Free Astronomy Magazine September-October 2022

37 SEPTEMBER-OCTOBER 2022 ASTRO PUBLISHING O rbits of 96 of the 109 known distant asteroids and extreme Trans-Neptunian Objects (eTNOs) with aphelion distances Q > 200 AU as of January 2021. Orbits are from JPL's Small Body Database based on the epoch 31 May 2020. The re- maining 13 asteroids are excluded from this diagram, as their orbits are too uncertain. The red orbits are those of eTNOs with longitudes of the pericenter aligned with the hypothetical Planet Nine, while the blue ones are those of eTNOs anti- aligned with Planet Nine. The pink orbits are those of the three known sednoids (Sedna, 2012 VP113, Leleakuhonua), while the brown ones are of the scattered centaurs and damocloids, with perihelion distances within the orbit of Neptune (q < 29.9 AU). For comparison, the predicted orbit of Planet Nine is shown in purple (a~500, e~0.25, i~20). [Wikimedia Commons] tence of Planet X without, however, excluding it definitively. In fact, it is still possible to hypothesize different combinations of mass, albedo and orbital properties capable of placing a possible ninth planet (not tenth, because Pluto is still officially classi- fied as a dwarf planet) beyond the capabilities of observation among the campaigns conducted so far. The main intervening factors that limit the chance of a discovery are the small apparent magnitude of a body so far away from the Sun, as well as its very slow apparent motion among the stars. We are talking about shifts on the celestial vault on the order of one arc minute per year if the planet is about 800 AU away from the Sun; proportionally less for increasing distances. At this point, it is obvious that the best strategy for discovering Planet Nine (as it is widely known today) is by compar- ing infrared images of the same areas of the sky acquired several years apart from each other. This strategy is the basis of a recent study conducted by Chris Sedgwick and Stephen Serjeant (School of Physical Sciences, The Open University, Mil- ton Keynes, UK), published in the Monthly Notices of the Royal Astro- nomical Society . Sedgwick and Serjeant compared the databases of two major surveys of the whole sky in the far-infrared, separated by 23.4 years (taking mid- points of the survey periods in each case). The two surveys are those car- ried out by the Infrared Astronomi- cal Satellite (IRAS), operational from