The solar system's greatest mystery may finally be solved

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.

This is where the new research gets ingenious. Led by Amos Chen from the National Tsing Hua University, the team realized that searching for Planet Nine's heat signature could be far more effective than looking for its reflected light. Here's why: When you double the distance from the sun, reflected light becomes 16 times fainter (following what scientists call an inverse fourth-power relationship). But thermal radiation, the heat that all objects naturally emit, only becomes four times fainter when you double the distance.

The researchers turned to data from AKARI, a Japanese space telescope that conducted the most sensitive all-sky survey in far infrared light, the perfect wavelength range to detect the heat signature of a cold, distant planet. Unlike ground-based telescopes that are hampered by Earth's atmosphere, AKARI could detect the faint thermal glow that Planet Nine should emit.

The team focused their search on a specific region of sky where computer simulations suggested Planet Nine was most likely to be found, based upon the orbital patterns of the Kuiper Belt Objects. They then faced the challenging task of distinguishing a slowly moving planet from the countless stars, galaxies, and cosmic debris that populate this region.

Planet Nine should appear stationary over the course of a single day but show detectable movement over months. By comparing AKARI observations taken at different times, they could identify objects with this specific type of motion while filtering out cosmic rays, background galaxies, and other false signals.

After this meticulous analysis, the researchers identified two candidates. Both objects appear in the predicted location and emit the amount of infrared light that theory suggests Planet Nine should produce. While this doesn't constitute definitive proof, it represents the most promising lead in the search for our solar system's hidden giant. Their work is published in the Publications of the Astronomical Society of Australia.

These discoveries mark an important milestone, but the journey isn't over. The candidates require follow-up observations with more powerful telescopes to confirm whether they're truly moving in ways consistent with Planet Nine, or whether they're imposters, perhaps background galaxies or other astronomical objects.

If confirmed, the discovery of Planet Nine would revolutionize our understanding of how our solar system formed and evolved. It would also demonstrate the power of thinking creatively about astronomical searches. Sometimes the best way to find something isn't to look directly at it, but to feel its warmth instead.