The Extrasolar Worlds of the Milky Way Galaxy

Will Buchman

An extrasolar planet can be defined as a planet located outside of our solar system. It is believed that there are between 100-400 billion exoplanets in the milky way galaxy. The discovery of exoplanets is actually relatively new, the first confirmed discovery of an exoplanet didn't actually happen until 1988! However, this is not considered to be the first definitive discovery of an exoplanet, this title belongs to an event that occurred in 1992. Two extrasolar planets were discovered orbiting a pulsar by radio astronomers Aleksander Wolszczan and Dale Frail. This discovery was so influential because it basically removed all doubt concerning the existence of exoplanets. This is the original press release regarding that event. Since these initial findings, the discovery rate of extrasolar planets has boomed. As of November 4th this year, more than 4,000 exoplanet candidates have been discovered. Even with all these new discoveries, we still don't know a whole lot about exoplanets. In fact, only a few months ago, NASA determined the true color of an exoplanet for the first time.

The picture above is a comparison of Earth and an artists rendering of what the famous exoplanet Gliese 581g may look like. Believe it or not, this picture is actually based in real science, artists must use information regarding exoplanets in order to determine their possible size and color. This video describes the process in more detail

There are many different processes by which exoplanets are discovered. A lot of these techniques have only been successfully used once or a few times. There are a few methods that are used quite a bit, including the transit method, the radial velocity method, and gravitational microlensing. These methods are employed because it simply isn't possible to directly image most exoplanets, only a few extrasolar planets have been found through direct imaging.

The transit method:

The way planets are identified through the transit method is by observing their star. When a planet crosses in front of its parent star, the observed brightness of said star will drop ever so slightly. A candidate must be observed transitting its star several times before it can be confirmed to be an exoplanet. This technique can be used to determine a planets radius, by measuring the amount that the star dims during transit. The transit method is far from the perfect method. For example, an exoplanet's orbit must be perfectly lined up with the observing point, many possible planets could slip through the cracks using this method. Another major flaw is that plenty of other things could cause a star's brightness to dip, there could be a high number of false detections as a result of this method.

The radial velocity method:

As you know, everything in the universe has its own gravitational force. This means that planets are exhibiting a gravitational pull on their stars. Ergo, a star with a planet will move in a small orbit in response to its planet's force of gravity. This effect can be seen in the animation below.

By measuring these wobbles, astronomers can calculate whether or not a star has a planet. This way of detecting exoplanets has proven to be exceptionally successful. However, as with all techniques, it has its flaws. Multi-planet and multi-star systems can give mixed results, which can lead to false positives. Another flaw of this technique is that it is difficult to use on stars with higher mass, seeing as how a planet's gravitational force will have much less effect on a supermassive star as opposed to a lower mass star. Despite these flaws, this method is still the most productive technique.

Gravitational microlensing:

In this class we learned that gravity can have an effect on light, this technique measures that effect. When light passes through the gravitational field of a star, it can potentially act as a lense, magnifying background light. In this case, a distant star. This technique is unique in that it is most efficient at detecting planets that have a smaller orbit. In fact, it was the first method used to detect Earthlike planets orbiting around main sequence stars like our own. There is one significant problem with gravitational microlensing. The lensing effect only occurs when two stars are exactly aligned, these events do not last long, and most notably, they do not repeat. This is an enormous disadvantage, it is impossible to observe this effect after it is over, meaning that it is harder to determine whether a candidate is a true extrasolar planet or not.

Now you may be asking yourself, how does this effect me? Why should I care about exoplanets? Well these discoveries actually have a number of direct impacts on us. For example, exoplanets likely hold the answer to the age old question, "are we alone in the universe?" Astronomers predict that there are more than 10 billion habitable Earthlike planets in our galaxy alone, what are the odds of life not developing on any of them? With all these discoveries, it is believed that it is no longer a matter of if we find a true Earth analogue, but when. Not only are exoplanets the key to understanding life in the universe, but they are also likely to play an important part in the future of mankind. With modern technology, colonizing exoplanets is but a pipe dream, but if humans want to survive, we will eventually have to leave Earth and find a new home. Therefore, it is of the utmost importance that we learn as much as we can about exoplanets.

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