Astronomy paper

Introduction: Searching for extra-solar planets is not a very easy task. Because most of the times we find them by not actually looking at them but by the effects they produce mainly on the star around which they are rotating. It is because planets look like just a dull spec of light through a telescope. The distance between us and a nearby planet is very large. In our scale model it will be like looking at a dust particle in Thunder Bay Ontario if we are sitting in Halifax with our telescope. That’s why astronomers have adopted various techniques to find extra-solar planets. In most of the techniques we focus on the star around which planets are rotating and by noticing the changes occurring in star we can decide whether that star has planet(s) or not. The following methods are used for that purpose.

1. Astrometry

2. Radial Velocity Method

3. Pulsar Timing

4. Transit Method

5. Gravitational Microlensing

Astrometry: This is the oldest method used to search extra-solar planets. This planet is basically used to find the mass of thee planet. This method is based on the gravitational effect of planet on its host star. As the planet rotates around its star, it applies gravitational force on the star and makes the star to move under the effect of its (planet) gravity. Thus, both the star and the planet revolve around a common center of mass also called center of gravity. Thus, the position of star always keeps on changing, which we can notice by using sensitive telescopes and thus by drawing 2D or sometimes by 3D graphs of position of star and measuring how the x and y intercepts are changing. How much the position is changed will decide the mass of planets. Massive the planet, grater will be the change in position.

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This method is most useful in the absence of atmospheric distortion. That’s why telescopes like Hubble can use this method more efficiently as compare to ground based telescopes.

Advantages:

1. This technique is very useful in detecting the planets with very large orbits cause they cause the star to change its position to a greater extent as the length of the orbit of star increases.

2. This technique is very useful in finding massive stars as they can cause the star to wobble to a much greater extent as compare to the small planets.

Disadvantages:

1. This method requires a long period of observations possibly years or sometime decades, as the star changes its position very slowly.

2. It is hard to find less massive planets with this method because they cannot change the position of star very much.

3. We cannot how many planets are rotating around the star.

4. This method is not suitable for ground based telescopes.

Radial Velocity Method: This method is also used to find the mass of planet. In this method we actually notice the change in the radial velocity of star with which it is coming towards or going away from us. We use Spectral lines of star to measure that. We use Doppler’s Effect (blue and red shift) to get the information from spectral lines. The radial velocity can be calculated from the displacement in the spectral lines. Spectrometers are the instruments used to measure the change in the redial velocity. HARPS (High Accuracy Radial Velocity Planet Searcher) is the most sensitive spectrometer. It can detect even a 1m/s change in the radial velocity of star. The mass calculated by this method will not be the actual mass. That will be the minimum mass. We have to use the Transit Method to calculate the actual mass.

This method can be used most accurately if the orbit of planet and the star we are looking for is perpendicular to our line of sight and the signal strength is high enough to get good precision.

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Advantages:

1. This method is distance independent i.e. it does not stop working at greater distances.

2. This is so far the best technique used to detect planets.

3. This method is easy to use for the massive planets with small orbits around the star.

Disadvantages:

1. We cannot find the true mass of a planet by this method.

2. Mass of planets with inclined orbits with respect to our line of sight cannot be measured with much accuracy.

Pulsar Timing: In this method neutron stars are used to find the planets. They emit radio waves extremely regularly as they rotate. These radio waves are so regular that a small change in them can be easily detected by the radio telescopes. Thus, if a planet will be rotating the pulsar, it will cause some interference and distortion in the radio waves by the effect of its gravity and by absorbing them too. Even the circumference of an orbit of a star can be measured by this method.

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Advantage:

1. This method can be used even for very less massive planets even with mass less than one tenth the mass of Earth.

2. Even the parameter, circumference of the orbit of planet can be calculated by this method.

Disadvantage:

1. As neutron stars are not very common, thus we cannot take a big advantage from this method.

2. Life is not possible on planets orbiting pulsars as such a planet will be exposed to dangerous high intensity UV,X-rays and gamma rays.

Transit Method: Transit method is used to determine the size thus the radius and diameter of the planet. Even the orbital period of planet can be determined by this method. For this method we use the brightness curve of the star. If the brightness curve shows dips in the curve at regular interval of time, then it shows that there is a planet revolving around the star. The amount the star dims is directly related to relative size of star and planet .The time difference between the occurrence of two consecutive dips will give us the orbital period of star.

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This method can be used fully accurately if the planetary transits are perfectly aligned with respect to our line of sight and if the orbits of planets are small.

Advantage:

1. We can calculate both size and orbital period of planer by this method.

2. By using both transit and radial velocity method we can calculate the density of planet as we have both radius and mass.

