Physics of Detecting Extraterrestrial Life
I intend to create a unit that highlights many aspects of physics in a manner that is current, engaging and enlightening for my students. This section will address the physics of extraterrestrial life and how we can detect it. SETI has identified a relatively noise-free frequency coined the "water hole" because it is in the neighborhood of the microwave emission lines of hydrogen and hydroxide, which would combine to form water. This frequency is our best bet for searching for a signal. In order to detect the signal we are looking for a focused signal most likely produced by a laser in the radio frequency range. The benefit of a laser is that the collimated beam does not disperse and can be detected at great distances. The detrimental aspect of this is that the signal is only detectable if you are in the narrow scope of the beam. In addition, it is essential to search millions of frequencies or channels to make sure that the signal is not miss! ed.
Methods of Detecting Extra Solar Planets
The "wobble" of stars is caused by the existence of planets. A star's wobble caused by the revolution of planets around it is the result of the fact that multiple bodies orbit around their center of mass. The center of mass moves in a straight line. The mass of a planet affects the wobble of the star although it is far less massive. In the case of the Solar System, the center of mass is located near the edge of the Sun in the direction of Jupiter. That is why this "wobble" is so difficult to detect and why it took thirty years of searching to identify the first verifiable planet. It also explains why the first several hundred planets were huge Jupiter like planets, since they would affect the motion of the star much more significantly than a small planet.
Although it is simplified, the one dimensional analogy is the situation in which a large adult sits on a see-saw with a very small child. In order for the child and adult to balance (which is equivalent to finding a stable orbit) the adult would have to sit in very close to the pivot point and the child would have to be as far away from the pivot as possible.
Astrometric Method
This method involves the visual detection of the motion of the stars wobble perpendicular to our own position. This is very difficult to do because the amount of motion is so slight relative to the size of the point sources of light. Often the wobble is less than the diameter of the star. This requires technological sophistication that is just now being developed.
Radial Velocity Method
This method of detecting the wobble is utilized when the star is moving parallel to us. It requires the use of the Doppler effect to determine the slight motion and resultant change in velocity towards and away from us. This motion causes a slight Doppler shift in the light emitted by the star. This is done by analyzing the spectrum of the light of the star. Two slices of a spectral line are analyzed comparing the Doppler shift in a slice of the rising curve to the Doppler shift of the falling light curve. When done with sufficient sensitivity, the variability of the stars oscillation can be detected. This oscillation is the result of an orbiting planet and the amount of Doppler shift enables the calculation of the mass of the planet. This method took thirty years to develop and it has been the method by which most extra solar planets have been detected including Gliese 581 c and d. This information is M p l a n e t(sin i), where i is the angle of inclination to ! us, which is typically unknown.
Transit Method
As in the previous method of planet detection, the transit method is used when the motion of the planet is parallel to us. However, the transit method is the exact situation when the angle of inclination is exactly 90 degrees, or in our plane of sight. This results in the planet eclipsing the star. The planet is very small in comparison to the size of the star so only a small amount of the light is blocked. The method of analyzing this light data is known as photometry. It is only recently that the technology enabled the devices to be sensitive enough to detect this slight variation in luminosity. There needs to be an exquisite precision. The method of photometry has recently achieved this level and can detect 1 part change in luminosity in 10 5 difference. By continuing to detect for the next transit of the star the period and mass of the planet can be determined. Also, it is possible using this method to determine if the planet is in the star's ha! bitability zone.
Coronographic Method
This method which is still in development involves the direct observation of the planet by optically blocking out the star's light. This is equivalent to trying to detect a firefly next to a search light. The benefit of this method is that it would allow for the direct observation of the planet and therefore allow for spectral analysis of the atmosphere of the planet. This information could be utilized to potentially determine the existence of life based on the abundance of certain elements and compounds which we know are produced by life processes.
