FromQuarkstoQuasars

It’s Dark Out There: The Black Planet

Click to see a larger image (CREDIT: David A. Aguilar, CFA)
Click to see a larger image (CREDIT: David A. Aguilar, CFA)

We have all been there. There’s that one person who is at the top of the class in school and college, always picked for that coveted opportunity at work, always has the most intelligent kids, and even our parents seem to favour them (okay, maybe I am being a tad dramatic). The point is, people often feel like they will always be the second-best. As it turns out, planets also have a hierarchy. And as is true of humanity, there always seems to be that one perfect one that blows all the rest out of the water…

I’m talking about a planet that has dethroned the reigning universe-champion as the darkest known planet. But this planet isn’t merely dark–it is as black as coal.

Astronomers have found that a planet, discovered way back in 2006, is the darkest planet ever seen by the human eye (well, technically we can’t see it with our own eyes; we use our telescopes, but still).

The planet, unimaginatively called TrES-2b, lies at a distance of 750 light-years in the direction of the constellation Draco (how many constellations are named after you, Harry Potter?). It circles the star GSC 03549-02811. No, that’s neither a registration number, nor an account number, nor some crazy telephone number, nor anything else that you might think it to be…except a name (scientific naming systems can be kind of boring, huh?).

The planet is so dark that it reflects less than 1% of the light it receives from its star. To put things into perspective, it’s even blacker than black acrylic paint! However, it’s not completely pitch black. The planet orbits its star at a distance of just about 3 million miles (that’s 9 times closer than Mercury is to our Sun at its closest). As a result, it gets hot–a blistering 1800° F/980° C. For comparison, Venus, the hottest and unofficially dubbed “hell” of our solar system, is just 863 °F or 462 °C. Ultimately, TrES-2b is so hot that it emits a faint red glow, much like a burning ember or the coils on an electric stove.

This planet’s darkness was discovered using the Kepler spacecraft, whose primary objective was to discover exoplanets (I say “was” because, unfortunately, we lost it due to technical problems in 2013). Exoplanets, or extra-solar planets, are planets outside our solar system. The spacecraft found them by precisely measuring the brightness of a distant star and the subtle dimness of that brightness when a planet passed in front of it.

Another rendering of the planet (Credit:  Starkiteckt on deviantart)
Another rendering of the planet (Credit: Starkiteckt)

Now, it so happens that our black beauty is tidally locked with its star. “Tidal Locking” is when a planet (or moon) always presents the same side to its parent star (or planet, respectively). Such is the case with our moon as well. And so, unsurprisingly, the planet shows changing phases as it orbits its star. Due to its changing phases, there were slight variations in the total brightness of the sun plus the planet as the planet passed in front of its sun. Astronomers measured these variations, and after careful observations and analysis, they found that what they were viewing was the smallest ever change in the brightness of a star caused by an exoplanet. These extremely small fluctuations proved that the planet is extremely dark – in fact, the darkest because the fluctuations were the smallest when compared to other observations that we have made. A more reflective planet would show larger brightness variations as its phase changed as it orbited around its star.

The darkness of the planet can be attributed, to some extent, to its peculiar atmosphere. Due to its very high temperature, this Jupiter-sized gas giant lacks reflective clouds. As a result, it does not reflect back the star’s light – unlike Jupiter, where huge, red-and-white storm clouds do this job pretty well. Moreover, the planet’s atmosphere consists of light absorbing chemicals like vaporised sodium and potassium, or gaseous titanium oxide. However, these reasons are still not enough to explain the extreme blackness of TrES-2b.

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