Absorption Features in an Extrasolar Planet Atmosphere - Research Paper Example

THE SEARCH FOR EXOPLANETS With respect to the search for exoplanets astronomers have made certain generalizations based both on factual observation, as well as limitations of instrumentality. An important strategy for planet detection must be the location of optimal configuration. At present, astronomers have uncovered several super earths, so named due to the superiority of size and mass with respect to our own planet – but those that we have presently discovered tended to orbit very close to their stars. In fact, all the super earths presently known to astronomers exist within a very limited, close range that is consistent between divergent star systems. This may not apply that no terrestrial planets, Earth like or larger than Earth could exist further out, but using purely telescopic methods there are limitations with regard to the detection of eclipses. Large, terrestrial planets outside our present optimal detection rate would eclipse their stars so infrequently as to be virtually undetectable by the present observational techniques. (Choi, 2012) Before evaluating the tools and strategies employed by astronomers in the search for exoplanets, it is necessary to define the parameters, as well as define the overall, eventual goal: The detection of earth-like planets. More than merely an item of curiosity, the search for new potential earths will help to answer fundamental questions about our own world: How common are the planets that maintain the stable environments for millions of years that Earth offers? If such planets are indeed common, then what is the frequency of life upon them? In addition, at what level of evolutionary/intellectual development? Answering questions like these puts Earth in perspective, with implications across the spectrum of scientific endeavor. A fundamental principle in the search is the definition of habitability. What makes a planet habitable by potential life; both in terms of its stellar position, and present conditions? The exoplanet task force defines the habitable zone of a star as the orbital region at which liquid water is stable on the surface of a planet; based on the amounts of radiation/heat the planet is calculated to receive. (Exoplanet Task Force, 2008) It is possible to speculate on other fluid media that might permit living chemistry; cooled hydrocarbons such as found on Titan for example, but since no such living systems are presently known to exist, we must begin by evaluating the prospects for life base on known principles. An additional caveat being that this intermediate area must retain the right range of temperatures for an extended eon. A giant star that burns through its fuel supply in less than a hundred million years would not be a viable prospect, even if a temperate orbital region can be deduced. Thus, the habitable zone of a star is where modern astronomers must begin their search. The presumption is that stars with the best prospects of hosting habitable planets would be F, G, K, or possibly M-class stars (Palma, 2011). Other stellar bodies besides these categories are predicted to have lifespans too short for anything approaching complex life. Many of the planets known to astronomers are super-earths, but this is not surprising given their nature. A larger mass (or volume) should be more visible using telescopic methods due to a large eclipse (transit) as the object passes between the earth-bound observer and its parent star. Furthermore, methods measuring stellar displacement, the wobble observed in stars that host planets will be more pronounced as the gravity of the orbiting object pulls against the star. In either case, super-earths are more visible. 2.) Photo from Palma, 2011. Palma C, 2011.The Habitable Zone. John A. Dutton, e-Education Institute. https://www.e-education.psu.edu/astro801/content/l12_p4.html. Accessed: 4/3/2012. Above is a comparison of three basic star types found in the Milky Way. The distance from the star where the radiation is too intense for water to exist as a liquid is seen in red. The blue orbit is too cold for liquid water, while the green region is known as the “Goldilocks zone", the approximate zone that our Earth occupies, which should stand the best chance of supporting life. (Palma, 2011) 3.) According to NASA, the number of known planets increases on a regular basis. Numerous websites exist which track new discoveries daily. (NASA, 2012) But terrestrial planets that might contain liquid water on the surface are of course, of