Interferometry
This method utilizes the wave characteristic of EM radiation to interact, thus creating constructive or destructive interference or perfectly canceling each other out. This is a method that enables extremely exacting measurement within the wavelength of the EM radiation utilized. As in waves in a pool created by multiple sources, the waves add and cancel resulting in patterns known as diffraction patterns. Interferometry is a powerful method that utilizes multiple smaller telescopes linked together by a detector. You achieve the same resolving power as a telescope that was the size of the separation of the small telescopes. However, in order to achieve this, the distance of separation must be maintained with incredible accuracy. You get great resolution but the distance must be maintained within a portion of a wavelength. Consequently, the first such telescopes were radio interferometers since the wavelength of radio waves can be miles. As the technology has progressed, thou! gh, we now have optical interferometers where the separation needs to be controlled within nanometers. Although interferometers offer incredible resolution, the amount of light collected is only the sum of the area of the telescopes. Therefore, for faint sources there are only a small number of photons collected. In addition to searching for planets, interferometery is useful in doing direct determinations of distances to astronomical objects and determining the Hubble constant with great accuracy.
Thermodynamics
The planetary equilibrium temperature, habitable zones and the greenhouse effect can all be determined. The temperature of a planet is based on the energy it receives from the luminosity of its star minus the energy that it radiates which is based on its surface area. A planet will achieve an equilibrium temperature. Greenhouse gases can raise the equilibrium temperature as a blanket raises the temperature of a body beneath it. The body radiates heat more slowly through the blanket material so a higher equilibrium point is attained at the lower atmospheric level, but regardless, equilibrium is achieved.
Electromagnetic Spectrum
The electromagnetic spectrum is the full spectrum of all energy photons as gamma rays, then x-rays, ultraviolet rays, visible light, infrared waves, and the lowest waves are radio waves. The velocity of all of this radiation is the speed of light. Therefore, the higher energy waves have higher frequency based on c=ΛΥ ; E=?Υ; E=?c/Λ; ?=4.135 x 10 - 1 5 eV-s.
Doppler Effect
The Doppler effect is the phenomenon that results from a moving source. A wave that is continually generated by a moving source will increase in frequency if it is moving toward you and decrease in frequency if it is moving away from you. In sound this is known as a change in pitch, either higher or lower respectively. In light, the change in frequency is the result of the constancy of the speed of light and its relativistic effects are detected as a change in color. This is the method used to determine that the galaxies in the Universe are moving away from us.
Spectroscopy
The study of spectrum enables astronomers to analyze the electromagnetic waves that reach us. The light contains an incredible amount of information. The quantization of light is the result that photons come in discrete packets. In addition, from chemistry we know that EM waves are emitted in quantized amounts that indicate the different potential orbits of the atom. There are discrete orbits so there are a limited, and specific number of energy states that electrons can occupy. Therefore, when electrons transition between these energy levels they either absorb or emit very specific amounts of energy. From a knowledge of a these chemical states and the periodic table, the electromagnetic radiation from space can be analyzed to determine what the composition of the matter that it interacted with consisted of. Radiation is absorbed by matter in these discrete energy levels, but at all other energy levels the atoms are transparent. Therefore, the photons of these exact energy l! evels capable of exciting atoms are absorbed. This causes there to be "holes" or dark bands in the electromagnetic spectrum. These bands act as a fingerprint for each element. A careful analysis of this fingerprint enables the chemical composition of far away matter to be determined! In addition, this spectrographic finger print is used to determine how much the EM spectrum might have been Doppler shifted as the result of the velocity of the source towards or away from us. The EM waves that are absorbed by the atoms are eventually reradiated, but they are radiated in all directions therefore resulting in the dark band in the spectrographic spectrum.
Land Based Telescopes
The disadvantage of land based telescopes is that they must get all of their information from EM radiation that is distorted by the atmosphere. The atmosphere is made up of a fluid of particles that diffract the EM radiation. In large part this can be accounted for. However, the turbulence of the atmosphere, known as seeing, results in a blurring of point sources so it puts a definite limit on the amount of resolution that can be achieved. For exacting measurements that are required for these faint and far away sources that may only activate a pixel on the receiving device the seeing is a devastating effect and results in a blurring that spreads the light over several pixels. In part, as a result of the "Star Wars" program, scientists have been able to resolve the relative distortion of the atmosphere, by analyzing the effects on laser light, however, the effect of seeing, although it can be mitigated cannot be removed and it is the limiting factor of lan! d based telescopes. Another disadvantage is that land based telescopes can only operate at night because the sky is too bright, are affected by local city light, and are periodically obscured by cloud cover. The down time for the telescope due to daylight and cloud cover prevent continual data collection which is necessary for some observational purposes.
