For the first time ever, astronomers have calculated the length of a day on an exoplanet. That may not sound like much, but it’s extremely tricky to do for something so far away. This particular exoplanet (a planet around another star) is called Beta Pictoris b and is around 63 light years away. The fancy name comes from the fact that it’s orbiting a star known as Beta Pictoris: “Pictoris” because it’s in the constellation Pictor and “Beta” because it’s the second-brightest star in that constellation. The planet gets the exciting name of “b” for simplicity of categorization because it was the first planet discovered around the star. Subsequent planets, if discovered, will be named “c”, “d”, “e”, etc. in order of discovery. There’s no “a” planet, as the star is considered the first “object” of the system.
OK, enough semantics, let’s get back to the science! Beta Pictoris b has been found to spin on its axis at 28,000 miles per hour, giving it roughly an 8 hour day. How do we know this, you might ask? Because we can actually see this planet directly — it was found using a technique called direct imaging. This enables scientists to see light coming from the planet which they can then study further by using the Doppler effect. Essentially, the planet radiates in infrared light (heated by its star). Astronomers can look at the light from the planet and break it down into its individual wavelengths via spectroscopy. It turns out Beta Pictoris b is a gas giant (bigger than Jupiter) with carbon monoxide in its atmosphere. The carbon monoxide molecules absorb the infrared radiation coming from inside the planet which results in a distinctive absence of light (an absorption line) in the planet’s spectra as seen here on Earth. Furthermore, as the planet rotates, this absorption line widens based on how fast it’s spinning. Based on the width astronomers detected, they calculated Beta Pictoris b has an 8-hour day. E voila!
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