1086F Origin and Geology of the Solar System
- Introduction
An exoplanet is a term used to describe a planet that is located outside the solar system that includes the Earth (exoplanet, 2013). Astronomer Peter van Kamp made the first announcement of the discovery of an exoplanet in 1963, however, that discovery failed to hold up over time and now has been conclusively disproved (Matson, 2013). The first exoplanet discovery that has stood the test of time was the discovery of two bodies orbiting a neutron star in a system named PSR 1257+12 (Wolszczan and Frail, 1992). Wolszczan and Frail calculated that the masses of these exoplanets would be at least 2.8 and 3.4 times the mass of the Earth. A third exoplanet in that system was confirmed in 1994 (Wolszczan). The first discovery of an explanet orbiting a star like the sun followed several years later, when Mayor and Queloz found an exoplanet they named 51 Pegasi b (Powell, 1996). As of July 2013, there are 921 planets listed at exoplanet.eu, a comprehensive listing maintained by an astronomer at the Paris Observatory (Schneider, 2013). The present paper will examine the method used to search for exoplanets, how the various planets are characterized, and some future developments that will expand the ability to discover exoplanets.
- Searching for exoplanets
Scientists are currently focusing on finding planets that occupy an area within the star system known as its “habitable zone” (NASA/Jet Propulsion Laboratory, 2013). The habitable zone is determined by looking at the amount of total radiation a star emits. Bigger stars burn hotter, so their habitable zone is farther out than smaller stars that do not burn as intensely. The reason why scientists are concentrating on looking for planets within a star’s habitable zone is that is where it is most likely to find a planet that could harbor life. Figure 1 illustrates the habitable zone of the Earth’s solar system and estimates habitable zones for stars of other sizes, using the area labeled “habitable zone” within the figure. Habitable zones define areas where liquid water could exist, given the conditions of the star and of the planet.
Figure 1. Illustration of Habitable Zone (“Habitable zone,” n.d.).
Mass of the planet that is being searched for is also an important consideration.
One type of planet that has been found is a type called a super-Earth. A super-Earth is a
planet outside the Earth’s solar system that has a mass that is greater than the Earth’s but is smaller than a gas giant planet such as Uranus or Neptune (Kramer, 2013). In particular, a discovery of three potentially habitable super-Earths was recently made around the star Gliese 667C, which is located 22 light-years from Earth.
Three organizations are searching for exoplanets and have different approaches. In particular, the Kepler Mission is looking for habitable planets (Ames Research Center, 2013), while the Search for Extraterrestrial Intelligence (SETI) is seeking evidence for life outside our solar system by trying to find a signature of its technology (2013), and the proposed TESS Mission, which stands for Transiting Exoplanet Survey Satellite, is a space telescope that will be used to search for exoplanets looking for shadows cast by a planet as it moves across the face of the parent star (After Kepler, TESS, 2013).
- How to characterize planets
Transits of a parent star are the first indication that a planet may be present. Transits that are measured by telescopes that are outside the Earth’s atmosphere are more sensitive than those taken by Earth-bound telescopes and thus can detect more faint changes in light (Seager, 2003). Another way that exoplanets can be detected is through seeing the effect of the exoplanet’s gravitation on the parent star. As larger planets have a greater gravitational effect, this method is good at finding larger planets, such as the super-Earths discussed above.
Beyond just detecting that exoplanets are present, a technique called spectography is used to characterize exoplanets. Specotgraphy is the process of looking at light emitted from a substance in order to determine what chemicals are present in the substance. That is because different chemicals have signature color fingerprints that are emitted. These colors are not visible to the naked eye, but can be seen using a machine called a spectrograph. Once the elements and their quantities are determined, scientists can figure out what kinds of compounds likely contained those elements (Klotz, 2010). Scientists have even measured the Earth’s spectography signature to help evaluate those that are seen from far exoplanets (Palle et al., 2009).
