Exoplanet search has hit a purple patch. NASA literally announces every other day that a new Super-Earth has been found. It has been difficult for me to keep track of every one of the new planets, which has resulted in me taking almost two months to write this post (I had to keep changing the theme of the post every time a new planet was found). In this post, I will be talking about the techniques that astronomers use to search for exoplanets and also talk about a few new planets that have been discovered.
A couple of decades ago, apart from the nine planets around our Sun (there were nine planets then), astronomers did not know about the existence of any other planet. It is obvious that other planetary systems must exist as our Sun is not unique. There are a few hundred billions stars in each of the few hundred billion galaxies in the universe. Instead of having nine planets, even if each one of the stars had a single planet revolving around them, there should be at least 100 sextillion (1 followed by 23 zeros!) planets in the universe. But astronomers couldn't find even a single planet. The reason planets are so hard to find is that they are small and at the same time, unlike stars, they are not bright. They reflect light from the stars but when they are looked at from a distance, the light from the stars overwhelm the light reflected by them. But astronomers have come up with a number of cool techniques to find exoplanets.
Here is a link to the wikipedia page on exoplanets which talks about all the detection methods. But I am going to talk about a few of the important methods that were used in the recent discovery of exoplanets.
1. Transit method: This is the method used by the Kepler mission to search for exoplanets. What the Kepler space observatory does is that it looks at a small patch in the sky, with a few hundred stars, and monitors their brightness. If a planet goes in between the us (Earth) and the star, the brightness of the star would decrease marginally as the star's light is obstructed by the planet. By calculating the decrease in brightness, it is possible to find the size of the planet, its distance from the star and also its orbit around the star. This method has been more successful in finding exoplanets and most of the recent discoveries have come from this mission. This method has a crucial drawback. This method would work only when the plane of the orbit of the planet is almost parallel to the earth's orbit around the sun. If the orbit it perpendicular, it would not be possible to use this method to detect planets.
2. Doppler method: Similar to how planets revolve around stars, stars also revolve around the galactic center. When objects move away from us, the light that we get from them shifts to higher wavelengths and when they move towards us, the light shifts to lower wavelengths. This phenomenon is called Doppler Effect. In astronomy Doppler effect has been used successfully to measure the velocities of stars revolving around the center of their respective galaxies. When planets are also are revolving around the stars, though small, the planet's gravity pulls the star by small amount which could slow down or speed up the star as it goes around the galaxy. Based on how the velocity fluctuates with time, it is possible to predict the size and the orbit of the planet around the star. This method works well in the case of planets large planets, like Jupiter, which have a greater effect on their star's velocity, due to their immense gravity.
3. Direct Imaging: This is is most difficult of all the methods. As mentioned above, planets appear as tiny dark objects in extremely bright backgrounds. Therefore finding them by looking at them directly is a really difficult task. Usually this method is used in conjunction with the other methods. But there is one advantage in using this method - it can be used to analyze the atmospheric composition of planets. This analysis is very important especially in the search for conditions that could support life.
Here is a link to the wikipedia page on exoplanets which talks about all the detection methods. But I am going to talk about a few of the important methods that were used in the recent discovery of exoplanets.
1. Transit method: This is the method used by the Kepler mission to search for exoplanets. What the Kepler space observatory does is that it looks at a small patch in the sky, with a few hundred stars, and monitors their brightness. If a planet goes in between the us (Earth) and the star, the brightness of the star would decrease marginally as the star's light is obstructed by the planet. By calculating the decrease in brightness, it is possible to find the size of the planet, its distance from the star and also its orbit around the star. This method has been more successful in finding exoplanets and most of the recent discoveries have come from this mission. This method has a crucial drawback. This method would work only when the plane of the orbit of the planet is almost parallel to the earth's orbit around the sun. If the orbit it perpendicular, it would not be possible to use this method to detect planets.
2. Doppler method: Similar to how planets revolve around stars, stars also revolve around the galactic center. When objects move away from us, the light that we get from them shifts to higher wavelengths and when they move towards us, the light shifts to lower wavelengths. This phenomenon is called Doppler Effect. In astronomy Doppler effect has been used successfully to measure the velocities of stars revolving around the center of their respective galaxies. When planets are also are revolving around the stars, though small, the planet's gravity pulls the star by small amount which could slow down or speed up the star as it goes around the galaxy. Based on how the velocity fluctuates with time, it is possible to predict the size and the orbit of the planet around the star. This method works well in the case of planets large planets, like Jupiter, which have a greater effect on their star's velocity, due to their immense gravity.
