Finding Exoplanets – Part 1: The Transit Method – Not Quite Point and Click!

With Kepler increasing the total number of potential planets discovered almost daily, (as of 3/10/2012 the total is at 2321 with 61 planets confirmed) it is becoming easier to take this exciting news for granted. There is a lot more going on behind the scenes to these discoveries than just pointing a telescope at the stars and waiting for the planets to go by. Kepler uses a technique called the transit method to identify potential planets, but there are other ways to find planets, which I will discuss in a future post.

The transit method requires that the planets revolve around their stars in a plane that is within our line of sight, so from our perspective the planet blocks the light coming to us from its star. In concept this is pretty straight forward and simple. But, as they say, the devil is in the details.

If you click on the image above you will see a simulation provided by the Kepler site of a planet transiting the star it orbits.  What’s interesting to note is the gradual slope of the curve as the planet first begins to cross the star or ingress and is repeated on the other side when the planet egresses the star.  Also, as the planet transits the face of the star the light curve is not flat but curved due to limb darkening effects.  The curve is not smooth, which indicates the variability in the brightness across the star’s surface.  Note that in most cases the amount of light blocked by the planet is only on the order of 1% – 2%.

Consider this.  As you watch the light dim from the star are you really seeing a planet passing in front of it or are you seeing the star itself dim because it is a variable star or has a large starspot (sunspot) on its surface that just came into view or some other phenomena is affecting the light you are seeing?

Maybe the dimming is due to the fact that this is not one star but two in a binary system – two stars orbiting each other in a plane that lies along our line of sight. What you perceive as dimming due to a planet could really be one star eclipsing the other. You would get the brightest image when they are not eclipsing, the dimmest when one is behind the other. These are known as eclipsing binaries. (Note that Kepler has discovered 2165 eclipsing binary stars as of this blog post)  And, what if it’s a trinary system – three stars orbiting a common center of mass?  Throw in a few planets and try to imagine the light curve for that system!

The dimming has to be periodic and repeatable to increase confidence that there really is something out there orbiting the star and not just an intervening asteroid or comet that happened to pass through your field of view. Kepler requires 3 to 4 events to record a potential exoplanet.  The first planets Kepler discovered had orbital periods of several days, which allowed astronomers to gather a set of data in a very short time.  These planets are very close to their star and are extremely hot.

Kepler has been observing for 3 years now, so it will be finishing up data sets on planets that are orbiting further from their star with orbital periods of a year or so.  The longer Kepler looks, the more planets it will unveil.

Through various techniques like spectroscopic analysis one can determine if the star is part of an eclipsing binary pair. Through other observations one can determine if the star is a variable star and if the dimming doesn’t reappear, then it may have been a starspot or other transient phenomena. If the dimming repeats and you collect a set of light curves that show how much light is blocked by the object you can then begin to determine some interesting properties about this object, like how big it is, what its orbital period is and distance from its parent star. But, these numbers don’t come without some hard work.

It’s easy to get a relative size of the planet to its star by how much light it blocks when the planet transits the face of the star.  But, one needs to take into account a number of physical phenomena that will affect the data that is collected. One of these is limb darkening – which is the effect that the star is not as bright at its edges (limbs) as it is at the center. This is due to the fact that one is not looking as deeply into the star at its edges as at its center. This means that the planet will block a greater percentage of light as it traverses the center of the star than at the edge.

Where the planet crosses the star will affect the shape and size of the light curve. Crossing directly over the equator of the star would produce the broadest, most shallow curve while crossing the star at higher latitudes would produce a narrower, deeper curve. This is a reflection of the tilt of the planet’s orbit relative to Earth. These will affect the calculations of how big the exoplanet is and how large its orbit is and must be taken into account.

Transit Light Curves - Image Courtesy of NASA/JPL

Starspots can also skew the data if they occur as the planet transits the face of the star. This is because they are dark and they will add to the amount of light perceived to be blocked by the planet, giving the impression that the planet is bigger than it really is.

An advantage of the transit method is that it allows astronomers to determine if the planet has an atmosphere. Using a spectrometer it is possible to determine the constituents of the planet’s atmosphere as the star’s light passes through the planet’s atmosphere as the planet passes over the edge of the star at the beginning and end of the transit.

The transit method provides information on how big the planet is, once the size of its parent star is known, which is a challenge in its own right.  Once the planet’s size is determined and coupled with mass data gleaned from another technique—radial velocity measurement—the density of the planet can be calculated. When the density is known, approximations can begin to be made about the composition of the planet. Is it gaseous, rocky, or somewhere in between? This information, along with the knowledge of how close to its star it orbits, which determines the temperature of the planet, will dictate what state water might exist if present.

The science of astronomy is an amazing example of how inventive and ingenious man can be.  We have harvested all we know about our Universe from just the light that comes to us through the vacuum of space.

(Check out part two: “It’s All About the Mass“)

Till next time,

RC Davison

One thought on “Finding Exoplanets – Part 1: The Transit Method – Not Quite Point and Click!

  1. Pingback: Finding Exoplanets – Part 2: It’s All About the Mass | Orbital Maneuvers' Blog

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