Explain whatmakes a planet habitable. In the context of currently known extra solarplanetary systemsdiscuss the likelihood of habitable planets in our Galacticneighbourhood. Describe some examples of recently discovered extra solar planets, whichare suggested tobe habitable and explain why.
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Centuries long environmental degradation of Earth, the scarcity of resourcesand the development of advanced technology has induced the search forextrasolar planets within our Galactic Neighbourhood and beyond. As there is nosubstantial evidence for the existence of life elsewhere, astronomers useconditions that support life on Earth as a habitability criteria in theirsearch for life. Being habitable refers to encompassing the fundamentalproperties to sustain life. Although there are several factors that determinehabitability, this essay will explore five key factors; liquid water, thehabitable zone, atmosphere, mass and magnetic field. The second part of theessay will discuss the likelihood of habitable planets in our Galacticneighbourhood.
The third part of the essay will delve into recently discoveredpotentially habitable extrasolar planets, which are Kepler-186f, TRAPPIST-1 e, f, g and Kepler-452b. WaterWater is almost a prerequisite for life due to being an excellentsolvent and polar molecule, thus enabling it to act as a transporter of vitalchemicals to and from cells of organisms. Its essentiality stems from theinteraction between liquid water and carbon-based molecules. As all life is aconsequent of this interaction, the presence of liquid water allows for the “ assemblyof complex organic molecules”, according to NASA.
Due to its polar and nonpolarcharacteristics, it is able to form strong hydrogen bonds and maintain itsliquid form in fluctuating climates, thus allowing it to sustain stable biomolecularand cellular structures. The absence of water deprives cells and organisms, large and small, the necessary chemicals and nutrients required for theformation and sustenance of life. Water also plays a crucial role in mantle convection and the maintenanceof tectonic plates.
Climaticthermostat: The significance of this lies in the ‘ Geochemical Carbon Cycle’which allows for the stabilisation of a planet’s temperature. Silicate weatheringfacilitates the process of recycling carbon from the atmosphere into the oceanin the form of rocks. Liquid water and tectonic plates then enables thesubduction of carbon into the mantle. As such, carbon dioxide is removed fromthe atmosphere, allowing less heat radiation to be trapped and the planet tomaintain cooler temperatures. Volcanic eruptions restore CO2 back into theatmosphere. Hence, the presence of liquid water is crucial for habitability as itsprovides favourable conditions which are conducive to life.
Atmosphere The presence and composition of an atmosphere plays a pivotal role indetermining the habitability of a planet for several reasons. Firstly, the presenceof greenhouse gases keeps a planet from freezing. Heat radiation from the Sunreflects off a planet’s surface (reflectivity is directly proportional to aplanet’s Albedo) and back into a space. Greenhouse gases in the atmospherecapture this heat, increasing the temperature of the planet and allowing waterto exist in a liquid state. This process facilitates the growth of life.
An atmospheretoo thick like Venus’s will retain a large amount of heat, making the surfacetoo hot for life. An atmosphere not dense enough like Mars’s, will only allowwater to exist in a solid state. (Put in evidence of Mars having an atmosphereand talk about Earth’s atmosphere containing oxygen) An atmosphere insulates a planet from harmful radiations likeultraviolet light emitted from celestial objects like our Sun.
More importantlythough, it provides a protective shield from other short wavelength light thatwould damage the DNA of organisms and thereby harm the presence of life. Anatmosphere also creates favourable conditions for the assembly of complexmolecules and provides consistent conditions for the stable development of organisms. Habitable ZoneIt is vital for a planet to maintain a specific range of surfacetemperature in order to be habitable. Along with the density of a planet’s atmosphere, a planet must also be situated within the ‘ habitable zone’ of its central star. The habitable zone refers to the region surrounding a star which allows aplanet to maintain temperatures which allows water to exist in a liquid phase. Temperatureinfluences the rate at which atoms and molecules move. A planet is able tomaintain water in a liquid form and hence sustain life in the temperature rangeof -15° C and 115°C.
Lower temperatures causes chemicals, atoms and moleculesto react slowly, turning liquid water into ice. Higher temperatures not only lead to evaporation, but causescarbohydrate molecules and proteins to split up and DNA and RNA to function abnormally. Depending on atmospheric pressure, temperatures above 125°C causes water to evaporate.
As such, a planet’s distance from the Sun plays a crucial role in determiningits surface temperature and thereby its ability to sustain life. Positioning of a planet in the HZ is also critical as the host star is asource of energy organisms. Organisms and biodiversity use light energy to photosynthesiseand produce oxygen. Too less energy won’t allow chemicals like iron and sulfurto provide energy to cells, whilst excessive sunlight will expose organismsharmful radiations. Mass and Magnetic FieldThe mass of a planet and the presence of a magnetic field are crucialdeterminants of habitability. The mass of a planet is directly proportional toits gravity. Thus, a low mass planet won’t be capable of retaining itsatmosphere, allowing gas particles to easily reach escape velocity and be lostto space. This process is only accelerated in the absence of a magnetic field.