3. Direct Imaging: This is is most difficult of all the methods. As mentioned above, planets appear as tiny dark objects in extremely bright backgrounds. Therefore finding them by looking at them directly is a really difficult task. Usually this method is used in conjunction with the other methods. But there is one advantage in using this method - it can be used to analyze the atmospheric composition of planets. This analysis is very important especially in the search for conditions that could support life.
The sensitivity of all the above methods depend heavily on the size of the planets. The larger the planet, the easier it is to find them. Amongst the above methods, the transit method used by the Kepler mission is the most sensitive method. An important point to note is that, all the detection methods are useful in detecting only those planets that are a part of planetary systems like our own solar system, that is, planets that go around their stars. Recent solar system simulations indicate that when our solar system formed there were more (at least one more) planets going around the sun. But at some point the planets other than the ones that we observe today were thrown out of the solar system due to gravitational imbalance. It is now believed that this is a common phenomenon and many such rogue planets might be wandering far away from the star near which they were formed. Since these planets are far away from any light source, it is not possible to detect them by any of the current detection methods.
In order to understand why a lot a new planets have been found in the last couple of months, imagine that an alien species in some other part of the galaxy is searching for other planetary systems using the transit method. If they find Earth, moving across the sun, due to Earth's period of revolution (which is about 365 days or one year), they would have to wait for a entire year before they would see Earth again. And it would take another year to see Earth again and confirm the data. Therefore it takes time to look for planets especially the ones which have a lengthy orbital period. Since the Kepler mission is about three years old it is in a position to verify the data that it has been collecting since its inception. That's why a large number of planets are being found at the same time.
Let me now talk about a few planets that have been detected. A planet named GJ667Cc orbits a star called GJ667C. This planet is about 4.5 times as massive as Earth and is in the habitable zone around its star. The habitable zone is a region where temperatures are suitable for liquid water to exist. We still do not know what is the composition of its atmosphere but depending on its distance from its star it is said to be in the habitable zone. Most of the exoplanets that have been detected so far are as big as the Jupiter. One reason for this is that, as mentioned earlier, larger planets are easy to find. But two new planets named Kepler20e and Kepler20f has been identified to be almost as big as Earth. These two planets are too close to their star and hence they are not in the habitable zone. But the discovery of these two planets has given hope to astronomers that Earth-like planets in the habitable zone can be detected.
Here is an excellent discussion on exoplanet search by Fraser Cain (@fcain), Phil Plait (@BadAstronomer), Nancy Atkinson, Emily Lakdawalla and Alan Boyle.
Exoplanet search generates a large amount of data, due to which it takes a very long time to analyze the data and publish the results. But a new initiative called Planet Hunters invites the public to be a part of exoplanet search. By creating a free account in their website anyone can analyze the data and search for new planets. People who have helped in the discovery of new planets have been included as authors in the papers with the results.
Let me now talk about a few planets that have been detected. A planet named GJ667Cc orbits a star called GJ667C. This planet is about 4.5 times as massive as Earth and is in the habitable zone around its star. The habitable zone is a region where temperatures are suitable for liquid water to exist. We still do not know what is the composition of its atmosphere but depending on its distance from its star it is said to be in the habitable zone. Most of the exoplanets that have been detected so far are as big as the Jupiter. One reason for this is that, as mentioned earlier, larger planets are easy to find. But two new planets named Kepler20e and Kepler20f has been identified to be almost as big as Earth. These two planets are too close to their star and hence they are not in the habitable zone. But the discovery of these two planets has given hope to astronomers that Earth-like planets in the habitable zone can be detected.
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| Credit:NASA/Ames/JPL-Caltech |
Exoplanet search generates a large amount of data, due to which it takes a very long time to analyze the data and publish the results. But a new initiative called Planet Hunters invites the public to be a part of exoplanet search. By creating a free account in their website anyone can analyze the data and search for new planets. People who have helped in the discovery of new planets have been included as authors in the papers with the results.
Currently, 759 planets have been identified (as of February 11, 2012) and this list is going to swell by the end of this year. Very soon Earth's twin will be discovered.
Nice one na :)
ReplyDeleteThanks! :)
ReplyDeleteAh, I finally got some of my questions answered!
ReplyDeletePrabitha
Glad I could help :)
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