Warmet

Meet Warmet, a rocky planet who balances the sunlight he soaks up with the heat he gives off.

A planet warms until it radiates away as much energy as it absorbs. Some sunlight is reflected straight back, measured by the albedo A. The rest heats the planet, which then glows in the infrared.

Balancing the two gives T_eq = T★√(R★/2d)(1 − A)^¼. For the Earth, the Sun's surface is 5772 K, its radius is 6.96 × 10⁸ m, the distance is 1.50 × 10¹¹ m and A ≈ 0.30. That gives about 255 K, or −18 °C.

The real average is about 288 K, or 15 °C. The extra 33 K is the greenhouse effect, from gases that trap some of the outgoing infrared.

Astronomers use this formula to guess whether newly found planets around other stars might be warm enough to hold liquid water. Without this natural warming, the average surface would sit well below freezing.