Limitations of the Transit Methods

        Even though the transit method for detecting planets around other stars is one of the most popular techniques that are being used there are limitations and there are challenges that have to be overcome to make this method even better. As I mentioned in my last post the smallest planet that has been found to date is Kepler 10-b. It's radius is 1.24 times that of the Earth which is promising but by far most of the planets that have been found are of the sizes of the gas giants with most being Jupiter sized and some even Super Jupiter sized. It is amazing that we are able to detect these planets but when we want to look for life we want to find smaller terrestrial like planets. These planets will be hard to find because they are a lot smaller than the star that they orbit, they also do not give of their own light and the sun gives off a lot more light than the planets.  This limits the ability for the transit method but there are other limitations as well. The first limitation is our limited angle. This means that only a fraction of those systems viewed will be tilted so that the planets will pass in front. This means that they have to be aligned to be able to catch a transit. The probability that they will align with a planet orbiting a sun-sized star at the same distance we are from the Sun is a very small .47%. This means that there is a very low chance that we would be able to catch a planet that is around a G type star that is in its habitable zone. This is discouraging but there is still a chance. Another one of the limitations is the high rate of false detections. This means that to confirm a sighting one has to see three or four transits. There can also be other techniques that can be used to confirm or to help confirm if there is in fact a planet. Also the transit technique can determine the planet’s size this method does not reveal its mass or details of its orbit as well as we have not yet been able to determine the atmospheric composition. This means that we have to use other methods to determine these other properties. For example if a planet was confirmed from the transit one can determine the radius of the planet. Then by using the Doppler Wobble method we can determine the mass. From these we can determine the density and then make models of what the planet could be made of and the different layers. There are limitations of this method but it is still a widely used method.

Information obtianed from: http://questional.com/blog/181-planet-hunting-techniques-transit-dimming-the-lights/

Kepler

          Kepler is a satellite that was launched March 7^th 2009 and has found many extrasolar planets; some have been confirmed while others have yet to be confirmed. The Doppler Wobble method could be used to confirm if these detected exoplanets are actually planets. This satellite was set up into the sky and it points at the same patch of stars night after night. The aim of this mission is to explore the structures of planetary systems. Kepler has been very instrumental in finding exoplanets using the transit techniques. This method has been more successful since the satellites are above the atmosphere and are able to get better images. 

Figure obtained from: http://blog.professorastronomy.com/2010/06/too-many-planets.html

        One can see from the image above that the ground based telescopes the image of the transit is not as well defined as the Kepler measurement.  So far Kepler has confirmed 25 planets and has 1235 planet candidates, this tally can be found on NASA’s webpage that is designated for Kepler. One of the confirmed planets that Kepler has found is Kepler-14b. This is a huge discovery because it is the smallest planet that has ever been discovered outside of our solar system. It is a rocky planet and is only 1.4 times the size of Earth. This discovers was based on data that was collected from May 2009 to January 2010. They first found Kepler-10 and it was a star that NASA identified as a possible star that could harbour a small transiting planet and this made it a priority. Even though the discovered this planet that is the closet planet to Earth in size that has been found they noted that it orbits Kepler-10 once every 0.84 days this means that the planet is 20 times closer to its star then Mercury. This means that Kepler-10-b is not habitable. With the large amount of candidate planets that Kepler has discovered I feel that it is just a matter of time before we find planets that may be possible to harbour life. 

Information obtained from:

http://kepler.nasa.gov/

http://www.nasa.gov/topics/universe/features/rocky_planet.html

Nanosatellites

        Over the past few weeks I have discussed the transit technique and how it is used to locate exoplanets. I have looked at that the concept of transits and the different light curves that can form. Last week I discussed the complex light curves that can arise when there are binary systems.  There is a prototype of a satellite that well be focused on finding Earth sized planets around sun like stars. These satellites are small and can range from 1 to 10kg and are made of four components. The prototype of the satellite is shown in the figure below.

