Question 13. Habitability of planets
One of the most commonly thrown around and debated questions within the scientific
world right now is whether there are other planets which can support human life. Scientists
are constantly researching evolution and have manifested many theories on how the world
began, but now are in a search for what’s next when the inevitable ‘end’ of Earth roams
above us. Due to reasons such as global warming and the mass destruction of land daily, the
desire for their search into other habitable planets has turned into a need. More intense
studies and use of vehicular research models such as rovers has seen an increase on our
understanding on the chemistry and nature of said planets.
In order to assess the habitability of planets, we need to first understand what makes a
planet habitable. Based on their research, astrobiologists claim that there are roughly 500
habitable planets within the Milky Way. The criteria for labelling a planet ‘habitable’ is quite
simple to understand when generalised. [1]
Planets need to be in the “habitable zone”. This zone ensures that the planet is not of a
close proximity to the Sun, or too far away from it either. This ensures that they are distant
enough from heavy heat and radiation, while being not far enough to not be freezing. The
“habitable zone” is an essential factor on a planets habitability as it will influence the
temperature on the planet, which is a vital determinant in whether life can be sustained. It
is understood that “for life to exist and be sustained, the suitable temperate range must be
between -15 degrees Celsius to 115 degrees Celsius”. [2] In order to understand this
statement, it can be explained through simple high school biology. Temperatures lower than
-15 degrees Celsius are far too low for basic chemical reactions to occur within living
organisms as there isn’t enough heat to provide energy for the reaction. This disables the
formation of crucial molecules and compounds, which are essential to continue life in a
sustainable manner. Temperatures above 115 degrees Celsius also has adverse effects on
living organisms. High temperatures can lead to protein denaturation. Protein denaturation
is the disruption and, in some events, the destruction of the structure of the protein. This
occurs due to the excess of heat, which leads to greater kinetic energy within the protein
structure causing the molecules to vibrate intensely and disrupt the bonds holding them
together. [3] High temperatures also have adverse effects on carbohydrates and genetic
material such as DNA and RNA, causing them to break apart. This ultimately will lead to the
culmination of life on the planet as organisms cannot survive and reproduce. [1]
Another factor which determines whether a planet can support life is the composition.
Gaseous planets such as Uranus, Jupiter and Saturn are known for not being able to support
life, or at least being able to support organisms similar to the ones on Earth due to its
abundance of Helium and Hydrogen. These gaseous planets have a lack of Carbon, Nitrogen
and Oxygen which make up the “holy trio” of gases needed for life. However, the suggestion
that gaseous planets cannot be habitable is still questioned and argued against as there isn’t
enough research to effectively prove this. [4] Carl Sagan, a renowned American
astrophysicist describes the possibility of life on Jupiter as “vast living balloons which can
stay buoyant by pumping heavy gasses from their interiors or by keeping their insides
warm”.
Another part of the planet within its composition is the molten core. If planets are big
enough to have a molten core, they are usually more susceptible to bearing life, as the core
provides a huge source of geothermal energy.[1] The molten core within Earth allows the
cycling of raw materials to occur which enables life on the planet to flourish, as well as
providing a magnetic field around the atmosphere, protecting us from harmful radiation
from the Sun. This is essential as an abundance of UV radiation can lead to increased
temperatures on Earth, leading to the aforementioned problems above. Increasing UV
radiation is also harmful for humans and other organisms, leading to issues such as skin
cancer.[5] Due to rising carbon emissions from corporations and countries around the
world, there have greater incidences of “ozone holes” in our ozone layer[6]. This is the layer
essential to ensuring limited UV rays pass through, while also blocking essential gases such
as Carbon Dioxide and Oxygen from leaving the atmosphere. This characteristic is important
because if excess radiation comes into contact with organisms DNA, it will lead to
irreversible damage to its structure and functions, impeding the planet’s ability to foster life.
A planets size can also have an impact on its ability to foster life. [1] Gravity is vital to
providing the scaffolding and structure for an atmosphere and is something that usually is
characterised by larger planets. Smaller planets such as Mars aren’t equipped with a large
enough gravitational pull to hold an atmosphere, which leaves its gases exposed to
radiation, while also dissipating heat out into space.
Another key factor in determining a planets habitability is its level of nutrients and
compounds essential to the growth and sustenance of life. Without a substantial availability
of nutrients, organisms would not be able to repair, grow and maintain their bodily
functions. Nutritional deficiencies will ultimately lead to the inability of growth and eventual
death of organisms. These nutrients initially were formed from water cycles, however
volcanoes are an important source of nutrients. [7] Eruptions leads to the dispersal of
copious amounts of ash, which contains a large number of nutrients needed for organisms.