special concern. NASA discovery mission number 10 is planned with the express purpose to survey our surrounding neighborhood in the Milky Way galaxy to perform a survey of habitable exoplanet candidates. This will give astronomers the ability to place the solar system in the proper context within the "continuum" of star systems in the Milky Way. (NASA, 2012) The Kepler mission seeks to complete its survey through the observation of planetary transit. Specifically, as exoplanets their host star, the eclipse of which causes a decrease in that stars observed brightness. From our perspective, most transits will change the brightness of the star by a degree of one in 10,000. (NASA, 2012) for this transition to be observable, the star system must be on the exact same elliptical plane of the galaxy as the observer. NASA calculates the probability that a given planet will orbit perpendicular to Earth, equals the diameter of the star divided by the diameter of the planets orbit. (NASA, 2012) Furthermore, the change in brightness must be consistent and repeatable before it can be affirmatively posited that it is in fact a planet, and the same planet. This is also the basis for the Transiting Exoplanet Survey Satellite, as described below. (Ricker et al. 2009) Other methods of searching for extraterrestrial life are more presumptive. SETI does not bother attempting to identify sources of liquid water, the objective is to "cut to the chase" by attempting to identify the activity of advanced, extraterrestrial civilizations. SETI attempts to identify radio transmissions that could only be the products of technological civilization. Here strategies range from pointing a receiver at individual stars that may be assumed to be possible candidates for alien life, in addition to a more passive approach that examines entire blocks of the sky for any sort of signal from an extraterrestrial source. (Seti.org, 2012) This approach has the advantage of an equal probability for the detection of life highly similar to our own, as well as divergent forms of life. As stated above, it is easy to speculate that living systems might use radically different biochemistry from our own, with only the earth as a guide it is difficult to determine these probabilities one way or another. Using finite resources, it makes sense to focus our efforts on systems with the greatest chance of earth like life, but should the potential for life be proved to be more diverse than we anticipate, then SETIs plan is just as likely to detect life forms of different chemistry. Something of a hybrid approach would be the transiting exoplanet survey satellite, known as TESS for short. This orbital device is intended for a two-year survey of the entire sky, cataloguing the transit effect on stellar luminosity. The proposed array is more of a catch-all than missions like Kepler, and should be capable of surveying 400 times more stars than that mission. (Ricker et al. 2009) TESS would have served as a complementary method for the identification of exoplanets, allowing astronomers to survey additional stars not targeted by the Kepler mission. (Gilster, 2009) PART TWO 1.) Once candidates for life have been identified, more refined tools are available to obtain additional evidence confirming the possibility. Spectrographic analysis can be utilized to measure absorbance of the surfaces of other bodies in our solar system, and theoretically - exoplanets. Any substance in any phase of matter will exhibit physical tendencies to absorb some portion of the electromagnetic spectrum. Sophisticated spectrographic analysis can delineate the chemical identity of a given surface based on the spectrum of radiation that the surface absorbs, and the extent to which it does so. This gives astronomers the tools needed to gain a rough impression of the surface composition of a stellar body. Chemical extrapolation based on spectrographics, combined with temperature data can identify the habitability of a given planet. Examples include numerous surveys in our own system, including scans of Mars, the ultraviolet reflective properties of which permit inferences about the planets composition to supplement probe data. (Barth &Hord, 1971) As data from the old Mariner mission indicates, it is also possible to determine atmospheric density and reflectivity; this allows calculations that can determine air pressure. (Barth &Hord, 1971) From here, it is possible to estimate other surface conditions; such as whether the planet might be a hothouse, Venusian world, or an airless ice dwarf - or earth-like. 