The solar system's greatest mystery may finally be solved

Artist's impression of giant eclipsing the central Milky Way, with a star-like Sun in the distance. Neptune's orbit is shown as a small ellipse around the Sun. Credit: Nagual Design

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For years, astronomers have been searching for a mysterious ninth planet lurking in the dark outer reaches of our solar system. Now, a team of researchers have taken a completely different approach to this cosmic detective story—instead of looking for reflected sunlight, they're hunting for the planet's own heat signature.

The story begins with a puzzle in the outer solar system. Scientists noticed that small icy bodies called Kuiper Belt Objects, which orbit far beyond Neptune, seem to be clustered together in unusual ways. Their orbits are aligned in patterns that shouldn't exist by chance alone. The leading and most tantalizing explanation.... a massive, undiscovered planet (dubbed "Planet Nine") is gravitationally shepherding these distant objects into their strange orbits.

If it exists, Planet Nine would be a true giant, roughly 5-10 times the mass of Earth, orbiting somewhere between 400-800 times farther from the sun than our planet does. At such an enormous distance, it would be incredibly faint and nearly impossible to spot with traditional telescope searches that rely on detecting reflected sunlight.