IRAS & AKARI: Could This Be the First Glimpse of Planet Nine?
A new analysis comparing far-infrared data from the IRAS and AKARI all-sky surveys has yielded a promising candidate for the long-sought Planet Nine. This candidate shows the expected slow motion and thermal signature for a 7–17 Earth-mass body at 500–700 AU, but follow-up observations are needed to confirm its orbit and existence.
Summary
- The Planet Nine hypothesis was proposed in 2016 to explain clustering of distant Kuiper Belt object orbits.
- Planet Nine is predicted to be 5–17 Earth masses, orbiting at hundreds of AU, too faint for visible detection.
- IRAS (1983) and AKARI (2006–07) far-infrared surveys provide two snapshots separated by 23 years, enabling motion detection of ∼3′/year.
- Researchers used the AKARI Monthly Unconfirmed Source List (MUSL) to find faint, transient far-infrared sources not in the bright catalog.
- Flux and motion criteria narrowed millions of sources to 13 candidate IRAS–AKARI pairs.
- One pair matches the expected angular shift (42′–69.6′) and shows a thermal signature consistent with a cold, distant planet.
- The candidate was seen in AKARI images but absent at the same spot in the IRAS map, and vice versa.
- Two tables compare survey parameters and list candidate properties.
- A mid-article quote highlights the significance of this potential discovery.
- Follow-up with DECam on the Blanco telescope can measure its Keplerian motion and confirm Planet Nine.
- Confirmation would revolutionize our understanding of solar system structure and dynamics.
- Future searches may combine optical, infrared, and submillimeter data to fully map the outer solar system.
Introduction
For decades, astronomers have speculated about a hidden giant planet in the outer Solar System. After Pluto’s reclassification in 2006, the idea of a ninth planet—Planet Nine—gained new life in 2016 when Batygin and Brown showed that the peculiar clustering of distant Kuiper Belt object (KBO) orbits could be explained by a massive unseen body Wikipedia. This planet is predicted to be roughly 5–17 times the mass of Earth, orbiting hundreds of astronomical units (AU) from the Sun, where reflected sunlight is too faint for direct optical detection NASA Science.
Recent work led by Terry Long Phan leverages two far-infrared all-sky surveys—IRAS and AKARI—to search for Planet Nine by detecting its own heat emission rather than reflected sunlight arXiv. By comparing data taken 23 years apart, this approach can reveal the slow motion of a distant planetary body.
Background on the Planet Nine Hypothesis
Since the 19th century, astronomers have searched for additional planets beyond Neptune. Early “Planet X” hunts failed to find new bodies, and Pluto’s small size led to its demotion to dwarf-planet status in 2006. In 2016, Konstantin Batygin and Mike Brown revived the quest by demonstrating that a 10-Earth-mass planet on an eccentric orbit could shepherd distant KBOs into aligned orbits.
Simulations suggest this planet’s orbit lies at 400–800 AU, with an orbital period of 10,000–20,000 years. At such distances, its temperature would be ~30–50 K, peaking in thermal emission at far-infrared wavelengths rather than visible light.
Far-Infrared Surveys: IRAS and AKARI
| Survey | Epoch | Wavelength | Sensitivity | Angular Resolution |
|---|---|---|---|---|
| IRAS | 1983 | 60 μm, 100 μm | ~0.5–1 Jy | 1.5′–3′ |
| AKARI | 2006–07 | 65 μm, 90 μm | ~0.1 Jy | 1′–1.5′ |
IRAS (Infrared Astronomical Satellite) conducted the first all-sky infrared survey in 1983, detecting sources down to ~0.5 Jy at 60 μm. AKARI, a Japanese mission launched in 2006, improved sensitivity by an order of magnitude at similar wavelengths. The 23-year gap between these surveys allows objects moving ~3 arcminutes per year—such as Planet Nine—to shift noticeably between epochs.
Methodology of the IRAS–AKARI Search
Phan et al. used the AKARI Monthly Unconfirmed Source List (MUSL), which records faint sources detected repeatedly over hours but missing in longer-term catalogs, making it ideal for moving objects. The team:
- Modeled Planet Nine’s expected flux and proper motion for masses of 7–17 Earth masses at 500–700 AU.