[9] Another important source of nutrients for survival is through the carbon cycle. As living
organisms die and decompose into matter, these decomposers will break down into Carbon
Dioxide(CO2) as well as other nutrients into the soil for use. The release of CO2 is essential
for photosynthesis to occur and enable the growth of plants. Without these vital nutrients,
organisms such as plants and animals would not be able to survive and develop into the
complex organisms they are right now. [8] Water is another important factor for a planets
habitability. Humans are made up of over 60% of water, which can easily validate how
essential it is for the sustainability of life. Water can exist in solid, gas or liquid form but in
order for it to promote life it needs to be in liquid form. This is because it is a universal
solvent, and is essential for the transportation of crucial molecules around an organisms
body. Its used for reproduction, evolution as well as the removal of excretory produces from
the organism. [9] Many other planets are known to have water within their structure,
however since it is in largely gaseous or sometime solid form, it cannot promote life. Mars
was originally to have only water in solid form on the planet, however recent discoveries
show that beneath the Martian surface, are very small water deposits. [10] There are no
large standing bodies of liquid water on the planet’s surface, much due to the atmospheric
pressure being over 600 pascals, which is slightly below water vapor pressure at its melting
point. However, astrobiologists suggest that about 3.8 billion years ago, Mars would have
had a denser atmosphere and higher surface temperatures, leading to the larger bodies of
water previously.[12] It is also claimed that around this time, there may have been a large
ocean spanning over 1/3 of the planet, where water has flowed across the surface for short
periods at various intervals throughout the history of the Martian planet. Other planets
where water may exist in liquid form are known as “Goldilocks planets” which are within the
Goldilocks zones. These are planets which are just the right distance from the galactic centre
and other obstacles to support liquid water and life. [13]
Does the Milky Way have more planets which can support and/or do already
have organisms living on it?
To start, the Milky Way is a galaxy that contains the Solar System. It has a radius of roughly
52,850 light years and over 300 billion stars, and within the centre of the galaxy is where the
entire galaxies rotation occurs. The galactic centre thus has extremely strong gravitational
forces and radiation, leading to the ionisation of gas clouds which surround the “black hole”.
This leads to the inability of life existence around any planets near this area due to the
extremely strong gravitational pull and exposure to crazy radiation. Planets near supernovae
and “globular clusters” cannot sustain life due to the three detrimental forms of radiation
(cosmic, gamma, x) released, which deplete the atmosphere and any chance of life.
However, outside of these dangerous regions, far way from the galactic centre are what’s
known as “Galactic Habitable Zones”. [10] This is a zone within the galaxy where it is most
likely to develop life, which considers factors such as metallicity and supernovae to calculate
which areas are more likely to develop terrestrial planets. Since our solar system is located
around 25000 light years away from the centre, near the Orion-Cygnus Arm, there is proof
suggesting that the solar system and nearby regions could foster life, as we are well
“protected” from these supernovae and catastrophes. [11] The Kepler Mission was rolled
out to survey the Milky Way’s habitable zones and find Earth-sized Goldilocks planets.
Launched on March 7 2009, around 4.5 years later, the Kepler recorded as many as 40
billion Earth-sized planets within these habitable zones, the nearest planet being 12 light
years away. [14]
Any habitable planets to our knowledge?
Proxima Centauri B is an exoplanet orbiting the habitable zone surrounding the red dwarf
star Proxima Centauri, which is the closest star to the Sun. [15] Due to its distance from the
star, it allows for the existence of liquid water. The planet is roughly 4.2 light years away
from Earth, while taking around 11.2 earth days to orbit the Proxima Centauri. [16] It was
discovered by the European Southern Observatory in August 2016, where scientists
estimate it consists of a rocky exterior. However, the true habitability of this exoplanet is
unclear and more learning towards being inhabitable, much due to the greater exposure of
radiation from its star, as well as its stellar wind pressures, rising 2000x stronger than
Earth’s solar wind.
Kepler 438b and 452b are two Earth-like exoplanets discovered by the Kepler Mission. [18]
Kepler 438b is slightly larger than Earth, but due to its proximity to its orbiting orange dwarf
star, it receives over 40% greater heat than Earth. However, it is still just within the
Goldilocks zone and has free flowing water. Accompanied with its rocky structure, free
flowing water is essential for habitability. However, being a more recent discovery and with
lack of understanding on the planet, the true extent of its habitability cannot be assessed
yet. [14] Kepler 452b, also known as “Earth 2.0 or Earth’s Cousin” is an exoplanet orbiting
the sun-like star Kepler 452, about 1402 light years away from Earth. Even it’s orbit around
its own star is around 385 days, similar to Earth’s 365 days. By being located within the
Goldilocks zones, liquid water and temperatures of between -8 to 17 degrees Celsius can
allow the sustainable development of life. Based on its small radius, Kepler 452b is likely to
be rocky – considering this and its higher mass than Earth, the gravitational pull on the
planet would make habitable possible for “another 500-900 million years, before the
habitable zone is pushed out of Kepler 452b’s orbit”.[17] The light spectrum that Kepler
452b receives is similar to that of Earth’s, allowing simple processes such as photosynthesis
to occur and creating functional cells and organisms. If we combine this, the light,
temperature, waterbody and rocky structure, it is not a stretch to say that Kepler 452b is
“Earths Cousin” or that it very well can foster life. [17]
References
[1]. What Makes A World Habitable? [Internet]. Lunar and Planetary Institute. 2017 [cited
20 January 2017]. Available from:
http://www.lpi.usra.edu/education/explore/our_place/hab_ref_table.pdf
[2]. Why water is important for life [Internet]. Ivyroses.com. 2017 [cited 20 January 2017].
