Deep in the outer reaches of the solar system — so far away from the known planets that the sun would barely be distinguishable from a nearby star — a massive, icy world may be lurking in the shadows, waiting to be discovered by humanity.
But the hunt for the solar system's potential ninth planet may soon be coming to a close. With the opening of the groundbreaking Vera C. Rubin Observatory in 2025, we may either finally find Planet Nine within the next few years — or rule out the idea for good, experts told Live Science. But other than the now-demoted Pluto, no full-fledged planets beyond Neptune or the Kuiper Belt — a massive ring of asteroids, comets and dwarf planets that orbit the sun beyond Neptune — have shown up since. And as astronomers mapped more of the outer solar system, it seemed increasingly unlikely that they were missing something as large as a planet.
Even after the pair realized a ninth planet was possible, they decided to sit on their findings until they could come up with another, less-controversial explanation. However, they then found four more TNOs with matching, misshaped orbits, which suddenly made a missing planet look like the most logical explanation.
These discoveries also constrain Planet Nine's potential size, its distance from the sun and its orbital trajectory through the solar system. Planet Nine's unusual orbit and extreme distance from the sun also raise the possibility that it could be a rogue planet — an interstellar world that was captured by the sun after being ejected from its star system. However, Brown and Batygin believe Planet Nine likely formed alongside the other planets in the solar system.
However, Brown and Batygin discount the notion that observational bias is creating the illusion of a ninth planet. Why haven't we found it?So, if Planet Nine does exist, why haven't we spotted it yet? In their most recent work, Brown and Batygin analyzed data from the Panoramic Survey Telescope and Rapid Response System at Haleakala Observatory in Hawaii and confidently ruled out around 78% of the suspected orbital pathway as possible hiding places for the planet.
This ground-based telescope, which will be equipped with the world's largest digital camera, will let researchers peer farther into the solar system than any of its predecessors allowed, similar to how the James Webb Space Telescope has enabled researchers to look farther across the observable universe than ever before. Related: 5 space discoveries that scientists are struggling to explain