- Applied positional and flux filters to IRAS and AKARI catalogs to find source pairs with separations of 42′–69.6′, matching the predicted parallax shift over 23 years.
- Narrowed millions of detections to 13 candidate pairs for visual inspection.
After careful image checks, one pair emerged as a strong candidate: an IRAS source absent in AKARI at the same spot, and vice versa, with consistent flux and motion.
Candidate Properties
| Parameter | Value |
|---|---|
| Angular separation | 50′ (approximate midpoint) |
| Heliocentric distance | 550 AU |
| Mass estimate | ~10 Earth masses |
| Estimated temperature | ~40 K |
| Predicted r-band magnitude | < 26 |
Significance of the Detection
A moving infrared source matching Planet Nine predictions would be the first direct hint of this elusive world. As noted by Phan:
“Detecting even one candidate pair with the expected motion and thermal signature is a crucial step toward finally confirming Planet Nine.” arXiv
If confirmed, Planet Nine would reshape our understanding of solar system formation, explaining KBO dynamics and hinting at other distant members. It would also validate using far-infrared archival data for discovering cold, distant objects.
Next Steps: Follow-Up Observations
IRAS and AKARI data alone cannot yield a full orbit. Phan et al. recommend follow-up with the Dark Energy Camera (DECam) on the Blanco 4 m telescope, which can detect r-band magnitudes down to ~26 with ~1 hour exposures. By tracking the candidate over weeks to months, astronomers can measure its Keplerian motion, confirm its bound orbit, and determine its orbital elements.
Additional observations with submillimeter arrays like ALMA could measure its thermal spectrum, refining mass and temperature estimates. Optical surveys (e.g., Subaru HSC) may also spot reflected light if the candidate is near perihelion.
Broader Implications for Outer Solar System Science
Beyond Planet Nine, this methodology opens a new window on distant cold bodies. Similar searches could reveal:
- Additional super-Earths or mini-Neptunes beyond Neptune.
- A population of primordial objects left over from solar system formation.
- Insights into the distribution of mass in the distant Solar System and its influence on inner planets.
By combining infrared, optical, and submillimeter data, astronomers can build a more complete inventory of the Solar System’s frontier.
Facts
- At 550 AU, light from the Sun takes over 2 days to reach Planet Nine.
- A year on Planet Nine would last ~13,000 Earth years.
- Its temperature (~40 K) is colder than Pluto’s average (~44 K) despite being larger.
- The motion of 3′/year equals about one full Moon diameter per year.
- IRAS was the first satellite to survey the sky in infrared, discovering over 350,000 sources.
References
- Phan, T. L., Goto, T., Yamamura, I., et al. “A Search for Planet Nine with IRAS and AKARI Data.” arXiv:2504.17288 (2025). arXiv
- “Is This the First Hint of Planet Nine?” Universe Today, Apr. 26, 2025. Universe Today
- Phan, T. L., et al. “A Search for Planet Nine with IRAS and AKARI Data.” arXiv PDF (2025). arXiv
- Batygin, K., & Brown, M. “Evidence for a Distant Giant Planet in the Solar System.” Caltech (2016). California Institute of Technology
- “Planet Nine.” Wikipedia. Last updated Apr. 2025. Wikipedia
- “Hypothetical Planet X.” NASA Science. NASA Science
- “Planet Nine – NASA Science.” NASA Science Editorial Team. NASA Science
- “A Search for Planet Nine with Far-Infrared All-Sky Surveys Data.” NTHU Indico. Indico
- ResearchGate. “A search for Planet 9 in the IRAS data.” (2024). ResearchGate
- Brown, M. E., & Batygin, K. “Planet Nine: The hunt continues.” AJ, 2019. Wikipedia
- “Subaru Telescope Horizon Limits.” NOIRLab. arXiv
- “Infrared Astronomical Satellite (IRAS).” NASA. arXiv
- “AKARI Far-Infrared Surveyor.” JAXA. arXiv
- “Dark Energy Camera (DECam).” CTIO. arXiv
