When the first planet outside our solar system, an exoplanet, was discovered, I was amazed. I think most people think of astronomy as being about building bigger and bigger telescopes, just like blowing up a photograph. But just like in the movie, it doesn’t work that way. Just in the most basic sense, imagine looking into a very bright light and how difficult it would be to see a small dark object right next to it. That ought to give you some idea of the problems.
The first confirmed exoplanet discovered was orbiting around Gamma Cephei. It was detected the same way about half of all exoplanets have been detected: Doppler spectroscopy. This is an amazing bit of science. But there are actually too many things to explain it all like centers of mass (stars aren’t stationary in their solar systems) and Doppler shifting of light (the color of light from objects moving towards or away from us gets changed). But the main thing is that the star sends out visible light and the planet sends out infrared light. Based upon the shifts, we can find out a certain amount about the planetary mechanics of the system.
A more obvious technique for locating exoplanets is the transit method. This is very simple, but it doesn’t always apply. If the planet crosses between the star and us, it blocks out some of the light. By seeing the difference, we can determine what the radius of the star must be. For example, the exoplanet that circles the romantically named HD 209458 star was first discovered with Doppler spectroscopy. But then other scientists went back and measured the brightness of the star and found that it decreased by a tad less than 2% every 3.5 days. Thus they could determine its radius to be about 30% greater than Jupiter.
This is all amazing stuff, but I read something today that shocked me, Length of Exoplanet Day Measured for First Time. I couldn’t imagine how they managed that feat. The reason it is possible to measure this value is because the planet, Beta Pictoris b, is one of the few exoplanets that we have managed to take direct pictures of. It’s a huge planet, of course: 65% larger radius than Jupiter’s and a mass between 5 and 10 times. Plus, it orbits about twice the distance from Beta Pictoris than Jupiter does from the sun. All of these things make it easier to see.
And now we know it rotates about twice as fast as Jupiter: about 100,000 km/hr at its equator. It likely spins so fast because it is a very young planet: just 20 million years old. The rotations of planets slow over time because of the gravitational pull on them by the sun, which eventually locks them so that the same side always faces the sun. This has happened to Mercury. And the same mechanism explains why the same side of the moon always faces the earth.
I’m not clear on exactly how they determined the rotation speed. I know the basics: they looked at the star light reflected of the planet. Since the planet has a lot of carbon monoxide in its atmosphere, they looked at the band of light that wasn’t there: the CO absorption line. But that band was broadened, again because of Doppler shifting. Based upon that, they could determine how fast the CO molecules were moving when they were reflecting the light.
This stuff amazes me. We little humans on our little planet are able to determine what is happening 63 light-years away. Of course, it’s still true that when it comes to terrestrial matters, we aren’t any more evolved than chimpanzees. But when we gaze at the stars or doodle our ideas, we are at our best.