Planets are frequently exposed to solar winds; charged particles (primarilyprotons) emitted by the Sun which can knock off gases from the upperatmosphere. The presence of a strong gravitational force in tandem with a magneticfield act as shield, preventing solar winds from penetrating the atmosphere. Thisallows a planet to retain its atmosphere and therefore be able to trap heat andmaintain water in a liquid state. (Wikipedia has condensed info on this) Other critical factors that determine habitability are: The spectral class of a planet’s host starprovides information about its photospheric temperature Low stellar variation allows organisms and biodiversityto adapt to a consistent environment. Abnormal and intense fluctuations in astar’s luminosity are associated with bursts of gamma rays and x rays, both ofwhich are harmful to life Volatile orbital and rotational motions causeinconsistent climatic conditions, making it difficult for life to adapt. Likelihood ofHabitabilityCenturies long environmental degradation on Earth coupled with the discoveryof thousands of other planetary systems like our Solar System has prompted thesearch for extrasolar planets. Given the colossal size of our galactic neighbourhoodand the infinite magnitude of our universe, astronomers deem the likelihood ofhabitable planets extremely high. At present, 3584 extrasolar planets have been confirmed by NASA, ofwhich 15 fall into the habitable category.
In fact, reports based on the Keplermission, which utilises the Transit and Doppler methods for detection, suggeststhat there could be more than 40 billion habitable, Earth sized extrasolarplanets in our galactic neighbourhood, 11 billion of these orbiting in thehabitable zones of their host stars. Despite the discovery of planets thatcould potentially bear life, whether life does or can exist is a differentquestion, and one that may take decades to answer due to a lack of advancedtechnology. Nevertheless, the search for more extrasolar planets and potential extraterrestriallife has intensified, with a particular focus on the planet’s position aroundthe habitable zone of its host star.
Astronomers acknowledge that beingpositioned within the habitable zone is only a start and that more advancedtesting is required to confirm that a planet ticks all the boxes for thehabitability criteria described above. Water has long been known as the prerequisite for life, but thediscovery of extremophiles has provoked the question of whether life can existwithout Earth like conditions. As such, astronomers are keeping an open mindabout the factors that determine the suitability of a planet accommodating life. Perhaps life can be sustained on extrasolar planets without meeting thehabitability criteria? About a dozen other planets have been found to orbit in their star’shabitable zone, like Kepler-22b and Kepler-62f, however they are all largerthan Earth, and many are thought to have a thick atmosphere of gas like Jupiterand Neptune rather than a solid surface. The Kepler spacecraft measures the size of a planet bythe fraction of starlight that it blocks as it transits across the face of thestar. Kepler-186f Earth Similarity index ESIAt 492 light years away in the Cygnus constellation, the discovery of extrasolarplanet Kepler-186f has seemed to create a strong buzz across the global space community. Although its mass, atmospheric composition and density are unknown, Kepler-186fis described as the first potentially habitable Earth-size planet.
It orbits a coolM-dwarf star in the habitable zone receiving mostly infrared light and has anorbital period of 130 days. Despite receiving approximately 30% of the sunlightEarth receives and locating on the outer edge of the habitable zone, itsslightly larger size than Earth could mean that Kepler-186f has a denseratmosphere, allowing it to retain more heat and sustain liquid water. Kepler-186f’s close proximity to its star, at 0. 36 AU, mean that itcould be tidally locked in the same way the Moon is tidally locked to Earth. This would result in one side of the planet beingexposed to majority of the heat whilst the other side remains frozen. However, its potentially thicker atmosphere would cause high speed winds to transfer theheat from one side to the other, ensuring consistent climatic conditions. (extremedifference in temp on both sides is likely)According to astronomers at the SETI institute, “ The first signs ofother life in the galaxy may come from planets orbiting an M dwarf”.
This islargely due to their abundance in our galaxy coupled with more frequent anddeeper transits, allowing for easier detection. Additionally, their slowerevolution period keeps their habitable zones constant for billions of years. Therefore, Kepler-186f’s strong resemblance to Earth like properties andits position in the habitable zone of an M-dwarf star makes it a primecandidate for habitability. TRAPPIST-1 February 2017 saw the remarkablediscovery of a planetary system of seven Earth-sized extrasolar planetsorbiting a cool M-dwarf star, TRAPPIST-1, 40 light years away. Theseven planets, TRAPPIST b, c, d, e, f, g and h, of which e, g and h reside inthe habitable zone, are all tidally locked to their star and are believed topossess Earth-like properties, including similar masses and rocky compositions.
