makes a planet habitable. In the context of currently known extra solar
discuss the likelihood of habitable planets in our Galactic
Describe some examples of recently discovered extra solar planets, which
are suggested to
be habitable and explain why.
Centuries long environmental degradation of Earth, the scarcity of resources
and the development of advanced technology has induced the search for
extrasolar planets within our Galactic Neighbourhood and beyond. As there is no
substantial evidence for the existence of life elsewhere, astronomers use
conditions that support life on Earth as a habitability criteria in their
search for life. Being habitable refers to encompassing the fundamental
properties to sustain life. Although there are several factors that determine
habitability, this essay will explore five key factors; liquid water, the
habitable zone, atmosphere, mass and magnetic field. The second part of the
essay will discuss the likelihood of habitable planets in our Galactic
neighbourhood. The third part of the essay will delve into recently discovered
potentially habitable extrasolar planets, which are Kepler-186f, TRAPPIST-1 e,
f, g and Kepler-452b.
Water is almost a prerequisite for life due to being an excellent
solvent and polar molecule, thus enabling it to act as a transporter of vital
chemicals to and from cells of organisms. Its essentiality stems from the
interaction between liquid water and carbon-based molecules. As all life is a
consequent of this interaction, the presence of liquid water allows for the “assembly
of complex organic molecules”, according to NASA. Due to its polar and nonpolar
characteristics, it is able to form strong hydrogen bonds and maintain its
liquid form in fluctuating climates, thus allowing it to sustain stable biomolecular
and cellular structures. The absence of water deprives cells and organisms,
large and small, the necessary chemicals and nutrients required for the
formation and sustenance of life.
Water also plays a crucial role in mantle convection and the maintenance
of tectonic plates. Climatic
thermostat: The significance of this lies in the ‘Geochemical Carbon Cycle’
which allows for the stabilisation of a planet’s temperature. Silicate weathering
facilitates the process of recycling carbon from the atmosphere into the ocean
in the form of rocks. Liquid water and tectonic plates then enables the
subduction of carbon into the mantle. As such, carbon dioxide is removed from
the atmosphere, allowing less heat radiation to be trapped and the planet to
maintain cooler temperatures. Volcanic eruptions restore CO2 back into the
Hence, the presence of liquid water is crucial for habitability as its
provides favourable conditions which are conducive to life.
The presence and composition of an atmosphere plays a pivotal role in
determining the habitability of a planet for several reasons. Firstly, the presence
of greenhouse gases keeps a planet from freezing. Heat radiation from the Sun
reflects off a planet’s surface (reflectivity is directly proportional to a
planet’s Albedo) and back into a space. Greenhouse gases in the atmosphere
capture this heat, increasing the temperature of the planet and allowing water
to exist in a liquid state. This process facilitates the growth of life. An atmosphere
too thick like Venus’s will retain a large amount of heat, making the surface
too hot for life. An atmosphere not dense enough like Mars’s, will only allow
water to exist in a solid state. (Put in evidence of Mars having an atmosphere
and talk about Earth’s atmosphere containing oxygen)
An atmosphere insulates a planet from harmful radiations like
ultraviolet light emitted from celestial objects like our Sun. More importantly
though, it provides a protective shield from other short wavelength light that
would damage the DNA of organisms and thereby harm the presence of life. An
atmosphere also creates favourable conditions for the assembly of complex
molecules and provides consistent conditions for the stable development of organisms.
It is vital for a planet to maintain a specific range of surface
temperature 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 a
planet to maintain temperatures which allows water to exist in a liquid phase. Temperature
influences the rate at which atoms and molecules move. A planet is able to
maintain water in a liquid form and hence sustain life in the temperature range
of -15° C and 115°C. Lower temperatures causes chemicals, atoms and molecules
to react slowly, turning liquid water into ice. Higher temperatures not only lead to evaporation, but causes
carbohydrate 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 determining
its surface temperature and thereby its ability to sustain life.
Positioning of a planet in the HZ is also critical as the host star is a
source of energy organisms. Organisms and biodiversity use light energy to photosynthesise
and produce oxygen. Too less energy won’t allow chemicals like iron and sulfur
to provide energy to cells, whilst excessive sunlight will expose organisms
Mass and Magnetic Field
The mass of a planet and the presence of a magnetic field are crucial
determinants of habitability. The mass of a planet is directly proportional to
its gravity. Thus, a low mass planet won’t be capable of retaining its
atmosphere, allowing gas particles to easily reach escape velocity and be lost
to space. This process is only accelerated in the absence of a magnetic field.
Planets are frequently exposed to solar winds; charged particles (primarily
protons) emitted by the Sun which can knock off gases from the upper
atmosphere. The presence of a strong gravitational force in tandem with a magnetic
field act as shield, preventing solar winds from penetrating the atmosphere. This
allows a planet to retain its atmosphere and therefore be able to trap heat and
maintain 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 star
provides information about its photospheric temperature
Low stellar variation allows organisms and biodiversity
to adapt to a consistent environment. Abnormal and intense fluctuations in a
star’s luminosity are associated with bursts of gamma rays and x rays, both of
which are harmful to life
Volatile orbital and rotational motions cause
inconsistent climatic conditions, making it difficult for life to adapt.
Centuries long environmental degradation on Earth coupled with the discovery
of thousands of other planetary systems like our Solar System has prompted the
search for extrasolar planets. Given the colossal size of our galactic neighbourhood
and the infinite magnitude of our universe, astronomers deem the likelihood of
habitable planets extremely high.