3. We can get an idea about the composition of atmosphere of planet by reading the spectral lines of stellar spectrum, as the upper atmosphere of planet absorb some of the light coming from the star. In some cases we can even measure the surface temperature of the star.

Disadvantages:

1. Its hard to observe those planets whose transits are not aligned with respect to our line of sight.

2. We cannot use only this method to get the data because this method has low value of accuracy. That`s why we have to use other methods too especially radial velocity method to get the confirmation of the data we got from transit method.

Gravitational Microlensing: This method is based on the gravitational effect of planet and star on the light of a distant background star. As light can be effected by the action of gravity on it. So, the same phenomenon occurs here. Light coming from a background star will be bended and will be magnified by the star and the planet that are present between us and the source star. The contribution of planet can be detected by using robotic telescopes which regularly keep on monitoring these kinds of events.

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But these kinds of events require a very improbable alignment between the source star, the lens star and us.

This planet is most useful to detect those planets which lie between Earth and Galactic centre in a perfect alignment, as galactic center provides us enormous number of background stars.

Advantages:

1. This is the only method of detecting Earth like planets around the ordinary stars.

Disadvantages:

1. As this method requires a perfect alignment, that`s why planets cannot be detected by this method very frequently.

2. The lensing that occur between the background star and the in between star and planet cannot be repeated as the alignment is not possible once gain. That`s why we can not use this method for a long time observation.

Recent interesting exoplanet discoveries:

1. In 2007 astronomers have found the most massive extra-solar planet HAT-P-2b. The mass of this planet is more than 8 times the mass of Jupiter, but in case of size it is slightly bigger than Jupiter. Thus, this planet is a big very-very dense ball with very high gravity. This planet has highly elliptic orbit and its orbital period is just 5.63 days. It is made up of Hydrogen and is almost at the boundary of becoming a star.

2. In April 2007, an earth-like planet was detected. Its name is Geliese 581C. This planet is revolving around its star in the habitable zone i.e. the distance between the star and the planet is almost exact same as the distance between Earth and the Sun. Astronomers are expecting water and probably life on this planet. This planet is ½ times bigger than our Earth.

3. In January 25, 2006, planet Super Earth (OGLE-2005-BLG390LB) is found by using the technique of Gravitational Microlensing. Its mass is almost 5 times the mass of Earth. This discovery is important in the way that we can now find small Earth like planets with using new techniques and good accuracy.

Ideas for future methods of searching Extra-solar Planets:

1.Observations from Space: Observations from space will be more accurate and sensitive than the observations done by ground based telescopes. Thats why our space agencies have designed the space probes that will use the same methods that we use on Earth e.g. transit method etc. In additions to that they can absorb infra red radiations coming from different planets to determine the temperature etc of the planet. For example: Kepler Mission is designed by NASA to detect exoplanets from space. The main hope from these missions is that they will be able to detect terrestrial earth-like planets.

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2. Eclipsing binary minima timing: This method is used for binary stars that orbit each other. As they have a fixed orbital period in which they complete their orbit around each other. But if a planet(s) is orbiting this eclipsing binary, it will interrupt the orbital period of the binary due to the effect of its gravity. Thus, the star wobbling will disturb the orbital period and the delay or the advancement in the orbital period will be the measure of mass of planet.

3. Polarimetry: This method will be used to find the planets by using polarimeters. As the light that comes from a star is un-polarized i.e. the light waves do not travel in a regular pattern. But if a planet will be orbiting the star, its upper atmosphere will absorb some of the light and also reflect it in a polarized fashion. Thus, we can detect the planet and even the composition of its atmosphere. By combining this method with transit method we can calculate the orbital period of the planet too.

The main questions that are arising for research :

1. Is life possible elsewhere in the universe?

2. Are there any planets with Earth like conditions?

3. Is it possible to make other planets habitable for us?

4. Is it possible to use the energy resources of other planets, asteroids etc and their transport to the planet Earth?

Bibliography

References:

• http://en.wikipedia.org/wiki/Methods_of_detecting_extrasolar_planets

• http://www.cfa.harvard.edu/news/2007/pr200711.html

• http://en.wikipedia.org/wiki/OGLE-2005-BLG-390Lb

• http://www.space.com/scienceastronomy/070424_hab_exoplanet.html

• Discussion with Prof. Douglas Pitcairn (Professor of Sky and Planets) Saint Mary’s University Halifax, NS, Canada.

• http://www.cnes.fr/automne_modules_files/standard/public/p1403_9ef5e2e97d4ce878ede92d956cd5fcadtransit.JPG