In recent years there have been a number of scientific
discoveries that have gotten the scientific community thinking seriously and
realistically about the possibility of life beyond our planet. In 2005 the
European Space Agency announced the discovery of ice on Mars. "Earth-like"
planets have also been discovered in nearby solar systems such as Kepler-62 (which
contains two solid planets in its habitable zone) and the planet Kepler-22b.
Although they may be unimaginatively named after the Kepler Telescope, the
discovery of these planets has led many to believe that there may be life
beyond Earth. In order to understand the possibilities of life on other
planets, scientists aim to understand life and the requirements for life by
studying living organisms here on Earth.
So let’s start by looking at the creation of planet Earth.
More than four and a half billion years ago the planets in our solar system
were formed when objects formed of ice, rock and dust, called planetesimals
collided with one another. The bigger planetesimals drew in the smaller ones
with a gravitational pull until the planets were formed. Earth would have
started as a ball of molten rock too hot to support organic material. So where
did all this life come from?
There is a growing field of science that aims to answer this
question; Astrobiology. That’s right, biology... in space. There are two astrobiological concepts that aim to answer
our questions about life on Earth and in space; Panspermia and abiogenesis. The
Panspermia Hypothesis is the theory that basic but tough living organisms can travel
between planets on comets and meteors, seeding new planets with life. Abiogenesis,
on the other hand, is a process by which biological life evolved from simple
organic compounds that were created when the Earth began to cool and form a
crust roughly 3.9 billion years ago. Since the discovery of extremophiles both
of these theories have been gaining credibility. They attempt to explain life
on Earth, but also to promote the idea of the possibility of extra terrestrial
life.
Extremophiles
Extremophiles are basic life forms that are incredibly tough. As their name suggests, it literally means “lovers of the extreme”. They thrive in conditions that we would consider uninhabitable, but also suffer in conditions we deem necessary for life, such as high levels of oxygen. There are many different types of extremophile, for example there are: thermophiles which thrive at 45-122 °C, xerophiles which can survive the driest of conditions including the driest desert on Earth, the Atacama Desert, piezophiles which thrive in extreme under-water pressure and even radioresistants which can survive ultraviolet and nuclear radiation. Amongst the most interesting of these are the polyextremophiles, which as their name suggests, can survive a range of these conditions.
Tardigrades, also called water bears or moss piglets, are polyextremophiles that can survive temperatures from absolute zero to almost 150°C. They can also withstand pressures up to six times stronger than those in the deepest oceans on Earth, and can even live through a huge amount of radiation exposure.
Tardigrades range in size from 0.1mm to 1.5mm so are visible to the naked eye. Under a microscope however, they look like this.
Panspermia
The Panspermia Hypothesis has a long history. It was first
discussed by the Greek philosopher Anaxagoras in the fifth century BC and has
made numerous returns as a topic of interest since. In the 1960’s the theory
was brought into modern science by Fred Hoyle and Chandra Wickramasinghe. In an
interview broadcast by the BBC Wickramasinghe talked about the statistical probability of life
on other planets. He argued that the amount of solar systems and planets in the
known universe was so great that there must be life out there based on the
numbers alone. On top of this he believed that life was being spread between
these planets by meteors. Unfortunately for Wickramasinghe statistical
probability is not proof, and there was also no evidence that any life forms
could survive travel in space at the time. The idea was therefore rejected and
ridiculed by the scientific community for a long time. This was all to change though. Scientists are
now discussing the Panspermia Hypothesis with new eyes. This is largely because
of knowledge we now have of extremophiles, first discovered in the 1980’s,
research into their impressive resistances has revived Panspermia as a credible
theory.
Fast forward to 2007. NASA sent tardigrades into space and
exposed them to the vacuum of space and the incredibly harmful ultraviolet rays
of the sun for ten days. A similar experiment was conducted by the European
Space Agency who called the experiment TARDIS (Tardigrades in Space) and it
replicated NASA’s results. These experiments proved that there are life forms
that can survive in space which gives The Panspermia Hypothesis much more
credibility. The tardigrade brings the Panspermia hypothesis back into the
realm of possibility. Whether they can survive indefinitely or not is still
under question, many scientists argue that any journey between solar systems
would still be too far, but one within a solar system, from Earth to Mars or
vice versa, could be possible.
If The Panspermia Hypothesis is correct, then life could be
much more common in the universe than we previously thought; but it also means
that life on Earth could have been brought here from somewhere else, which is a
tough concept for the human mind to grasp. The problem with the Panspermia
Hypothesis is that even though it is all logically possible, it is quite
speculative and we have no strong evidence that it has actually happened. In
its defence it does not need to have happened on Earth for it to be possible in
the rest of the universe, but it is not the only theory that astrobiology has
to offer about life on Earth and beyond.
Abiogenesis
Abiogenesis is the process by which the building blocks of
life (probably amino acids) are naturally created from non-living organic
compounds. It has been suggested that it starts with self replicating
molecules, possibly created by extreme doses of radiation or heat. By showing
that life can sprout on a planet with the right conditions it simultaneously argues
for the possibility of extra terrestrial life, but without all of the messy
space travel issues. If it can happen on Earth, then logically it should be
possible on the “Earth-like planets” we have discovered. Abiogenesis is easier
for us to grasp than Panspermia because it argues that life on Earth actually started
on Earth. In logical terms it has similarities with Evolution Theory so we can
relate it to knowledge that we are already comfortable with. There is also some
experimental evidence. Amino acids have
been created synthetically in conditions similar to those on Earth roughly 4
billion years ago. There have been multiple
experiments which have done this and many scientists believe that this is how
life starts on planets as they begin to cool.
Even as the Earth began to cool, it was still far too harsh
for complex life forms, or even most basic life forms to flourish. This is why
extremophiles were most likely some of the first living organisms. It is
thought that they may have started deep in the oceans (making them piezophiles)
and that this could happen on other planets too, even within our own solar
system. It doesn’t just have to be piezophiles in the deep oceans though.
Remember that xerophiles can survive in extremely dry conditions meaning that
the whole idea of needing liquid water on a planet for there to be life could
be wrong. By changing the requirements that we place on planets in order for
them to bear life, it becomes much more likely that there is life beyond Earth;
especially considering that abiogenesis could occur on a large number of
planets.
The discussion of extra-terrestrial life is now at a
rational and reasonable stage, no longer constricted to conspiracy and science
fiction. Aliens now are no longer the large headed, grey stereotype of the 20th
century. Scientific discussion of aliens is now about those that live in the world
we see through a microscope. Both the
Panspermia Hypothesis and Abiogenesis have experiments which back up their
theories, but neither have empirical proof. The thing is, in order for aliens
to exist neither of those things need
to have happened on Earth. They only need to be possible. As long as they are
possible life could be seeded all over the universe.
