Discovering Planet Nine: Astronomers Uncover Clues in 23 Years of Data

What just happened? Astronomers may have found strong evidence for a mysterious Planet Nine, a large planet thought to orbit far beyond Neptune. This new object appears to be a crucial piece of the puzzle, but scientists will need to conduct more observations with powerful telescopes to confirm what it is and where it’s headed.

The idea of Planet Nine isn’t new. It was first suggested by Michael Brown and Konstantin Batygin from Caltech in 2016. They proposed that this hidden planet could explain why some distant objects in the Kuiper Belt, a region past Neptune filled with icy bodies like Pluto, have unusual orbits. Now, a study led by Terry Long Phan at National Tsing Hua University has taken a significant step forward, using infrared data collected over two decades.

Phan’s team analyzed archival data from NASA’s Infrared Astronomical Satellite (IRAS), launched in 1983, and Japan’s AKARI satellite, active from 2006 to 2011. They looked for objects that were visible in 1983 but had shifted position by 2006. At Planet Nine’s expected distance, any movement would be slow—about three arcminutes per year—making it tricky to spot.

To track this strange motion, they compared both datasets on the same day each year. They confirmed potential movement by ruling out nearby objects moving quickly. Ultimately, they discovered a faint point that had moved about 47.4 arcminutes, aligning with what they expect from Planet Nine’s orbit over 23 years. However, more observations are still needed to understand its exact path.

“Once we know where to look, modern telescopes can find it,” Phan said, adding that the follow-up observations will need to cover a significant portion of the sky to account for any changes since 2006.

Interestingly, the estimated size of the object could be larger than Neptune. This is surprising because researchers initially thought it would be smaller, like a super-Earth. Previous searches, such as NASA’s Wide-field Infrared Survey Explorer (WISE), had ruled out Jupiter-sized planets at specific distances. However, a planet the size of Neptune could still remain hidden.

Phan believes earlier surveys may have missed this object because it has shifted since 2006. If this candidate is confirmed, its orbit would be highly unusual, ranging from about 280 to 1,120 astronomical units from the Sun—much farther than Neptune’s 30 AU distance. This peculiar path raises intriguing questions about how it came to be there.

One possibility is that Planet Nine originally formed closer to the Sun, near where the gas giants like Jupiter and Saturn exist. Over time, gravitational interactions could have pushed it outward. Alternatively, it might be a rogue planet that was captured by the Sun at an early stage in the solar system’s history.

Researchers haven’t been shy about looking for Planet Nine in the past. In 2021, Michael Rowan-Robinson from Imperial College London identified a potential object in IRAS data, estimating it to be three to five times the mass of Earth, located about 225 AU away. However, other datasets, including AKARI, didn’t confirm this. Phan argues that his candidate is more notable since it appears in both IRAS and AKARI datasets.

While the find is exciting, it remains uncertain. More data is needed to clarify the candidate’s orbit. Fortunately, advances like the soon-to-launch Nancy Grace Roman Space Telescope and the new Vera C. Rubin Observatory are expected to significantly aid in the search for Planet Nine.

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