A new paper conducted a "needle in a haystack" type search for a hypothetical Planet 9, a hypothesis which Wikipedia explains as follows:
Planet Nine is a hypothetical planet in the outer region of the Solar System. Its gravitational effects could explain the unlikely clustering of orbits for a group of extreme trans-Neptunian objects (ETNOs), bodies beyond Neptune that orbit the Sun at distances averaging more than 250 times that of the Earth. These ETNOs tend to make their closest approaches to the Sun in one sector, and their orbits are similarly tilted. These alignments suggest that an undiscovered planet may be shepherding the orbits of the most distant known Solar System objects. Nonetheless, some astronomers question the idea that the hypothetical planet exists and instead assert that the clustering of the ETNOs orbits is due to observing biases, resulting from the difficulty of discovering and tracking these objects during much of the year.Based on earlier considerations, this hypothetical super-Earth-sized planet would have had a predicted mass of five to ten times that of the Earth, and an elongated orbit 400 to 800 times as far from the Sun as the Earth. The orbit estimation was refined in 2021, resulting in a somewhat smaller semi-major axis of 380+140−80 AU. This was more recently updated to 460 +160−100 AU.
Konstantin Batygin and Michael E. Brown suggested that Planet Nine could be the core of a giant planet that was ejected from its original orbit by Jupiter during the genesis of the Solar System. Others proposed that the planet was captured from another star, was once a rogue planet, or that it formed on a distant orbit and was pulled into an eccentric orbit by a passing star.While sky surveys such as Wide-field Infrared Survey Explorer (WISE) and Pan-STARRS did not detect Planet Nine, they have not ruled out the existence of a Neptune-diameter object in the outer Solar System. The ability of these past sky surveys to detect Planet Nine was dependent on its location and characteristics. Further surveys of the remaining regions are ongoing using NEOWISE and the 8-meter Subaru Telescope. Unless Planet Nine is observed, its existence is purely conjectural. Several alternative hypotheses have been proposed to explain the observed clustering of trans-Neptunian objects (TNOs).
The paper narrowed the search down to one candidate for further examination, at the low end of the range of closeness to the Sun, and smaller, than some hypotheses. The paper and its abstract are as follows:
I have carried out a search for Planet 9 in the IRAS data. At the distance range proposed for Planet 9, the signature would be a 60 micron unidentified IRAS point source with an associated nearby source from the IRAS Reject File of sources which received only a single hours-confirmed (HCON) detection. The confirmed source should be detected on the first two HCON passes, but not on the third, while the single HCON should be detected only on the third HCON. I have examined the unidentified sources in three IRAS 60micron catalogues: some can be identified with 2MASS galaxies, Galactic sources or as cirrus. The remaining unidentified sources have been examined with the IRSA Scanpi tool to check for the signature missing HCONs, and for association with IRAS Reject File single HCONs. No matches of interest survive.For a lower mass planet (< 5 earth masses) in the distance range 200-400 AU, we expect a pair or triplet of single HCONs with separations 2-35 arcmin. Several hundred candidate associations are found and have been examined with Scanpi. A single candidate for Planet 9 survives which satisfies the requirements for detected and non-detected HCON passes. A fitted orbit suggest a distance of 225+-15 AU and a mass of 3-5 earth masses. Dynamical simulations are needed to explore whether the candidate is consistent with existing planet ephemerides. If so, a search in an annulus of radius 2.5-4 deg centred on the 1983 position at visible and near infrared wavelengths would be worthwhile.
Michael Rowan-Robinson "A search for Planet 9 in the IRAS data" arXiv:2111.03831 (November 6, 2021) (Accepted for publication in MNRAS).
It is expected to be 600 times more faint for an Earth based observer than Pluto. The fact that it is a struggle to determine if there is a decent sized planet in the Solar System, while it is possible to see far more distant galaxies and stars, also illustrates the extent to which even state of the art astronomy techniques are far more limited in their ability to see objects in space that are not stars than they are in their ability to see stars and diffuse radiation or gravitational waves.