What Self-Luminous Planets are Like

The planets that we are familiar with in our own solar system have evolved, aged, and cooled, for over 4.5 billion years since the Sun and planets formed. What do planets look like at younger ages? Can we use the light that a planet emits to understand its past history?
[left] Jupiter as it appears to our eyes, in reflected sunlight. [right] Jupiter as it appears at a wavelength of 5 microns. These long wavelengths are far beyond visible light, in the thermal infrared.When we look at a planet like Jupiter with our eyes, the light that we see is Sunlight that is reflected back to us at Earth, scattered by clouds and gasses in the planet’s atmosphere. But what would Jupiter look like if we instead could see only its thermal “heat” emission, far beyond the visible spectrum? We can use infrared light detectors on telescopes to see this thermal emission, which comes from the deep interior below the visible clouds. We can use this light to precisely “take Jupiter’s temperature” in a way that you can’t do with reflected Sunlight. It turns out that our 4.5 billion-year-old planets are cold enough that their thermal “heat” radiation is quite ...