Available from: http://www.ivyroses.com/Biology/Why-is-water-important-to-life.php
[3]. Strobel N. Life Beyond the Earth [Internet]. Astronomynotes.com. 2017 [cited 20
January 2017]. Available from: http://www.astronomynotes.com/lifezone/s4.htm
[4]. Principles in Biology - energy of life [Internet]. Whatislife.com. 2017 [cited 20 January
2017]. Available from: http://www.whatislife.com/principles/principles05-energy.htm
[5]. How Ultraviolet Light Reacts in Cells | SciBytes | Learn Science at Scitable [Internet].
Nature.com. 2017 [cited 20 January 2017]. Available from:
http://www.nature.com/scitable/blog/scibytes/how_ultraviolet_light_reacts_in
[6] Seager, Sara (2013). "Exoplanet Habitability". Science. 340 (577): 577–
581. Bibcode:2013Sci...340..577S. doi:10.1126/science.1232226. PMID 23641111.
[7] "Goal 1: Understand the nature and distribution of habitable environments in the
Universe". Astrobiology: Roadmap. NASA. Archived from the original on 17 January 2011.
Retrieved 11 August 2007.
[8] Staff (1 September 2018). "Water worlds could support life, study says - Analysis by
UChicago, Penn State scientists challenges idea that life requires 'Earth clone'". EurekAlert.
Retrieved 1 September 2018.
[9] Williams M. What Are The Benefits Of Volcanoes? - Universe Today [Internet]. Universe
Today. 2017 [cited 20 January 2017]. Available from:
http://www.universetoday.com/32576/benefits-of-volcanoes/
[10]. Su Mvan Eldik C. Gamma rays from the Galactic Centre region. Comptes Rendus
Physique. 2015;16(6-7):686-703.
[11]. Galactic Habitable Zones - Astrobiology Magazine [Internet]. Astrobiology Magazine.
2017 [cited 20 January 2017]. Available from: http://www.astrobio.net/newsexclusive/galactic-habitable-zones/
[12] Choi, Charles Q. (21 August 2015). "Giant Galaxies May Be Better Cradles for Habitable
Planets". Space.com. Retrieved 24 August 2015.
[10} Jump up to:a b c d e Prantzos, Nikos (2006). "On the "Galactic Habitable Zone"". Space
Science Reviews. 135 (1–4): 313–322. arXiv:astroph/0612316. Bibcode:2008SSRv..135..313P. doi:10.1007/s11214-007-9236-9.
[13]. The Habitable Exoplanets Catalog - Planetary Habitability Laboratory @ UPR Arecibo
[Internet]. Phl.upr.edu. 2017 [cited 21 January 2017]. Available from:
http://phl.upr.edu/projects/habitable-exoplanets-catalog
[14]. Jenkins J, Twicken J, Batalha N, Caldwell D, Cochran W, Endl M et al. DISCOVERY AND
VALIDATION OF Kepler-452b: A 1.6 R ⨁ SUPER EARTH EXOPLANET IN THE HABITABLE
ZONE OF A G2 STAR. The Astronomical Journal. 2015;150(2):56.
[15]. The Five Most Earth-Like Exoplanets (So Far) [Internet]. IFLScience. 2017 [cited 20
January 2017]. Available from: http://www.iflscience.com/space/five-most-earthexoplanets-so-far-0/
[16]. ESO Discovers Earth-Size Planet in Habitable Zone of Nearest Star [Internet]. NASA.
2017 [cited 22 January 2017]. Available from: https://www.nasa.gov/feature/jpl/esodiscovers-earth-size-planet-in-habitable-zone-of-nearest-star
[17] "NASA Exoplanet Archive – Confirmed Planet Overview – Kepler-452b". NASA Exoplanet
Archive. 2015. Retrieved 23 July 2015.
[18] "NASA's Kepler Mission Discovers Bigger, Older Cousin to Earth". National Aeronautics
and Space Administration. 23 July 2015. Retrieved 10 June 2016.