At present, 3584 extrasolar planets have been confirmed by NASA, of
which 15 fall into the habitable category. In fact, reports based on the Kepler
mission, which utilises the Transit and Doppler methods for detection, suggests
that there could be more than 40 billion habitable, Earth sized extrasolar
planets in our galactic neighbourhood, 11 billion of these orbiting in the
habitable zones of their host stars. Despite the discovery of planets that
could potentially bear life, whether life does or can exist is a different
question, and one that may take decades to answer due to a lack of advanced
Nevertheless, the search for more extrasolar planets and potential extraterrestrial
life has intensified, with a particular focus on the planet’s position around
the habitable zone of its host star. Astronomers acknowledge that being
positioned within the habitable zone is only a start and that more advanced
testing is required to confirm that a planet ticks all the boxes for the
habitability criteria described above.
Water has long been known as the prerequisite for life, but the
discovery of extremophiles has provoked the question of whether life can exist
without Earth like conditions. As such, astronomers are keeping an open mind
about the factors that determine the suitability of a planet accommodating life.
Perhaps life can be sustained on extrasolar planets without meeting the
About a dozen other planets have been found to orbit in their star’s
habitable zone, like Kepler-22b and Kepler-62f, however they are all larger
than Earth, and many are thought to have a thick atmosphere of gas like Jupiter
and Neptune rather than a solid surface.
The Kepler spacecraft measures the size of a planet by
the fraction of starlight that it blocks as it transits across the face of the
Kepler-186f Earth Similarity index ESI
At 492 light years away in the Cygnus constellation, the discovery of extrasolar
planet Kepler-186f has seemed to create a strong buzz across the global space community.
Although its mass, atmospheric composition and density are unknown, Kepler-186f
is described as the first potentially habitable Earth-size planet. It orbits a cool
M-dwarf star in the habitable zone receiving mostly infrared light and has an
orbital period of 130 days. Despite receiving approximately 30% of the sunlight
Earth receives and locating on the outer edge of the habitable zone, its
slightly larger size than Earth could mean that Kepler-186f has a denser
atmosphere, allowing it to retain more heat and sustain liquid water.
Kepler-186f’s close proximity to its star, at 0.36 AU, mean that it
could be tidally locked in the same way the Moon is tidally locked to Earth.
This would result in one side of the planet being
exposed to majority of the heat whilst the other side remains frozen. However,
its potentially thicker atmosphere would cause high speed winds to transfer the
heat from one side to the other, ensuring consistent climatic conditions. (extreme
difference in temp on both sides is likely)
According to astronomers at the SETI institute, “The first signs of
other life in the galaxy may come from planets orbiting an M dwarf”. This is
largely due to their abundance in our galaxy coupled with more frequent and
deeper transits, allowing for easier detection. Additionally, their slower
evolution period keeps their habitable zones constant for billions of years.
Therefore, Kepler-186f’s strong resemblance to Earth like properties and
its position in the habitable zone of an M-dwarf star makes it a prime
candidate for habitability.
February 2017 saw the remarkable
discovery of a planetary system of seven Earth-sized extrasolar planets
orbiting a cool M-dwarf star, TRAPPIST-1, 40 light years away. The
seven planets, TRAPPIST b, c, d, e, f, g and h, of which e, g and h reside in
the habitable zone, are all tidally locked to their star and are believed to
possess Earth-like properties, including similar masses and rocky compositions.
According to SETI astronomer Jeff Coughlin, the seven rocky planets being close
to one another allows for the “accidental transport of life by comets…”, meaning
that if one planet is habitable, it is likely that life would spread easily to
the other planets.
what is most intriguing about this discovery is that the 3 habitable planets
may potentially contain substantial amounts of water. A combination of low
energy UV rays and high energy X-rays are known to split up water molecules and
heat a planet’s upper atmosphere, commonly referred to as photodissociation.
Since hydrogen is very light, it is able to escape the atmosphere and be
detected as an indicator of water vapour. This is precisely what was observed
by the Hubble Space Telescope, leading astronomers to hypothesise that whilst
the inner planets of TRAPPIST-1 had likely lost all liquid water, planets e-g may
have retained some on their surface.
Although there are many habitability characteristics
that TRAPPIST-1 e, f, and g possess, their proximity to their sun may have
exposed them to magnetic solar flares, stripping off its atmosphere and boiling
its oceans, precisely what Mars experienced.
NASA believes to have found the most Earth-like planet so
far, Kepler-452b, earning the title of ‘Earth 2.0’ and being classed in the ‘Super
Earths’ category. Kepler-452b shares many similarities with Earth, including orbiting
A G2 type yellow dwarf star like our Sun and having an orbital period of 385
days. However, Kepler 452-b being 60% larger than Earth and its parent star
being 6 billion years old poses a few problems. Given that a rocky surface is
essential for life, at that size, it is likely that the planet is a mini gas
giant like Saturn or Neptune. Kepler-452 being one billion years older than our
Sun, it is possible that is evolving into a red giant, and could potentially instigate
a runaway greenhouse effect, similar to what Venus experienced. On the contrary,
its old age presents a significant opportunity for Kepler-452b to have
Nevertheless, being 1400 light years away from Earth, extracting
accurate information about Kepler-452b’s features is difficult and requires
more advanced technology.
warming and climate change are accelerating the search for life beyond our
Solar System. Recently discovered extrasolar planets like Kepler-186f, TRAPPIST-1 e, f, g and Kepler-452b have
provided hope for astronomers about the increasing likelihood of life outside
Earth. Enhanced detection techniques and advanced astronomical equipment are
allowing for more viable solutions to research and explore extrasolar planets
orbiting their parent stars.
However, the discovery of organisms that thrive in physically
and geochemically extreme conditions; extremophiles, has prompted astronomers
to not be confined to searching for conditions that support life on Earth. Nonetheless,
astronomers remain hopeful about the possibility of human occupation on a
planet outside Earth, particularly as Stephen Hawkins once declared, “I don’t
think the human race will survive the next thousand years, unless we spread