According to SETI astronomer Jeff Coughlin, the seven rocky planets being closeto one another allows for the “ accidental transport of life by comets…”, meaningthat if one planet is habitable, it is likely that life would spread easily tothe other planets. However, what is most intriguing about this discovery is that the 3 habitable planetsmay potentially contain substantial amounts of water. A combination of lowenergy UV rays and high energy X-rays are known to split up water molecules andheat a planet’s upper atmosphere, commonly referred to as photodissociation.
Since hydrogen is very light, it is able to escape the atmosphere and bedetected as an indicator of water vapour. This is precisely what was observedby the Hubble Space Telescope, leading astronomers to hypothesise that whilstthe inner planets of TRAPPIST-1 had likely lost all liquid water, planets e-g mayhave retained some on their surface. Although there are many habitability characteristicsthat TRAPPIST-1 e, f, and g possess, their proximity to their sun may haveexposed them to magnetic solar flares, stripping off its atmosphere and boilingits oceans, precisely what Mars experienced. Kepler-452 b NASA believes to have found the most Earth-like planet sofar, Kepler-452b, earning the title of ‘ Earth 2. 0’ and being classed in the ‘ SuperEarths’ category. Kepler-452b shares many similarities with Earth, including orbitingA G2 type yellow dwarf star like our Sun and having an orbital period of 385days.
However, Kepler 452-b being 60% larger than Earth and its parent starbeing 6 billion years old poses a few problems. Given that a rocky surface isessential for life, at that size, it is likely that the planet is a mini gasgiant like Saturn or Neptune. Kepler-452 being one billion years older than ourSun, it is possible that is evolving into a red giant, and could potentially instigatea runaway greenhouse effect, similar to what Venus experienced. On the contrary, its old age presents a significant opportunity for Kepler-452b to havedeveloped life. Nevertheless, being 1400 light years away from Earth, extractingaccurate information about Kepler-452b’s features is difficult and requiresmore advanced technology. ConclusionGlobalwarming and climate change are accelerating the search for life beyond ourSolar System. Recently discovered extrasolar planets like Kepler-186f, TRAPPIST-1 e, f, g and Kepler-452b haveprovided hope for astronomers about the increasing likelihood of life outsideEarth. Enhanced detection techniques and advanced astronomical equipment areallowing for more viable solutions to research and explore extrasolar planetsorbiting their parent stars.
However, the discovery of organisms that thrive in physicallyand geochemically extreme conditions; extremophiles, has prompted astronomersto not be confined to searching for conditions that support life on Earth. Nonetheless, astronomers remain hopeful about the possibility of human occupation on aplanet outside Earth, particularly as Stephen Hawkins once declared, “ I don’tthink the human race will survive the next thousand years, unless we spreadinto space.” Referenceshttps://www. scienceabc. com/nature/what-makes-a-planet-habitable. htmlhttp://www. bbc. com/news/science-environment-33929851https://www. ncbi. nlm. nih. gov/pubmed/11539468http://www. skyandtelescope. com/sky-and-telescope-magazine/what-makes-a-planet-habitable/http://scienceline. ucsb. edu/getkey. php? key= 4535https://www. space. com/30172-six-most-earth-like-alien-planets. htmlhttp://www. astronomy. com/news/2017/04/exoplanet-guidehttps://www. wonderslist. com/10-most-potentially-habitable-planets/https://www. nasa. gov/press-release/nasa-kepler-mission-discovers-bigger-older-cousin-to-earthhttps://www. washingtonpost. com/news/speaking-of-science/wp/2015/07/24/nasa-estimates-1-billion-earths-in-our-galaxy-alone/? utm_term=. fd16f3104526http://www. news. com. au/technology/science/space/fresh-hope-that-trappist1-planets-contain-water/news-story/9bae3f2edf7cd43b222e7a46544acf8chttp://earthsky. org/space/first-hints-of-water-on-trappist-1-planetshttps://www. space. com/39211-trappist-1-exoplanets-solar-system-discovery-2017. htmlhttp://www. dailygalaxy. com/my_weblog/2017/08/the-habitable-zone-planets-of-trappist-1-a-solar-system-twice-as-old-as-ours-life-there-could-have-s. htmlhttps://io9. gizmodo. com/what-are-extrasolar-planets-1706656300? IR= Thttps://www. centauri-dreams. org/? p= 31658https://en. wikipedia. org/wiki/Exoplanet#Confirmed_discoverieshttps://www. space. com/25541-alien-planet-kepler-186f-facts. htmlhttps://www. seti. org/seti-institute/kepler-186f-first-earth-sized-planet-orbiting-in-habitable-zone-of-another-starhttps://www. scienceabc. com/nature/what-makes-a-planet-habitable. htmlhttp://www. bbc. com/news/science-environment-33929851https://www. ncbi. nlm. nih. gov/pubmed/11539468http://www. skyandtelescope. com/sky-and-telescope-magazine/what-makes-a-planet-habitable/http://scienceline. ucsb. edu/getkey. php? key= 4535https://en. wikipedia. org/wiki/Planetary_habitability#Suitable_star_systems