With space travel becoming a more regular event with us travelling further and
for longer, it’s not insane to wonder whether we would be able to survive on
another planet. There is a lot of factors to consider when determining whether
a planet is habitable so for now, we will just be focusing on the composition.
For a planet to be habitable, it must be terrestrial. Terrestrial planets all have
the same basic structure, a central metallic core with a surrounding silicate
mantle. To sustain life, a planet requires a rapidly rotating magnetic field to
protect it from flares from nearby stars. This magnetic field is believed to be
generated by electric currents in the conductive metallic core of the planet.
The electric currents are created by convection currents due to heat escaping
from the core. These convection currents occur due to the temperature
difference between the solid inner core, the molten core and the cooler outer
crust of the planet. The silicate mantel of the planet is an important aspect of
its habitability. It lies between the molten core and the outer crust of the
planet, and it acts as an insulator for the molten core. This is important
because if the core is cold, no magnetic field is generated, and the planet will
no longer be protected from flares. For example, the Earth’s magnetic field
protects the Earth from harmful particles, such as solar radiation, from the
sun.
There are four elements needed for life to exist: carbon, hydrogen, oxygen and
nitrogen. These elements are also the most common chemically reactive
elements in the universe. For example, these four elements together comprise
over 96% of the Earth’s collective biomass. Oxygen is the only element that’s in
abundance in the planets crust. This is because many of the elements, such as
hydrogen and nitrogen, along with their simplest and most common
compounds, such as carbon dioxide, carbon monoxide, methane, ammonia,
and water, are gaseous at warm temperatures. They were trapped under the
crust when planets were formed and then later formed the planets’
atmosphere when they are released through volcanoes and other plate
tectonics. Plate tectonics, caused by the pressure as the elements expand as
gases, are crucial to habitability as they cool the deep interior as hot plumes
well up to the surface and cold plates drop down to the core-mantle boundary
by cycling material between the surface and interior. This cooling drives
convection in the metallic outer core, bringing us back to the above, in which
convection produces the geomagnetic field that shields the atmosphere and
protects the surface from the solar wind. One of the factors in determining
whether a planet, may be habitable or not is the existence of water. Water is a
key part in the habitability of a planet as it is composed of two (hydrogen and
oxygen) of the four elements most vital for life.
The habitability of a planet also depends greatly on the mass of the planet. A
planet with low mass is not suitable for habitation because low mass planets
have smaller diameters and thus higher surface-to-volume ratios than their
larger cousins. Such bodies tend to lose the energy left over from their
formation quickly and end up geologically dead, lacking the volcanoes,
earthquakes and tectonic activity which supply the surface with the four vital
elements needed to sustain life. For a planet to be habitable, its mass should
be roughly within one order of magnitude of the Earth’s mass.
The habitability of a planet also depends on whether they have not accreted
the gaseous outer layers of hydrogen and helium found on gas giants. A gas
giant does not have a surface and their gravity is huge. This means that they
are not viable for hosting life. A gas giant is a giant planet composed mainly of
hydrogen and helium. Gas giants are sometimes known as failed stars because
they contain the same basic elements as a star.
As we can see the habitability of a planet depends greatly on its composition
and that there is a lot of factors that prohibit life but our knowledge of the
universe is expanding so maybe one day we will find a planet that ticks all the
boxes for habitability