Figure obtained from: http://nextbigfuture.com/2011/09/exoplanetsat-and-other-things-to-do.html

        When the satellites are put into orbit around the Earth they will be concentrating their lenses at the brightest stars in the sky. Their hope is to be able to get light curves from planets that orbit around these stars. These satellites will get into their orbits by being piggybacked by space crafts. The CubeSats are confined to orbits that are low the Earth because there is no propulsion systems that are available that would fit into this small scale. There has not been a launch of these satellites yet but they are supposed to start to launch these in 2013.  There are benefits to these satellites over others. One of these advantages is that they are cost efficient and if damaged they can just sent another one up. One may also think that because of the small size of these satellites they would be a better chance for them to get damaged. This is not true the fact is that because they are so small there is less of a chance that they will get hit. They also have a huge amount of technologies that fit into the small satellite. They have CCD as well as CMOS and fine and rough pointing so that they are able to get the transit curves of earth like planets. They also have heaters that control the heat to as much as 10mK which reduces the noise.  In the future the satellites may be able to do follow ups to other satellites surveys to see if there are exoplanets around stars. They may also be able to see the composition of the atmosphere around the planets. Here is another example of how cutting edge the transit method is becoming.

What the transit method has Found

        The method that I described last week is the transit method of a single star system. However, there are binary star systems in which transits can occur. There are projects one known as the TEP network that is trying to observe a transit of a planet in this type of star systems.  They can measure this transit because the planet would transit in the plane of the star system. This means that the planet would transit both components. When this transit occurs there can be a variety of shapes on the transit graphs they are more complex than just the one star system.  This arises because the planet is moving around both of the stars, but the stars are moving around each other too.  There are two examples of the type of graphs one can get from the transition of the binary star system. 

Figure obtained from: http://www.iac.es/proyecto/tep/transitmet.html

        The first graph is of the normal transit of binary systems. It has two dips this is the transit of each star individually and are separated by a few hours. The second graph is a single long transit when the planet transits the two stars at once. These are just two of the configurations that can occur with two star binary systems. There are many other possibilities of the transit of planets around these binary star systems. one of the discoveries the binary star system was found by Kepler. Kepler-16 is a binary star system. The two stars that make up the binary system are dwarf stars, one is a class K orange dwarf and the second is a class M red dwarf. The planet that was found to orbit these two stars is a gas giant it is slightly smaller than Saturn and completes a nearly circular orbit every 228.8 days.  This is just one of the many exo-planets the Kepler has found.  

Information obtained from:

http://scienceblogs.com/SETI/2011/09/kepler-16_exoplanets_around_bi.php

http://www.iac.es/proyecto/tep/transitmet.html

Transit Method

        Last week I took a look at one of the methods which astronomers use to find planets around stars. I discussed the Stellar Wobble technique, which is what in the past has been used to hunt for extra solar planets. There are a few drawbacks to this method. One of the drawbacks is that it is tedious; one has to keep returning to this star and measure the motion.  The other drawback is the cost, it is expensive to take the spectra and see the red and blue shifts. There is no other way in which to obtain the information. Therefore, the transit method is a way that one can determine if there may be a planet that is orbiting a star. One can find the size of the planet though this method.  This method is less tedious and also cheaper. One only has to take a picture with a telescope and see if the brightness of the star has changed. The transit method is a method that measures the dimming effect a planet will have on the star when the planet passes in front of the star. This dimming effect is not a large one but with the technology we have one can determine this low amount of dimming. Kepler is a satellite that is in orbit right now and it is looking at the same patch of sky day after day and is looking for this effect. It has found a large number of possible planets around stars since it has been launched. The figure below shows the transit method. 

