According to NASA, 1,703 planets orbiting 1,033 stars have been found so far, but very little is known about these planets beyond some deductions based on their orbits. That veil is lifting though, as demonstrated by Dutch astronomers who have for the first time measured the length of an exoplanet’s day. Using the European Southern Observatory’s Very Large Telescope (VLT) in Chile’s Atacama desert, the team has learned that the young extrasolar gas giant Beta Pictoris b has an eight-hour day and spins faster than any planet in the Solar System.
Beta Pictoris b was discovered six years ago orbiting the naked-eye star Beta Pictoris, some 63 light years away in the constellation of Pictor. It was one of the first exoplanets imaged directly and the closest one to its star imaged directly at a distance of 8 AU (743 million mi, 1.2 billion km). It’s remarkably young as stars go – only 20 million years old. It’s 16 times larger, 3000 times more massive than the Earth, and has a day that lasts only eight hours. According to the team from Leiden University and the Netherlands Institute for Space Research (SRON), it also spins at nearly 100,000 km/h (62,000 mph) at the equator, which makes it faster than any planet in the Solar System. It’s also still cooling and shrinking, so unless other factors slow it, it will spin even faster in the future.
One interesting point is that the rotation of Beta Pictoris b matches a pattern seen in the planets of the Solar System: The larger the planet, the faster the rotation. Six of the planets in our system match this pattern, with the only exceptions in Mercury and Venus. Astronomers say that their rotation is slower than both Earth and Mars because of the effect of solar tides, which have acted as a brake on their spin.
“It is not known why some planets spin fast and others more slowly,” says team-member Remco de Kok, “but this first measurement of an exoplanet’s rotation shows that the trend seen in the Solar System, where the more massive planets spin faster, also holds true for exoplanets. This must be some universal consequence of the way planets form.”
Beta Pictoris b’s day was calculated from observations taken by the VLT cryogenic high-resolution infrared echelle spectrograph CRIRES using a technique called high-dispersion spectroscopy. Since Beta Pictoris b can be seen directly, the VLT can take spectrographs of its atmosphere. The scientists at the ESO studied the absorption signal from carbon monoxide. If the planet were standing still, the absorption lines in the spectrum would sit in the same place as one taken from a lab sample, but if it moves, the lines will shift due to the Doppler effect. The same would be true if the speed of the planet around the star was taken into account.
However, the absorption lines of Beta Pictoris b still show a shift that indicate that one side is moving in one direction and the other side in the other direction. In other words, the planet is rotating. By calculating these shifts, astronomers can calculate how fast it’s spinning. It’s a technique already used to calculate the spin of stars.
“We have measured the wavelengths of radiation emitted by the planet to a precision of one part in a hundred thousand, which makes the measurements sensitive to the Doppler effects that can reveal the velocity of emitting objects,” says lead author of the team’s results Ignas Snellen. “Using this technique we find that different parts of the planet’s surface are moving towards or away from us at different speeds, which can only mean that the planet is rotating around its axis“.
Mid-infrared E-ELT Imager and Spectrograph (METIS) principal investigator Bernhard Brandl says that the findings of Beta Pictoris b will open new opportunities for mapping exosolar planets. “This technique can be used on a much larger sample of exoplanets with the superb resolution and sensitivity of the E-ELT and an imaging high-dispersion spectrograph. With the planned Mid-infrared E-ELT Imager and Spectrograph (METIS) we will be able to make global maps of exoplanets and characterize much smaller planets than Beta Pictoris b with this technique.”
The team’s results were published in Nature (PDF).
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