Transit spectrographic analyses are used to look at the changes in light that comes from a planet as it travels in front of and behind its parent sun in order to determine what elements are present on the planet (Seager, 2003). When the planet is behind its sun, the light that is coming from the planet is gone. Thus, by comparing these two sets of light rays, scientists can determine what light is coming from the planet itself. This is useful because scientists expect that there should be chemical fingerprints within the light coming from a planet if there is life on its surface (Klotz, 2010). For example, they look for the light fingerprint for oxygen or for methane as examples of where life may be present. That is because both of these gases would be expected if life was perturbing the chemistry of a planet, yet not necessarily expected if the planet was just existing without the impact of life upon it (Klotz, 2010).
Another set of important characteristics can be determined just by doing careful measurements of the likely size and location of the exoplanet as well as the characteristics of the parent star. These measurements can increase or greatly decrease the chances of there being life, at least life as it is understood on the Earth. For example, if the parent star of an exoplanet is a red dwarf, that mere fact has significant impact on the exoplanet. Because red dwarfs are so dim, it would have to orbit relatively closely in order to have enough heat to support life. Further, it would likely have to have a reasonably thick atmosphere to keep the heat in and protect it from solar bursts (Major, 2013).
A second problem is that being so close to such a massive thing as a red dwarf would mean it would not have tides like our Earth, but be “locked.” However, scientists have stated that this problem would not prohibit life if there was a thick atmosphere or a deep ocean to move the heat around the planet’s surface (Aguilar, 2013). In all of these cases, scientists are emphasizing that the planet need not be a clone of Earth in order to harbor life and add that the time that a planet might have around a red dwarf, up to 10 billion years, could increase the chance that the right conditions were present for life to have evolved (Major, 2013).
Figure 2. Artist’s impression of a rocky planet orbiting a red dwarf (Major, 2013).
- Looking forward: proposed satellites
There are at least two proposed satellite telescope projects that are expected to expand the search for exoplanets. The first of these is the James Webb Space Telescope (JWSP) will be carrying two specific tools or instruments into orbit. The first tool is called MIRI, that stands for mid-infrared instrument. This instrument has a camera and a spectrograph and, as expected from its name, focuses on light that comes from the mid-infrared part of the light spectrum. This part of the spectrum has wavelengths from 5 to 28 microns, where the red shifted light of far-away galaxies, new stars, and extremely faint comets falls (Mid-Infrared Instrument, n.d.). For example, the mid-infrared spectrum of transiting exoplanet HD 209458b was useful in helping to characterize this exoplanet (Swain, Bouwman, Akeson, Lawler, and Beichman, 2008).
A second instrument carried by the JWSP is a NIRSpec instrument, or a spectrograph that will be used to divide the light coming from distance objects into their spectrum. This spectrum is the fingerprint discussed above that indicates the chemicals that are present on that object, such as an exoplanet. This instrument is special in that it can get over 100 spectra simultaneously using an array system of microshutters, thus allowing many more objects to be observed at the same time or specific ones looked at very carefully, by controlling what shutters are open and which are closed (NIRSpec, n.d.). Scientists would use this information to determine whether the object they were looking at had the possible light signature of an exoplanet that is harboring life.
Another project in the works is the proposed ATLAST telescope, or Advanced Technology Large-Aperture Space Telescope (ATLAST, n.d.). Like JWSP, this telescope is also a satellite that would be orbiting the Earth. It would have either an 8 meter primary mirror or a 16 meter segmented mirror, which would allow it to have 5-10X better resolution than JWSP and 2000 times better than the Hubble Space Telescope. Although well suited for looking for signs of life in space, ATLAST also has the ability to look at objects like dark matter, galaxies, and what is found between galaxies. It is seen not only as a way for looking for life on exoplanets, but also as the next step in the process of using telescopes to understand the basic physics surrounding the formation of the universe (ATLAST, n.d.).
