At the edge of the solar system, far beyond Neptune's orbit, something is knocking icy chunks off their paths. Their orbits are stretched and tilted in the same way, as if an invisible hand arranged them in a pattern. Astronomers suspect a ninth planet is hiding there — massive, five to ten times heavier than Earth, but so dim that no telescope can see it.

But even an invisible object can give itself away by heat. The paper proposes looking for Planet Nine by its infrared glow, which arises when particles of dark matter slam into it. This mysterious substance emits no light, but when it collides with ordinary matter, it can transfer energy, like an invisible wind that warms a rock.

If dark matter has been falling onto the planet for billions of years, it would heat up. Calculations show that with a dark matter density of about 1.32×10⁻¹⁷ g/cm³ (roughly a handful per cubic kilometer) and weak interactions (10¹⁰ times weaker than full absorption), the planet's surface could reach 200 kelvins: that's minus 73 degrees Celsius, warmer than Pluto. The planet's radiation would peak at a wavelength of 1.44×10⁻³ cm, in the infrared range.

Such a signal could be picked up by future infrared telescopes. If Planet Nine really exists, dark matter could help find it, and also tell us how often invisible matter collides with ordinary matter.