How can we find exoplanets?

Life in the solar system getting you down? Looking for other intelligent lifeforms to hangout with? Maybe you need to visit another planet. Question is, where are they? As Douglas Adams noted in The Hitchhikers Guide to the Galaxy:

“Space,” it says, “is big. Really big. You just won’t believe how vastly, hugely,
mindbogglingly big it is. I mean, you may think it’s a long way down
 the road to thechemist’s, but that’s just peanuts to space, listen…”

Best know where you going before you set out, eh? Although there may be lots of “rogue” planets who’ve escaped the attentions of their parent star and are currently arrowing across interstellar space, we’re going to focus on ones that orbit stars. Extrasolar planets, or exoplanets for short, are planets that don’t orbit our sun, but instead orbit other stars.

So how can you find them? In lots of ways. I’m going to discuss two. Doppler spectroscopy and the transit method. Sounds complicated. That’ll just be scientists giving fancy names to things to look good during department funding rounds.

First, Doppler spectroscopy. So you’ve heard of the Doppler effect, right? That sudden shift in pitch of an ambulance’s siren as it changes from getting closer to moving away. As the ambulance approaches the sound waves are bunched up. As it recedes they’re stretched out. Hence the different pitch. Light behaves in the same way. The only reason we don’t experience it in our day-to-day lives is because light travels way way faster than the objects that emit the light (think of cars headlights etc)–and so the Doppler shifts are tiny and unnoticeable. With careful enough measurements using scientific equipment, however, we can detect it.

Spectroscopy just means the study of spectrums. The white light of the sun is actually composed of a wide range of wavelengths–including all the colours of the rainbow. If we pass this white light through a prism we’ll see all the different colours as the white light gets split up. Now the cool thing is that elements in the sun’s atmosphere absorb some of this white light at very precise wavelengths. These are absorption lines and every element–every compound in fact–has these lines. When we split up the white light of the sun, we see dark lines where light of that wavelength has been absorbed by these elements. Using these as markers we can then determine if an object is moving away or towards us from Doppler shifts of these lines.

Objects like distant stars which will distinctly “wobble” if they’ve got a sizeable planet in orbit. Say what? Back to our solar system. We think of the Earth and Mars and the rest of our local planets orbiting the sun, but what’s more precise is that the planets and the sun orbit their mutual centre of mass. Since the sun is so massive compared to the planets, this point lies very close to the heart of the sun, but nonetheless it’s not exactly in the middle. Our sun actually carves out very small circles.

And if distant stars have planets so must they. Therefore, by studying the star’s Doppler spectroscopy over a long period of time–enough for a potential planet to orbit several times–and seeing if shifts back and forth (redshift and blueshift) regularly, we can predict the existence of an exoplanet. Phew!

And the transit method? That’s much simpler. Like a fly crossing a lightbeam and making the light fractionally lees bright, a planet crossing the path between a distant star and ourselves will slightly dim the star. By analysing how much dimming occurs and how fast it occurs, we can estimate the size and distance of the planet from the star. Clever stuff.

Can you think of any potential problems with these two methods? Can you think of any other ways to find exoplanets?

Image Credit: Wiki Commons


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