How does an astronomer detect a planet-sized object orbiting a star light years away from us here on Earth? This question is worth contemplating, given that when we examine stars through large telescopes, they often look like dots in far off distances. Since planets have roughly a small fraction of the mass of a star, the nuclear fusion that makes stars burn, does not occur in planets. As a result, planets not only become incredibly tiny but also extremely dark compared to stars, which also explains why we face extreme difficulties in trying to detect them from here on Earth.
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| An artist's depiction showing the multiple planet systems around a star |
One of the instrumental ways in which astronomers have been able to examine these types of planets is through direct imaging. Just as its name suggests, astronomers are able to find these exoplanets in the easiest way imaginable: by simply seeing them. These cases, however, are extremely rare for a couple of reasons. First, in order to be able to distinguish a planet from its star, the planet would need to be decently far away from it. However, suppose a planet is much too far from its star: then what happens? Unfortunately, it would likely not reflect enough of the star’s light in order to become visible to us here on Earth.
Although smaller than stars, exoplanets are still extremely large in nature, and so they tend to be very hot. This means that that they give off their own type of infrared radiation, which is able to be detected by telescopes and used to distinguish them from their nearby stars. In particular, planets that find themselves orbiting brown dwarfs can also be detected much more easily. In fact, brown dwarfs are not technically considered stars because they are not massive or hot enough to generate their own types of fusion reactions, which in turn, means that they give off very little light. Direct imaging has also been used to detect a handful of rogue planets, the types that float freely in the galaxy, rather than orbiting a nearby star. This example, however, is not as common as the planets that orbit brown dwarfs.
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| A brown dwarf that is accompanied by an object 5–10 times the mass of Jupiter | . |
Another significant way in which astronomers have been able to detect extrasolar planets is through indirect methods. It is safe to say that astronomers have had a much better success rate at detecting extrasolar planets through these means. Rather than directly detecting the planet and knowing it is there, astronomers infer its existence by observing any noticeable effects that it has on its parent star. Astrometric methods look for any slight motions of the star (or wobbles) about the center of its mass. Another tool in indirect observation is known as the transit method. If we on Earth examined a small planet orbiting a far off star in the distance, we would occasionally notice that the planet happens to pass between us and its star, which as a result will block some of its starlight. By noting that this dimming was occurring and with enough frequency to attract our notice, we would be able to likely infer the presence of the planet, despite us not being able to see it.
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| An artist's rendering of an exoplanet orbiting two stars. |
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| Explaining the Transit Method in all its glory. |
This method, while it may seem questionable in its use, has actually been responsible for the majority of exoplanets we have discovered to date. Astronomers have been able to observe these patterns through telescopes in order to analyze this data more thoroughly. Every so often, the amount of dimming caused when planets pass between us on Earth and their star, tells us roughly the size of the planet. How can astronomers know this? Simple. If they happen to know the size of a star, as well as the planet’s distance from it, then they can just as easily observe how much of a percentage of the star’s light the planet blocks. Then, from that step, astronomers can finally calculate the radius of the planet based on those numbers. It is also worth noting that while the transit method has had some success, there are some disadvantages in its use, as well. There is a very low chance that the perfect alignment, one that lines the planet up correctly to pass in between us and its star, will take place - and this chance decreases the farther out it tends to orbit.
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| Studying Pulsars up close. |
One last method that astronomers use for indirectly detecting exoplanets is through Pulsar Timing. Pulsars are neutron stars that emit radio waves fairly regularly as they rotate in the universe. Just as an ordinary star, a pulsar moves in its own orbit if it has a nearby planet. Astronomers can then make calculations based on this type of pulse-timing, which can help reveal the restrictions of that particular orbit. Even though this method was not originally designed to detect planets, it is capable of finding those that are much too small for any other method to root them out. Moreover, it has the ability to detect planets which are relatively far away from the pulsar, as well.
Just as there are some downsides to the transit method, there are two negative sides to pulsar timing. Because pulsars tend to be relatively rare, special circumstances often require a planet to form around a type of pulsar. What does this mean? Essentially, it is extremely unlikely that a large number of planets will ever be found this way. Moreover, life would not be able to survive on planets that orbit pulsars either, primarily due to the intensity of high-energy radiation that exists on them.
Since the first exoplanet was discovered in 1988, astronomers have been able to find hundreds more of them using direct and indirect detection methods. These discoveries keep coming at a faster pace than ever before. It is clear from exoplanet detection not only how far we have had to come in order to discover these planets, but how much more we will still have to do.
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| How far we've come, but how much more we have to do. |
Works Cited
http://www.nbcnews.com/id/27702538/ns/technology_and_science-space/t/first-ever-images-taken-extrasolar-planets/#.WE7x-hh-XEY
https://www.spacetelescope.org/images/opo1003e/
http://www.techtimes.com/articles/178982/20160922/hubble-telescope-spots-exoplanet-orbiting-two-stars.htm
http://www.australianscience.com.au/science-2/discovery-of-exoplanets-tbc/
http://www.sci-news.com/astronomy/article00638.html
http://pics-about-space.com/exoplanet?p=2#
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