Free Astronomy Magazine September-October 2022

39 SEPTEMBER-OCTOBER 2022 ASTRO PUBLISHING databases of IRAS and AKARI high- lighted more than 500 possible can- didates, sources that mostly appeared to be at distances fewer than 1000 AU and to have masses similar to or smaller than that of Neptune, both values exactly in the expected range for Planet Nine. However, when the team manually examined the individual infrared sources, they found that none of them were convincing. Most were located in-or-near faint integrated flow nebulae, structures also known as “galactic cirrus clouds.” These are diffuse clouds of interstellar gas not of the many possible combinations of a planet of 5-15 Earth masses, between 200 AU and 1000 AU from the Sun, with an or- bital period between 10,000 and 20,000 years. According to the two Caltech re- searchers, the most likely combination is that of a planet as large as Neptune with an orbit at an average distance from the Sun of about 700 AU. This distance cor- responds to the lower limit adopted by Sedgwick and Serjeant in their study, since for shorter distances a planet of that size would have al- ready been located based on the perturbations of the orbits of many TNOs. The two researchers also set the upper limit of the survey to around 8000 AU based on the sensi- tivity limits of the IRAS and AKARI detectors and the distance of the potential target, whose proper mo- tion, at greater distances, would be too small to be detectable. Surpris- ingly, the comparison between the easily detectable at visible wave- lengths, but relatively bright in the far-infrared. Nothing to do, there- fore, with nearby Neptunian-sized planets. Although the work of Sedg- wick and Serjeant did not produce the desired results, the idea of com- paring databases of surveys widely spaced over time appears to be the main road to follow in the search for Planet Nine. The only alternative is to calculate its position in the sky starting from the strange orbital concentrations of some TNOs. Cur- rently, the selection of TNOs is still too limited to follow this path. E xamples of images from IRAS and AKARI showing “Planet Nine” candi- dates within galactic cirrus clouds, which are almost certainly the cause of the far- infrared flux. The cir- cles indicate the coordinates of the targets: red for IRAS; blue for AKARI. The size of the circles is arbitrary. IRAS images are in J1950 equato- rial coordinates; AKARI are in ecliptic coordinates. [C. Sedg- wick and S. Serjeant] !