Light Curves!

We've already learned about the Kepler Space telescope and how it searches for planets. The basic idea, just to refresh your memory, is to watch a star for a long period of time and see if it changes in brightness periodically. If it does, then the dimming of the star might be due to a planet crossing in front of, or transiting, the star and blocking some of its light! When astronomers do this sort of thing they construct what's called a "light curve". A light curve is just a graph with star brightness on the vertical axis and time on the horizontal axis. Each time you measure the star's brightness you put a dot on the graph corresponding to the time you took the measurement and how bright the star is. A real example of some light curves constructed from Kepler data are shown below.

 The star appears to be at normal brightness, or 1 on the graph, most of the time. But once in a while we see a tiny drop in brightness (less than 0.1%). This drop in brightness occurs once every few days and lasts for a few hours. Not only can light curves reveal the existence of a planet, but they can also tell us the size of the planet, how far away it is from its host star, and where it sits in orbit! As you can see from the cartoon in the image, smaller planets block out less light, so we see a smaller dip in the light curve. Also, planets that cross in front of the star near the top or bottom rather than the middle have a thinner dip in the light curve. If we know how big the star is (and often we do) and we know the time between light curve dips, we can calculate the period of the planet (i.e. how long it takes for the planet to revolve around the host star one time) and from that get the distance between the star and the planet.

The coolest thing about light curves and planet hunting is that you don't need a big, fancy  and expensive telescope to do it. The very first discovery of an exoplanet was made using a digital camera and a 4inch wide backyard telescope! With just basic equipment and some astronomy knowledge you can make light curves and hunt for planets right in your own backyard.

Image Credit: NASA/Kepler

Direct Imaging of Exoplanets

It's not easy to take pictures of exoplanets for two reasons:

1) They are small and far away

2)They are very dim compared to the star they revolve around

Imagine you're standing in the end zone of a football field at night. Taking a picture of an exoplanet would be like trying to take a picture of a small moth fluttering in front of a stadium light at the other end of the field. The stadium lights are way too bright, and the moth is too small and far away to make out any details. The most common method used to "image" exoplanets is to look at light curves, which I will discuss on Thursday's post, but astronomers have actually discovered a way to take pictures of some exoplanet systems. To do this they use an instrument called a Vortex Coronagraph. This device was actually experimentally discovered by Grover Swartzlander, a physics/imaging science professor at the Rochester Institute of Technology! Simply put, the device "blocks" all the light from a star, allowing any planets around the star to appear very bright. It's a bit more complicated than it sounds, as the device doesn't really block the light, it uses mathematical tricks to polarize the light and cause the light waves to cancel each other out. 

The picture above is of the exoplanet system around star HR8799 taken using a Vortex Coronagraph on the Hale 1.5m telescope. The star is where the green X is, and as you can see, it's light has been canceled out so that we can see three planets. Only a few exoplanets have been imaged this way, but as more discoveries are made and technology advances, we hope to directly image as many systems as possible.