Exoplanet Out for a Quick Spin

Astronomers have used a new technique to measure — for the first time — the spin of an extrasolar planet.

In the disk of dust and gas that surrounds a newborn star, grains glom together into rocks, then boulders, then planetesimals, and then ultimately planets. At the end of the day (a.k.a. millions of years), the original disk’s angular momentum is imprinted in every planet’s individual rotation and spin.

But the exact details remain hazy. In our solar system, the gas giants spin more rapidly than the inner planets. Jupiter’s day, for example, lasts only 10 hours. There seems to be a rough correlation between mass and spin, but astronomers have had only eight planets to study.

To extend beyond the solar system, we’ll need to measure the spin of faraway, faint exoplanets. Now a research team led by Ignas Snellen (Leiden Observatory, The Netherlands) has pioneered a new technique capable of taking on this daunting task, and they’ve aimed their telescope at the young exoplanet Beta Pictoris b.

This exoplanet has 4 to 11 times Jupiter’s mass and takes about 20 years to circle a star 63 light-years from Earth. Still a youngster, astronomically speaking, it radiates in the infrared due to heat leftover from its formation. Now, Snellen’s near-infrared observations show this planet is spinning significantly faster than any planet within the solar system. At 25 km/s, it's much greater than Jupiter’s spin at 13.3 km/s and Earth’s 0.5 km/s.

This is expected — the result released today in Nature follows the rough correlation we see in our own solar system. Still, “it is quite surprising that there is a nice relation between planet mass and spin velocity,” says Snellen. “One would expect that the accretion process during formation, which is expected to give a planet its angular momentum, was quite different for rocky planets like the Earth than for gas giants like Jupiter and Beta Pictoris b.”

Watching a Planet Spin

Measuring the spin of an exoplanet is deceptively simple in concept. When observing a spinning exoplanet, half of the planet rotates away from Earth, while the other half rotates toward Earth. The spectrum of the side spinning away from us will show redshift (spectral lines will stretch to redder wavelengths), whereas the side spinning toward us will show a spectral blueshift.

The combined redshift and blueshift ends up broadening any line in the exoplanet’s spectrum, and the greater the broadening, the faster the spin.

But tweezing out the spectrum of a distant and therefore faint exoplanet from its parent star is challenging to say the least.

“Two techniques have been very successful in finding exoplanets: the radial velocity method, which uses high-precision spectroscopy to measure regular motions of the star induced by the planet, and high-contrast imaging, which spatially separates the planet from the star,” says Snellen. “We devised a technique that combines both these methods, filtering out the planet’s light both spectroscopically and spatially.”

The team applied this technique while observing Beta Pictoris b with the Very Large Telescope (VLT), four 8.2-meter telescopes in Chile. The observations were finished in less than an hour, a surprisingly short time for such a challenging observation.

Testing Theories of Planet Formation

Like most of the planets in the solar system, Beta Pictoris b’s spin appears to be perpendicular to its system’s plane of formation. That’s because we’re probably observing the disk edge-on and these new measurements suggest we’re observing the planet equator-on (a pole-on planet would show a negligible spin), says expert Travis Barman (University of Arizona).

But the ultimate test of this system’s geometry will come in 2017 or 2018, when the planet has a chance of transiting its host star.

In the meantime, this new technique could be applied elsewhere. Snellen’s team only reduced the starlight by a factor of 20. But the Gemini Planet Imager (which saw first light last November) and SPHERE (planned to see first light this May) — two instruments designed to directly image exoplanets — will reduce starlight by factors of 1,000 to 10,000.

“We ‘only’ have to couple such instruments to a high-precision spectrograph to make use of our technique and to make it hundreds of times more powerful than now,” says Snellen. “Rocky planets like Earth in the habitable zones of nearby stars will be a few thousand times more difficult to see, but are within reach of the future extremely large telescopes.”

Reference:
I. Snellen et al. “Fast Spin of the Young Extrasolar Planet Beta Pictoris b.” Nature, April 30, 2014.