The Search for Exoplanets
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- Daniel Davies
Throughout the history of humanity, numerous scientists have been intrigued by the search for inhabitable exoplanets. To date, such ventures are still in the developmental stages and the discovery of life beyond earth and the solar system , regardless of how primitive it may be, may soon be regarded as a fundamental breakthrough in astronomy (Jones & Mukai, 2007). The fundamental properties to be considered if a place is habitable are: presence of organic materials, water and energy sources. Habitability of the planet also takes into consideration, nearness of the planet to the sun, the duration of existence of life and not necessarily, that life is present or has ever been. The search for life in exoplanets has been ongoing for over a decade (Su, et al. 2011). The search has been difficult and lengthy and there have been no results showing the existence of life outside earth. The S.E.T.I or "Search for Extra Terrestrial Intelligence" project is probably the most famous experiment within this focus of study.
Why the search for inhabitable exoplanets?:
With the powerful view of telescopes, scientists have been able to view beyond our solar system. Planetary scientists have continuously been searching for exoplanets (planets beyond our sun) that can support life. However, they cannot view these planets in detail as they are so faraway (Su, et al. 2011). The closest known exoplanet orbiting star is called Epsilon Eridani, which is 63 trillion miles away – it is close to 14,000 times away from Neptune, the farthest planets in our solar system (Horner & Jones, 2010).
Due to the great distance, exoplanets cannot be viewed directly since the amount of light they reflect is too faint to be detected from a far off distance (Horner & Jones, 2010). Alternatively, scientists infer the presence of an exoplanet from the changes in wavelength of light illuminating from the star, this may be caused by the gravitational pull from an orbiting planet or the regulation of brightness of the star due to transiting planets.
When the Fermi Paradox was proposed initially, most people thought planets were rare. Nevertheless, since then the astrological tools have discovered the existence of numerous exoplanets. However, with each new discovery of an earth-like planet for instance, Epsilon Eridani, it becomes less likely that there could exist a planet apart from the Earth that can support life.
Using methods such as those stated above, scientists have been able to discover hundreds of exoplanets and the first detection was in 1995. Once they detect one, they begin evaluating it to ascertain whether it can support life. Further, they analyze the light spectrum that radiates from the star in order to reveal the properties of the planet. The hunt is still on for planets, which are about the size of earth and those that are at the right distance from the sun. This is done in a region known as the habitable zone, or sometimes referred to as the "Goldilocks Zone". The habitable zone is the belt bordering a star where temperatures are optimum for liquid water. The Earth lies within the habitable zone of the sun, which is the star. Beyond the habitable zone, life is impossible as it could be too cold and frozen to support life. Therefore, a planet that lies between a star and a habitable zone would be too hot and steamy to support life.
Ideally, most astronomers like to know more about the atmosphere of viable habitable exoplanets. In light of this, they study the molecular makeup of the planet in search for traces of greenhouse gases that have escaped that could be an indicator of an inhabitable planet or they can be able to pick up traces of oxygen, water, carbon dioxide, methane that indicate a planet is habitable (Horner & Jones, 2010).
The notion that planets beyond the Earth can support life is an ancient discovery. Since the late 20th century, there have been two breakthroughs in this field. Through observation and exploration by robotic spacecrafts of other planets and orbits within the solar system has provided scientists with vital information regarding habitability criterion and provided geophysical conditions for comparing life on Earth and on other bodies.
Exoplanets were discovered in the 1990s and has been fast paced thereafter hence providing information for the possibility of extraterrestrial life. The findings have further confirmed that the sun is not peculiar among the stars and planets and hence this has expounded the possibility of life beyond the solar system.
The earth is the only known planet in the universe that harbors life, despite recent evidence to suggest organic materials were found on Mars, this is only an indicator of what might have been. Nevertheless, there are estimates of habitable areas around other orbits. Additionally hundreds of exoplanets have been discovered, which has created new insights into other habitable places in the universe. In November 2013, the Kepler Space Mission data released stated that there were about a billion earth-sized planets that were orbiting within habitable zones within the Milky Way Galaxy.
Methods used in the search for inhabitable exoplanets:
Exoplanets do not emit any light of their own and are obscured by other brighter stars, this makes them difficult to detect. Moreover, normal telescopes cannot be used to view them. This is called Direct Imaging and is not the best way to identify exoplanets , however the technology and ideas behind it are seeing an advancement and could produce promising experiments in the future. Therefore in order to identify exoplanets, a number of techniques are used to detect them and the impacts that they have on the stellar system. Below is a summary of common methods used to detect inhabitable exoplanets (Horner & Jones, 2010).
Pulsars refer to neutron stars that have misaligned magnetic and spin axes. As the pulsars rotate, they emit flashes of radio waves that travel to earth at regular intervals. The radio flashes can be detected and timed. The intervals between the pulses are very regular and are more accurate than the ticks of an atomic clock. A planet that orbits around a pulsar will cause slight variations to the timing of the flashes, which can be used to detect it. The first exoplanet that was orbiting around a pulsar PSR B1257+12 was detected through pulsar timing in 1992.
