"The thermal cloak is like clothes for vehicles, buildings, spacecrafts, or even extraterrestrial habitats to keep cool in summer and warm in winter," says senior author Kehang Cui, a materials scientist at Shanghai Jiao Tong University.

To dampen natural temperature fluctuations, the cloak isolates the car—or any other object beneath it—from the surrounding environment. The cloak has two components: an outer layer which efficiently reflects sunlight and an inner layer that traps heat inside. Whatever heat the outer layer does absorb is emitted in such a way that it can be readily dissipated to outer space. This design earns it the name of Janus thermal cloak, inspired by the two-faced Roman god Janus.

"The cloak works basically the same way the earth cools down, through radiative cooling" says Cui. "The earth is covered by the atmosphere, and the atmosphere is transparent to a certain range of electromagnetic energy we radiate."

While this process is desirable in the summer, it would make the car colder during winter months. "You have to develop something that can turn on and off by itself without external energy input, and that's extremely difficult," says Cui.

Cui and his team designed the cloak to automatically counteract this effect in the winter. The cloak employs an effect called "photon recycling"—essentially, any energy that is trapped under the cloak will bounce back and forth between the car and cloak rather than escape to the surroundings outside.