Figure obtained from: http://www.jodcast.net/archive/201107Extra/

        

        Like every method there are disadvantages. Some of these included that these transits are only observable for planets whose orbits are aligned with the observer. Another disadvantage is that this method can suffer from false detections. For example the find a two star system or the star has the equivalent of our stars sun spots. This can make the results seem to be a planet but it is false. This means that there has to be another technique to determine if there is in fact a planet. This is where if the Stellar Wobble and the Transit Method are partnered together then it would save money and time. The transit method could be used to find possible extrasolar planets and then the stellar wobble would be able to confirm that they are in fact planets. This means that the expensive method would only be used on stars that have evidence for an orbiting planet.

Information obtained from:
 http://novacelestia.com/space_art_extrasolar_planets/detect_extrasolar_planets.html

Use of a Stellar Wobble to Locate Planets around Other Stars

       I have done a quick survey of some of the bodies in our solar system that could of harboured life or that is promising to harbour life. I am going to take a break from the survey of our solar system. Today I am going to talk about one method that is useful in finding planets indirectly. The method that I am going to discuss is called the Stellar Wobble. This model is useful in finding large planets that are close to the star.  The Stellar wobble is a measure of the variation in position of a star. This is because the force of gravity of the planet acts upon the star. The diagram below shows this interaction. This interaction causes the star and the planet to orbit the center of mass and thus the star appears to wobble. If the planet did not have a large gravitational effect on the sun then the center of mass would be located in the star and would not appear to wobble. Therefore, this method is only useful when trying to locate large planet that are also close enough as mentioned above. 

Figure obtained from: http://www.astro.wisc.edu/~townsent/static.php?ref=diploma-2

        This wobble is visible when looking at the Doppler shift in the star’s spectrum. This Doppler shift can be seen when the star is moving towards or away from the observer. When the star is moving toward the observer then one will observe a blue shift in the spectrum. This will shift the features of the spectrum to shorter wavelengths. If the star was moving away it would be red shifted and the spectral lines will be shifted to longer wavelengths. From this information we can concluded that there is at least one planet orbiting the star, one can also determine the mass of the planet and its distance away from the star. It gets more complicated when there are multiple planets that are acting on the star but with mathematical modeling one can determine how many and their mass, and distances.

Figure Obtained from:http://spiff.rit.edu/classes/phys230/lectures/planets/planets.html

        In order to determine if the star is in fact moving or wobbling there has to be tons of observations that have to be conducted. One has to check every few days and see how the star is moving. This method to find stars is tedious but I found that it was a very interesting way in which extra solar planets could be found.

Information obtained from http://www.grantchronicles.com/astro43.htm

The Chance of life on Europa

          So far I have commented on the vastness of our universe and have toned down the survey for life to just our solar system. I have looked at the only two planets that fall within or that are close to the habitable zone for G type stars.  Earth, Mars and maybe Venus are part of this zone and that is what has led me to take a closer look at these planets. However, these are not the only bodies in our solar system which should be taken into consideration for life. Jupiter is a planet that has four large moons. Europa especially contains a necessity for life. Under the frozen surface of Europa there is thought to be liquid water that has peaked astronomers interest. There are still questions about how think the layer of ice is. There are plans to use radar to determine this. However, as we know presently all life forms need water to survive and thus this large ocean may be a place where life could evolve. Also when one looks back in our own history here are many believers that life started in the oceans of Earth and then eventually moved onto the land. The main debate as mentioned above is the thickness of the ice.  This is because the thickness is an indirect measure of tidal heating.  Tidal heating is crucial for determining whether there is volcanism on the sea floor. I feel that this is one of the few possibilities that life could exist within our solar system but there are other factors that need to be considered. For instance if the ice is thick then the sunlight will not be able to penetrate through the layer of ice. If this happened then there would be no chance of organism that uses photosynthesis. Also even if there are macroscopic life forms that develop there may not be larger life forms or intelligent life. Even though there may not be intelligent life scientists are very interested in Europa.

Some information obtained from : http://www.lpi.usra.edu/resources/europa/thickice/