A mirror is supposed to reflect back all of the light that hits it, but, being made of atoms, mirrors are not perfect. They typically only reflect about 95% of the light that hits them. Fresh-fallen snow also reflects a lot of the light that hits it–80-90%–but in a diffuse way that makes it appear white instead of reflecting an image. This percentage of diffuse reflection is called albedo.
White materials have a high albedo, while dark materials have a low albedo. For example, sand has an albedo of around 40%; for leaves, it’s around 20%; and charcoal, probably the darkest substance you’ll encounter on a regular basis, has an albedo of just 4%.
But some planets are much darker. TrES-2b (also known as Kepler-1b) is a hot jupiter. It orbits extremely close to its star–so close, in fact, that it glows red hot. And yet TrES-2b is one of the darkest objects ever observed in the universe. How is this possible? For that matter, how do we even know that?
TrES-2b eclipses its star as seen from Earth; when it crosses the face of its star, it blocks some of the light, but something similar happens when it goes behind the star. Now, the starlight reflecting off the planet goes away, as does the light the planet emits due to its heat. This is a much smaller change in the total amount of light we see from that solar system (we can’t see the star and planet separately), but we can still detect the change using the exquisitely sensitive telescope on board the Kepler spacecraft. Since we know the planet’s approximate temperature, we can guess how much of the light is reflected and how much is emitted as heat.
It was predicted that for many hot jupiters, the planets’ atmospheres would fill with a very dark haze of sodium and potassium metal, with an albedo of just 3%, but TrES-2b proved to be even darker. The planet reflects less than 1% of the light that falls on it, rivaling the blackest material we can create in a laboratory, even with nanotechnology.
Why is TrES-2b so dark? We’re not really sure. It probably has something to do with that sodium and potassium haze, and maybe titanium oxide (a dark chemical that is known to exist in the coolest stars), but the exact combination of chemicals that can make a planet that dark eludes astronomers.
Our telescopes have only scratched the surface of what’s out there. There’s a lot more planetary weirdness still to come.