The benefit of land based telescopes is that they are much cheaper and can be maintained and upgraded easily. However, for extra solar planet searches they are reaching the limits of their capability. The Keck interferometer is an example of the best ground based telescope. More can be found out about this telescope at http://planetquest.jpl.nasa.gov/Keck/keck_index.cfm.
Allen Telescope Array- SETI
Array Telescopes are a method of interferometrically combining smaller telescopes together. It is possible if delicately coordinated to get the equivalent gathering aperture as significantly larger and monumentally more expensive telescopes. SETI, the Search for Extra Terrestrial Intelligence, has developed and begun deploying the ATA, Allen Telescope Array, which is a huge array of interlocking 6 meter satellites that are the equivalent of a one hectare telescope that will allow for the surveillance of tens of thousands of channels from 0.1 Hz to 16 GHz. For more information visit http://en.wikipedia.org/wiki/SETI#Allen_Telescope_Array.
Space Telescopes
The search for extra solar Earth like planets has become scientifically respectable as is indicated by the incredible amount of research being done in this field. The website dedicated to this field and the technological project underway is run by NASA and is called Planet Quest: the Search for Another Earth and can be accessed at http://planetquest.jpl.nasa.gov/index.cfm. This is an incredible site for specific information about existing and planned satellite projects, educational resources and background information.
SIM- Space Interferometery Mission
Interferometric telescopes utilize interferometry to make very exact measurements. The SIM telescope is an example of this type which is scheduled to be launched in 2012 and its sensitivity will enable it to detect planets as small as the Earth. Currently we can achieve 1 thousandth of an arc second. The SIM will be able to achieve 1000 times better resolution, or 1 millionth of an arc second. The website to find out more information is http://planetquest.jpl.nasa.gov/SIM/sim_index.cfm.
Transit- Kepler Satellite
The transit technique utilized by the Kepler satellite will enable scientists to determine whether a planet is in its star's habitable zone. Check out the website at
http://planetquest.jpl.nasa.gov/Kepler/kepler_index.cfm.
Coronagraph Telescope- TPF: Terrestrial Planet Finder
Coronagraphy is a method of attempting to detect planets directly by blocking out the stars EM radiation by "eclipsing" it optically. This is an extremely sophisticated method, but there is a satellite telescope being developed for deployment as the successor to SIM. By receiving EM radiation directly, it allows spectroscopy to be used to analyze the chemical composition of the planets atmosphere. Learn more about this mission at http://planetquest.jpl.nasa.gov/TPF/tpf_index.cfm.
Types of Orbits for Satellites
Low orbit
These satellites have a 90 minute orbit at about a few hundred kilometers and above and some need constant boosting or their orbit degrades and they reenter the atmosphere. The range is from 100- 1000 kilometers and above 700 kilometers there is no need to reboost. Our telecommunication satellites are in low orbit because the time for relaying the signal is short enough at the speed of light not to be problematic. These satellites experience high friction from the atmosphere and consequently require constant "boosting" to maintain their orbit.
Geosynchronous Orbit
Orbiting at the same rate as the Earth allows a satellite to stay above the same location. The orbit is 23 hours 56 minutes. These orbits are permanent which is great for us since they do not require energy to maintain their orbit, however, the detrimental side of this fact is that failed satellites in this orbit turn into space debris that stays there forever. This orbit is not used for telecommunication satellites because there is a quarter of a second delay in the two way transmission. This orbit is at approximately 23,000 miles altitude.
Inner-Lagrangian Orbit
This orbit maintains the same relative position between the Sun and the Earth. There are three of these positions. This orbit is in relative equilibrium and stays relatively stable.
Calculating Orbits
Kepler's Laws P 2=a 3; "P" is the planet's sidereal period in years and "a" is planet's semimajor axis, in AU.
Comments: