Day of the Exoplanet

For the first time, astronomers have determined how fast an exoplanet is spinning. To save you the dramatic tension, I’ll tell you right off the bat: 25 kilometers per second, or about 28,000 mph. That’s fast. With some reasonable assumptions, that means the planet has a day that’s about eight hours long, a third of an Earth day, which is pretty amazing given that the planet is 10 times the mass of Jupiter!

The planet in question is Beta Pic b, a massive gas giant orbiting a star about 63 light-years away. The planet was discovered in 2003 and its existence confirmed in 2009. This planet has been a boon to astronomers; it was one of the very first to have been directly imaged by telescopes, meaning we can actually see it in pictures. The image above shows it clearly near its parent star, Beta Pic.

Not only that, but over time we’ve seen it physically move around its star. That tells us its orbital period (its year) is 17-20 Earth years long. While getting the year of the planet was quite a treat, getting the length of its day is substantially harder. To do this, astronomers took advantage of the Doppler shift, the same effect that changes the pitch (the wavelength of the sound) of a motorcycle as it passes you (the familiar “EEEEEEEE-yowwwwwww” sound). When the motorcycle approaches you, the pitch is higher, and when it moves away the pitch drops.

The same thing happens with light: If a source approaches you the wavelength changes, shifting toward shorter wavelengths (what astronomers call the blue end of the spectrum, so we call this blueshifting). If it’s heading away, the wavelength of light gets longer (redshifting). If the source is moving quickly enough, this can be measured.

The astronomers used a spectrograph, a device that breaks up light into thousands of individual colors (called a spectrum), to observe Beta Pic b. The atmosphere of the planet has some carbon monoxide in it, and that molecule absorbs light at a very specific wavelength. It’s like a filter that blocks the light at a narrowly defined color, making a dark line in the spectrum. This is useful, because as the planet moves, the wavelength of the absorption line in the spectrum changes due to the Doppler shift. That’s what astronomers used to tag the velocity of the planet.

The CO absorption dip they found for Beta Pic b was blueshifted by about -15 km/sec (the minus sign indicates velocity toward us). That’s just what you’d expect for a planet at that distance from its parent star in a circular orbit. So right away, they could measure the orbital speed of the planet.

But there’s more. Since CO absorbs light at a specific color, you’d expect to see a sharp drop in the spectrum. But that’s not what they saw! Instead, the dip is broadened, smeared out. That, it turns out, is the key to its rotation.

Imagine a spinning ball. When you look at it, the left side (say) is spinning toward you, and the right side spinning away. If you could very carefully measure the light from it, the left side would be blueshifted, and the right side redshifted. The light from the middle wouldn’t be shifted at all. So you’d see velocities ranging from the maximum blueshift, through zero, out to the maximum redshift. The profile you’d see would be smeared out … just like that of Beta Pic b.

When they measured how broad the absorption profile of CO was, they found it must be spinning with a velocity of about 25 km/sec. If you were standing (or, really, floating) on the planet’s equator, you’d be moving at 90,000 kph (56,000 mph)!

To get the length of the day you have to figure out how big the planet is. For example, the Earth spins at 1,700 kph, and has a radius of 6,371 km. That means its day is 24 hours long, give or take (I’ll leave that math as an exercise for the reader, but remember the circumference of a circle is twice the radius times pi).

We don’t really know how big Beta Pic b is, but theoretical models give it a radius of about 1.65 times that of Jupiter, or 115,000 km. Given the velocity measured, that means its day would last a mere eight hours. Imagine: This is a planet 18 times wider than Earth, yet spinning so rapidly its day is a third as long!

Actually, this fits with what we know: In our solar system, the bigger the planet, the faster it spins. We think that’s left over from when the planets formed billions of years ago from the disk of material around the Sun. As bits and pieces coalesced to form the planets, their rate of spin increased, just as water in a sink speeds up as it approaches and spins down the drain. Planets that are bigger had more material fall in, so they sped up more than smaller planets.

And Beta Pic b isn’t done yet. It’s very young, only a dozen or so million years old, and young planets are hot. Since it’s mostly gas, that means as it ages it will cool and shrink. That will spin it up even faster, as an ice skater spins faster when she brings her arms in. In a few hundred million years, Beta Pic b may have a day only three hours long.

You think there’s not enough time in a day now to get anything done. It could be way worse. Imagine a three-hour day!

If it’s spinning that rapidly, Beta Pic b must be really oblate, oval-shaped (Saturn and Jupiter are noticeably noncircular due to their rapid spins). And it’ll get even wider at its equator as it ages. Hmmm … if there’s a metaphor there I may be missing it.

All of this is astonishing to me. Twenty years ago we didn’t even know if there were planets orbiting Sun-like stars at all. Now we know of 1,700 and counting. We’ve directly photographed a dozen or so of them. And this one, Beta Pic b, we’ve seen moving around its star. We know a bit about what’s in its atmosphere (carbon monoxide), we know its orbital speed, and have now measured its spin and inferred the length of its day! And mind you, we’ve learned all that from more than 600 trillion kilometers away.

If anyone tells you science isn’t cool, you now have one more arrow in your quiver to rebut them. Science is cool. Astronomers take the measure of the Universe by the starlight that falls to Earth, and discover what things are like on alien worlds. That’s about the coolest thing there is.