Astronomers from National Tsing Hua University (NTHU) in Taiwan have proposed an innovative method for searching for the hypothetical Ninth Planet by focusing on its thermal radiation rather than reflected light. Their study, published in Publications of the Astronomical Society of Australia (PASA), identified two promising candidates that may be this long-sought celestial body.

What Is the Ninth Planet?

The Ninth Planet is a hypothetical massive object believed to exist beyond Neptune’s orbit, in the distant reaches of the Solar System. Its existence was proposed in 2016 by scientists Michael Brown and Konstantin Batygin to explain the unusual clustering of trans-Neptunian object orbits in the Kuiper Belt. The planet is estimated to be 5–10 times more massive than Earth and to follow a highly elongated orbit 400–800 astronomical units (AU) from the Sun.

Due to its extreme distance, the planet reflects very little sunlight, making it nearly invisible in the optical range — a major challenge for its detection over the past decades despite indirect evidence supporting its existence.

Infrared Approach Using AKARI

Led by Amos Chen, the NTHU team employed a new approach based on detecting thermal radiation, which is emitted by cold, distant objects. Unlike reflected light, which weakens 16-fold with a doubling of distance, thermal radiation only diminishes by a factor of four, making it more suitable for identifying distant bodies.

The researchers analyzed data from the Japanese infrared satellite AKARI, which conducted the most sensitive far-infrared sky survey between 2006 and 2011. AKARI is capable of detecting faint thermal emissions that the hypothetical planet would produce. Scientists compared images taken at different times to identify moving objects — specifically those that remain nearly stationary over a day but show noticeable displacement over months, consistent with expected motion for the Ninth Planet.

Two Candidates for the Ninth Planet

After carefully analyzing data from the predicted region of the sky, the team found two candidate objects emitting infrared radiation matching theoretical expectations and located within the predicted zone. However, their motion and orbital characteristics still need to be confirmed using more powerful telescopes, such as the Subaru Telescope in Hawaii or future facilities like the Vera Rubin Observatory and the Nancy Grace Roman Space Telescope.

Comparison with Previous Studies

In 2021, British astronomer Michael Rowan-Robinson proposed a Ninth Planet candidate using IRAS data (1983), estimating a mass of 3–5 Earth masses and a distance of about 225 AU. However, this object was not confirmed by subsequent observations, including those from AKARI. The new candidates found by the Taiwanese team are considered more compelling, as their presence is confirmed in two independent datasets.

In addition, a 2024 study led by Terry Long Fan of NTHU identified another object that shifted 47.4 arcminutes over 23 years between IRAS and AKARI data. This candidate appeared more massive than expected — possibly larger than Neptune — which contradicts the idea of the Ninth Planet being a super-Earth.

The Significance of a Potential Discovery

If either of the candidates is confirmed as the Ninth Planet, it would be one of the most significant astronomical discoveries of the 21st century. Its existence could explain orbital anomalies in the Kuiper Belt and offer insights into how the Solar System formed. For instance, the planet may have been pushed to a distant orbit by gravitational interactions with Jupiter and Saturn, or even captured from another star system in the early Solar System.

Discovery of such a planet could also affect our understanding of planetary system formation around other stars, particularly those hosting massive bodies on elongated orbits.

Challenges and Future Prospects

Despite the promising results, scientists stress that the identified objects are not yet definitive proof of the Ninth Planet. Further observations are needed to rule out the possibility that they are background galaxies or unrelated astronomical objects. Upcoming tools like the Vera Rubin Observatory (set to begin operations in 2025) and the Nancy Grace Roman Space Telescope (also planned for 2025) will greatly improve the chances of confirming or refuting these candidates.

Previous searches, including data from the WISE telescope, have ruled out Jupiter- and Saturn-sized planets within 256,000 and 10,000 AU, respectively, but could not detect Neptune-sized or smaller bodies — leaving open the possibility that the Ninth Planet does exist.