Astrobiology is the ultimate study of life! This multidisciplinary science considers the
concept of life throughout the universe: From understanding the origins of life on Earth, to
space missions searching for signs of life on other planets in our solar system, efforts to detect 
and consider the possibility of life in other solar systems and galaxies, and planning for
the future of human life beyond Earth.

Why is Earth so special?
The Earth is the only planet in our solar system that is within the 'habitable zone', a region 
just the right distance from the Sun where a planet of Earth's mass can retain an atmosphere 
and support liquid water. Why is this so important? Earth is of just the right mass to retain 
the heavier gases like carbon dioxide, nitrogen and oxygen in its atmosphere. If Earth had a 
lower mass it would be like our Moon, unable to keep any gases around its rocky surface. 
If Earth were of greater mass then all gases would be trapped in the atmosphere, creating
volatile conditions unsuitable for life. Certain chemical elements, such as carbon, hydrogen, 
nitrogen and oxygen, are essential for life and they all require liquid water to facilitate 
biologically important chemical reactions. The presence of liquid water is regarded as a 
pre-requisite for life, and detecting it is the goal of current astrobiology space missions.


How did life arise on Earth?
Before life as we know it could exist on Earth, conditions had to favour the creation of 
self-replicating molecules that enabled information to be passed on between 'generations'. 
The first were probably chain-like molecules of amino acids or RNA. These self-replicating
molecules were then encapsulated in a membrane to create the first rudimentary cells, where
the molecules could replicate in an enclosed, more controlled environment. Such life may
have evolved deep under the oceans at hydrothermal vents, where volcanic fluids rich in
reduced chemicals served as an energy source, or in shallow geothermally heated waters on
Earth's surface, where these chemicals or sunlight could have provided an energy source for 
growth and replication. We know from microfossil evidence that living cells existed on
Earth some 3.5 billion years ago, and these bear a striking resemblance to microorganisms we 
find today in geothermal springs. These environments may be good analogues for early life
on Earth and are the subject of research by astrobiologists as models for primitive life on
other habitable planets.

Could life exist on other planets in our solar system?
There is much interest in our closest planetary neighbour Mars, with two space missions
currently exploring Mars surface. One of the most important goals is to search for signs of 
past or present liquid water on Mars, because if liquid water were present for sufficient time
then perhaps life also evolved on Mars. Most scientists believe Mars is now 'dead', but 
many believe that it once supported life, and 'biosignatures', chemical or fossil evidence for 
this may exist. The most likely place to find extant life, other than Earth, in our solar system
may be Jupiter's ice moon Europa. Whilst the moon has a frozen icy surface, a liquid ocean
beneath could have just the right conditions for life to have evolved. If we do find evidence
of life on other planets, this will lend weight to the theory of Panspermia. This theory 
suggests that living cells or self-replicating molecules travel from one planet to another 
within asteroids and facilitate the spread of life.

Could life exist beyond our solar system?
Our solar system is a tiny component of our galaxy, and in turn our galaxy is one of millions 
in the universe. It is therefore possible that planets equally as habitable as Earth orbit other
stars in distant galaxies. Astronomers interested in astrobiology are currently searching for 
Earth-like planets orbiting distant stars. Astrobiology implications for the future of human life
One of the questions all of us have probably asked ourselves is whether we will encounter 
intelligent alien life. Another branch of astrobiology, through a programme called SETI
(Search for Extra Terrestrial Intelligence) is concerned with trying to locate radio signals
emitted by alien civilizations. So far there has been no success, but consider that even the
earliest radio emissions from our civilization have only travelled about 50 light years into
space, it may take some time before we hear the aliens, and they can hear us! 
Eventually mankind will need to find a new planet to live on, as we know for a certainty that
in time our Sun will destroy our solar system. A part of astrobiology is therefore concerned
with considering how humans, and other organisms that we may need to sustain us, can cope
with life in space and on alien worlds. 

Astrobiology in Hong Kong
One of the most important goals of current research in astrobiology is to identify 
the diversity of life in environments that are thought to be similar to primitive life on Earth or
other planets. The Extremophiles Research Group at The University of Hong Kong studies 
microbial life in geothermal springs where water temperatures reach 90°C, since life on Earth
may have originated in volcanically heated pools of water. They also study peculiar microbes
living within rocks in the deserts of northwestern China, where lack of water and high UV
exposure create conditions on Earth that most closely resemble those on Mars. You can
learn more about the research by visiting this website: www.extremophiles.net.