title: A Search for Planet Nine with IRAS and AKARI Data

Authors： Terry Long Phan*, Tomotsugu Goto, Issei Yamamura, Takao Nakagawa, Amos Y.-A. Chen, Cossas K.-W. Wu, Tetsuya Hashimoto, Simon C.-C. Ho, Seong Jin Kim

 Published in Publications of the Astronomical Society of Australia (PASA) on May 23, 2025

• E-print version: https://doi.org/10.48550/arXiv.2504.17288 

• PASA version: https://doi.org/10.1017/pasa.2025.10024

Since the demotion of Pluto from a planet to a dwarf planet in 2006, our solar system has been maintaining its status with eight planets. However, when analyzing the orbits of minor objects in the Kuiper Belt, including Sedna and Sedna-like bodies, astronomers later found that there is a peculiar orbital clustering of these objects not only in argument of perihelion, but also in physical space (see Figure 1). Imagine a situation where people are walking in a crowded shopping mall in the same way and looking in the same direction. It is almost impossible if nobody/nothing forces them to do that. Apply the same logic here, orbital clustering cannot occur randomly due to chance unless it is related to an unknown dynamical mechanism. Simulations from Batygin & Brown (2016) suggested that the gravitational influence of a giant distant planet with a mass of ≥ 10 Earth masses and a semi-major axis of 700 AU can maintain this orbital clustering. That is how they came up with the Planet Nine hypothesis. The most important task is to confirm the existence of Planet Nine by observations.

There were various Planet Nine searches using optical surveys (e.g., Zwicky Transient Facility, Dark Energy Survey, Pan-STARRS1). However, it turns out that they only constrained the possible orbital parameters of Planet Nine without successfully proposing any Planet Nine candidates. One possible explanation is that the Sunlight, travelling from the Sun to Planet Nine and reflecting to the Earth, degenerates as a function of d^-4 (d is the heliocentric distance of Planet Nine). However, the thermal emission in infrared wavelengths from Planet Nine only degenerates as a function of d^-2. Consequently, infrared surveys conducted with space telescopes offer a promising chance for detecting Planet Nine. Instead of using only one infrared survey (IRAS) like Rowan-Robinson (2021), we came up with the idea of combining two far-infrared all-sky surveys (see Figure 2), which are IRAS (1983) and AKARI (2006). The 23-year epoch difference of the two surveys is large enough to detect the slow motion of Planet Nine (~ 3 arcmin/year at 500 AU).

We searched for sources that slowly move from an IRAS position to another AKARI position after 23 years. First, we estimated the expected orbital motion and flux of Planet Nine by assuming its black-body radiation with a mass of 7 – 17 Earth masses, a distance of 500 – 700 AU and an effective temperature of 50 K. One important thing is to make sure that the expected fluxes are above the detection limits of IRAS and AKARI. Next, we excluded too bright and stationary sources from the IRAS and AKARI datasets. The Galactic plane and Galactic bulge regions significantly affect the flux measurements; therefore, they were also excluded by our algorithm. Based on the list of single detections in IRAS and AKARI, we found 13 IRAS/AKARI candidate pairs whose angular separations correspond to a distance of 500 – 700 AU. Only one promising candidate “survives” after the visual image inspection step, as shown in Figure 3. However, more follow-up observations are needed to identify the full orbit of our candidate. The Dark Energy Camera (DECam) mounted on the Blanco 4m telescope in Chile offers a better chance for the Planet Nine search with its larger field of view. As long as we know the position of our candidate, the deeper surveys are powerful even in optical wavelengths.

The search for a mysterious planet beyond Neptune has become increasingly compelling as numerous astronomers contribute to this endeavor. If confirmed, its discovery would significantly reshape our understanding of the solar system’s history. Indeed, there are two possible scenarios for Planet Nine. The first one is that Planet Nine could have formed in the inner solar system at the early stages, then got ejected by the gravitational influence of other gas giants such as Jupiter and Saturn. The other one is that Planet Nine is a free-floating planet, “recently” captured by our solar system. Furthermore, the size of discovered exoplanets ranges between super-Earth and sub-Neptune. However, there are unfortunately no such objects in our solar system, suggesting Planet Nine, if real, could be the first member of this class. In my point of view, before doing research about stars, galaxies, or even the whole universe, it is better to understand our own home – the solar system. I believe that numerous mysterious celestial objects hidden in the dark region of the outer solar system are waiting for us to uncover in the near future.

Contact info: