Hunting Planet Nine: A Far-Infrared Search with IRAS and AKARI

In their study, Terry Long Phan and collaborators set out to investigate one of the most intriguing mysteries of our solar system: the existence of Planet Nine, a yet-unseen planet theorized to orbit far beyond Neptune. The idea comes from unusual patterns in the orbits of distant Kuiper Belt Objects (KBOs) that are difficult to explain without the gravitational pull of a hidden, massive planet. While some debate remains about whether these patterns are real or simply due to observational bias, the possibility of a distant giant planet has motivated many searches. Since Planet Nine would be extremely far from the Sun, it would be very faint in visible light but potentially detectable in infrared wavelengths, where it would emit its own thermal radiation.

Using IRAS and AKARI Surveys

To search for Planet Nine, the team used data from two space-based infrared all-sky surveys: the Infrared Astronomical Satellite (IRAS) launched in 1983 and AKARI launched in 2006. These missions are ideal because their 23-year separation provides enough time for a distant object like Planet Nine to show noticeable movement across the sky. Phan’s team focused on specific catalogs from these surveys, particularly using the AKARI-FIS Monthly Unconfirmed Source List (AKARI-MUSL), which is better suited for detecting faint and slowly moving objects that might otherwise be missed.

Predicting Motion and Brightness

The researchers first estimated how fast Planet Nine would appear to move over 23 years, depending on its distance, and calculated how bright it would be in the infrared assuming it has properties similar to Uranus or Neptune. Using these predictions, they applied strict criteria to sift through millions of sources in the IRAS and AKARI catalogs, filtering out known stars and galaxies, noisy data, and sources too close to the crowded center of the Milky Way. After narrowing down their sample, they matched objects from the two catalogs that moved the right amount and had fluxes and colors consistent with a cold, distant planet.

Finding and Inspecting Candidates

After a detailed search, they identified 13 promising candidate pairs. Through careful inspection of infrared images, they found one particularly strong candidate: an object detected by IRAS that appears missing in the same spot in AKARI images 23 years later, and vice versa. Further analysis showed that the AKARI source behaved like a slow-moving object, strengthening the case that this could be a real planetary body. However, with just two detections, it is impossible to fully confirm an orbit or rule out other explanations like faint background objects.

Next Steps and Future Prospects

The authors stress that follow-up observations are critical. They suggest using the powerful Dark Energy Camera (DECam) to try to spot the object again. With its wide field of view and sensitivity, DECam could track the object's motion and determine whether it follows the kind of orbit expected for Planet Nine. Confirming Planet Nine’s existence would not only solve a major puzzle about the outer solar system but would also reshape our understanding of how planets form and migrate.

Source: Phan