- Conclusions
As can be seen by the enormous growth in exoplanets recorded this month alone on the exoplanet.eu webstie, the exoplanet field is in a stage of high growth and discovery. From basic observations made about exoplanets crossing their parent suns, and the spectrum of light emitted from them, scientific guesses are being made about what these planets may be like. By focusing on what is seen as the habitable zone and using more than one approach to analyze the available data, scientists hope to be able to find the first supportable signs of life outside the Earth’s solar system. Just counting the possible planets orbiting red dwarfs – stars too dim to be seen by the naked eye from Earth -- the number of possible planets is incredibly large. As the new projects such as the JWST and ATLAST put even more sensitive instruments for measuring signs of life on exoplanets, it appears to only be a matter of time before man can confidently say that the Earth is not the only planet in the universe harboring life.
- References
After Kepler, TESS (2013). Astronomy & Geophysics. 54 (3) : 3.8. <http://dx.doi.org/ 10.1093/astrogeo/att066>
Aguilar, D. (2013). Earth-like planets are right next door. 6 February. Harvard Smithsonian Center for Astrophysics.
< http://www.cfa.harvard.edu/news/2013/pr201305.html>
Ames Research Center (2013). Kepler: A search for habitable planets.
< http://kepler.nasa.gov/>
ATLAST (n.d.) Space Telescope Science Institute. <http://www.stsci.edu/institute/atlast>
"exoplanet." Dictionary.com's 21st Century Lexicon. Dictionary.com, LLC. 19 Jul. 2013. <Dictionary.com http://dictionary.reference.com/browse/exoplanet>.
Habitable zone. (n.d.). Department of Astronomy & Astrophysics. Penn State University.
< https://www.e-education.psu.edu/astro801/content/l12_p4.html>
Kramer, M. (2013). Found! 3 Super-earth planets that could support alien life. Space.com. < http://www.space.com/21706-habitable-alien-planets-gliese-667c.html>
Klotz, I. (2010). Earth-like planet discovery buoys search. Discovery.com.
< http://news.discovery.com/space/alien-life-exoplanets/earth-like-planet-gliese-581g-life.htm>
Major, J. (2013). Earthlike exoplanets are all around us. Universetoday.com. 6 February. < http://www.universetoday.com/99784/earthlike-exoplanets-are-all-around-us/>
Mid-infrared instrument (n.d.). The James Webb Space Telescope. NASA. <http://www.jwst.nasa.gov/miri.html>
NASA/Jet Propulsion Laboratory (2013, July 17). In the zone: How scientists search for habitable planets. ScienceDaily. <http://www.sciencedaily.com /releases/2013/07/130717175438.htm>
NIRSpec (n.d.). The James Webb Space Telescope. NASA. <http://www.jwst.nasa.gov/nirspec.html>
Palle, E., Osorio, M., Barrena, R., Rodrigues, P., and Martin, E. (2009). Earth’s transmission spectrum from lunar eclipse observations. Nature. 459: 814-816.
Powell, Corey S. (1996). A parade of new planets. Scientific American. May 27, 1996.
< http://www.scientificamerican.com/article.cfm?id=a-parade-of-new-planets>
Schneider, J. (2013). The extrasolar planets encyclopaedia. Paris Observatory.
< http://exoplanet.eu/>
Seager, S. (2003). The search for extrasolar Earth-like planets. Earth and Planetary Science Letters. 208: 113-124.
SETI (2013). Our work. SETI.org. <http://www.seti.org/node/647>
Swain, M., Bouwman, R., Akeson, R.., Lawler, S., and Beichman, C. (2008). The mid-infrared spectrum of the transiting exoplanet HD 209458b. The Astrophysical Journal.674 (1): 482.
Wolszczan, A. (1994). Confirmation of Earth-mass planets orbiting the millisecond pulsar PSR B1257+12. Science, 264 (5158): 538-542.
Wolszczan, A. and Frail, D.A., (1992). A planetary system around the millisecond pulsar PSR1257+12," Nature, 355: 145.