A planet that orbits a stars exerts a little gravitational pull that makes the star wobble a little about the barycentre which is the system’s centre of mass. If the planet’s edge is aligned to the Earth, this wobble can be observed as a ‘Doppler’ shift in the light emitted by the earth. When a star is travelling away from the viewer, the wavelength's of the emitted light shift with respect to the velocity the star is moving at , hence the wavelengths shift to the "red" end of the spectrum and towards us the "blue" end of the spectrum. The planet’s gravitational pull is minute and hence, very accurate spectroscopic measurements are needed. On measuring the radial velocity it is therefore possible to determine the exoplanet’s orbital period, however the size of the planets cannot be determined. Astrometry:
This technique utilizes extremely definite measurements of the position of stars in order to detest the tiny shifts that are caused by orbiting planets. This method is highly effective for planets that orbit face-on where the motion positioning is at its greatest, nevertheless, the measurements are difficult to obtain.
Einstein’s theory of relativity deduces that massive foreground objects bend the light from background objects by their pull of gravity. The bending of light causes a ‘lensing’ effect that magnifies the background objects that are distant, allowing the curving of light so that distant planetary transits can be observed. Photometry:
Many scientists utilize this transit technique. It shows that when a planet passes in the anterior of its parent’s star edge, there is a decrease in the brightness of the star that can be detected. The periodic decreases in brightness may indicate the presence of an exoplanet, the measurements of light curves and spectral type of the star may give an indication of the size and orbital duration of the planet. When this method is combines with Radial velocity a number of parameters can be inferred accurately including the mass of the planet, which can determine the composition of the planet. The presence of methane and oxygen in an exoplanets atmosphere could be a strong indicator of a bio signature or evidence of past or present life in an exoplanet (Jones & Mukai, 2007). Methods today are used to gain a good estimate to the temperature and size of an exoplanet so as to ascertain whether liquid water can exist on the surface of the exoplanet, this is one of three criterion to ascertain whether a planet can support life.
Figure(1): Shows a plot of results from a dwarf star in another solar system , I have highlighted the transits with the blue segments , taken from my planethunters.org, account
Figure 1 clearly shows the dips in output from the light source, which is the dwarf being observed. The dips are huge indicators to an exoplanet transiting the star. Photometry is by far the most common form of finding new exoplanets.
Impacts of the results:
The results show that powerfully life on Earth highly affects the environment, and the feedback cycle loops. The impact of earthly life on cloud cover is an example. Clouds are composed of either water or carbon dioxide and therefore have a huge impact on the habitability of the planet through the greenhouse effect and that albedo. (Albedo is the surface reflectivity. High albedo surfaces reflect back most of the light and heat that falls on them, while low albedo surfaces absorb energy (Chambers, 2006). High albedo clouds maintain the coolness of a planet by reflecting instead of absorbing stellar energy.
From research on earth, plant life contributes to more cloud cover. Likewise, airborne microorganisms in an exoplanet’s atmosphere can be seeds that lead to more cloud formation. The effect can lead either to cooling of the albedo or to warming because of the greenhouse effect (Fujiwara, et al., 2010). In both cases, clouds or a lack therein may change the habitability, hence altering the planet’s temperature in one way or another.
Life may also alter the carbon cycle. Plants on earth affect the quantity of carbon dioxide in the air, calcareous plankton have had major changes in the Earth’s carbon cycle (Fujiwara, et al., 2010). The systems have effects on the heat trapping properties of the atmosphere and alter the chemistry of the atmosphere and hydrosphere.
Once exoplanets become hydrated, their impact will shift from affecting the delivery of volatiles to arid lands and hence will affect developments in life. Due to developments in our world, such as industrial revolution and technological advancements(Alvares et al, 2008), a number of species of biodiversity have become extinct and a great number of micro-organisms contributing to geochemical cycles have been extinguished (Chapman & Morrison, 2004). Although these are believed to have been cause by a myriad of other factors, a few of these have been caused as a result of the collision between the earth and other small heavenly bodies. At face value, most people assume that more favorable life conditions would be developed if a host planet were discovered, however, mass extinctions will have to occur in order to trigger an influx into these alien bodies.
Ever since it was discovered that collisions with asteroids and comets could lead to significant threats to human life on Earth, the idea that the impact rate on those bodies on earth would be much greater were it not for the protective influence of planet Jupiter (Chapman & Morrison, 2004). Were it not for the mass and placement of Jupiter within the solar system , the earth could have been more punishing throughout its evolution, which would have entirely prevented the evolution of life on earth. The earth is a habitable planet that is an incredibly rare and unique place and that life should be very scarce or even non-existent anywhere else in the universe.
It is now well acknowledged that asteroids and comets have bombarded the earth for decades. When it was first discovered that the impacts of craters on earth were the effects that resulted from the collisions between the earth and other solar systems, the greatest majority of the objects crossing the earth were comets (Chapman & Morrison, 2004). A significant fraction of the earthly bodies were ejected from the solar system due to the distant perturbations by the Jupiter leaving a significantly, greater number of transiting orbits that could threaten the earth’s ecosystems.
At face value, most people assume that more favorable life conditions would be developed if a host planet were discovered, however, mass extinctions will have to occur in order to trigger an influx into these alien bodies. The quest for an inhabitable exoplanet is a threat to humanity as it could trigger harmful effects on earth such as the collision of comets and the earth, which could lead to drastic effects such as volcanic and crater eruptions, which will have drastic effects on humanity. This means that the search for an exoplanet is indeed a search for our own preservation, eventually the earth will be swallowed by the sun , the human race will need somewhere else to live. The big question on people's minds is , will we ever get there ? The answer to this is in the near future , certainly not, but in the far future , perhaps. With the promise of new technology coming , the ability to predict rapid changes in our space transportation may be obsolete, in truth we don't know.
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