of only ∼1.5 × 10−3 by chance. None of the known biases is able to provide such an alignment. Numerical integrations are used to demonstrate that a planet by itself can reduce the perihelia of comets in its orbital plane to sufficiently small values so that they could be discovered from the Earth. To maintain the observed flux of comets in the stream requires a parent population of ∼3 × 109 objects on orbits close to the planet's orbital plane. There is a need for a sample of long-period comets that is free from unknown or hard-to-model selection effects. Such will be provided by the European Space Agency satellite GAIA, which will discover ∼1000 long-period comets during its 5-yr mission. This may finally bring to fruition the long tradition of looking for the effects of perturbers in cometary catalogues.
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