2.) Transit spectroscopy, an outgrowth of the visual methods used to detect exoplanets can give insight on atmospheric composition. As planets eclipse their star, a portion of the star’s light will be partially absorbed by any atmosphere present. Those rays that reach a distant observer that were NOT absorbed will give data concerning the chemical composition of the atmosphere; as various molecules absorb different spectra of light. This allows inference on the existence of an atmosphere, its thickness, and chemistry. (Barman, 2007) 3.) Red dwarfs would seem reasonable for searches of exoplanets; simply because there are so many of them. As the most common star-type in the galaxy of billions, (van Dokkum& Conroy, 2010) it would seem that through pure brute numbers some habitable world must exist around such a star, somewhere. But there are problems; such a planet is unlikely to be earth-like. With a miniscule heat output compared to our own sun, a planet would need to be close, probably close enough to be tidal-locked, preventing a normal day-night cycle. Perhaps some life is possible under these conditions, but their environment would not mirror our own. Although, one might speculate that a day-night cycle could be achieved by a large moon, orbiting a super-earth – tidally locked to that planet. As it rotates around a planet, a moon might potentially arrive at a day cycle anyway. Here, there might also be radiation issues; solar flares would be more potent closer to a star. If that star’s energy output is far less than that of our own, the radiation risk is reduced, but localized flares and coronal eruptions might produce ejecta that could yield damage to sensitive life forms, or possible delicate circuitry of an advanced civilization. PART THREE 1.) Aiding in the search is the James Webb Space Telescope, an infrared optimized telescope; which should have the ability to interpret red-shift and thus gather data about the early universe. This will aid in our understanding of galaxy and planet formation, thus giving us a better idea how and where to search. (JWST, 2012) 2.) The Advanced Technology Large Aperture Space Telescope is a telescoping mirror array system with an optimized architecture that will yield images 2,000 times more sensitive than Hubble. It is projected that ATLAST will have a resolution able to detect complex gravitic interactions that will improve our understanding of star formation, thus helping us find more planets. (stsci.edu, 2012) References Barman T, 2007.Identification of Absorption Features in an Extrasolar Planet Atmosphere. The Astrophysical Journal, 661: L191–L194, 2007 June 1. 2007. The American Astronomical Society. All rights reserved. Printed in U.S.A. Barth CA, Hord CW. 1971. Mariner Ultraviolet Spectrometer: Topography and Polar Cap. Science 16 July 1971: Vol. 173 no. 3993 pp. 197-201 DOI: 10.1126/science.173.3993.197 Choi CQ, 2012. Super-Earth alien planets too big to share life with neighbors, say researchers While meteors may spread life across planets, Super-Earths have too much pull to share. Space on msnbc.com. http://www.msnbc.msn.com/id/46893009/ns/technology_and_science-space/. Accessed: 4/3/2012. Exoplanet Task Force, 2008. Worlds Beyond: A Strategy for the Detection and Characterization of Exoplanets. Report of the Exoplanet Task Force.Astronomy and Astrophysics Advisory Committee. Washington, D.C. May 22, 2008. Gilster P. 2009. TESS Mission Fails to Make the Cut. Centauri Dreams, the News Forum for the Tau Zero Foundation. http://www.centauri-dreams.org/?p=8340. Accessed: 4/3/2012. JWST.NASA.GOV. 2012.The James Webb Space Telescope.NASA. http://www.jwst.nasa.gov/ Accessed: 4/3/2012. NASA, 2012.Kepler, A Search for Habitable Planets. http://kepler.nasa.gov/Mission/QuickGuide/. Accessed: 4/3/2012. Palma C, 2011.The Habitable Zone. John A. Dutton, e-Education Institute. https://www.e-education.psu.edu/astro801/content/l12_p4.html. Accessed: 4/3/2012. Ricker GR, Latham DW, Vanderspek RK, et al. 2009. The Transiting Exoplanet Survey Satellite (TESS). American Astronomical Society, AAS Meeting #213, #403.01; Bulletin of the American Astronomical Society, Vol. 41, p.193 Seti.org. 2012.SETI 101.A History of SETI. http://www.seti.org/node/662. Accessed: 4/3/2012. stsci.edu 2012. ATLAST The Advanced Technology Large Aperture Space Telescope. Space Telescope Science Institute. http://www.stsci.edu/institute/atlast. Accessed: 4/3/2012. vanDokkum PG, Conroy C, 2010. "A substantial population of low-mass stars in luminous elliptical galaxies.Nature. 2010 468(7326):940 Read More