Our galaxy is 120,000 light years in diameter. It contains up to 400 billion stars – and astrophysicists are getting closer to knowing where, in this vastness, to look for other life.
One of the more improbable aids for exploring far distant space in the hope of locating extraterrestrial life is a map: a chart that, give or take a few hundred million kilometres, says ‘this is where the aliens live’.
Given that any sign or signal sent from prospective neighbours in our Milky Way galaxy could be a billion or so years old by the time we saw it, the map would more likely be telling us where the aliens used to live. But such a map would still be a leap forward for astronomers searching for signs of other life, which is why astrophysicists in Melbourne and the UK are drawing one.
The project is a collaboration between the University of Central Lancashire’s Professor Brad Gibson and the director of the Monash Centre for Astrophysics, Professor John Lattanzio, along with Monash University honours student Kate Henkel. It arises from a wider investigation of galaxy evolution, particularly chemical evolution, seeking knowledge about where and when in space and time stars similar to our sun have evolved with the capability of supporting Earth-like planets.
The trio’s work is creating a scientific basis not only for pondering the existence of other life, but also for pointing to where it might be. In a galaxy 120,000 light years in diameter and home to between 200 billion and 400 billion stars, that is no mean feat.
Professor Lattanzio explains that the key to what they are doing is the improving knowledge of our own Milky Way’s evolution: its origins in stellar explosions called supernovae; how the stars produce different elements from nuclear fusion in their centres; and how this matter is returned to the galaxy when the star dies and is subsequently incorporated into the next generation of stars.
Any planets that form with these stars as their hosts will reflect the star composition. This means that as the composition of the galaxy changes over its 12-billion-year history, so does the composition of the planets within it.
“So what we are really mapping is the history of the composition of the galaxy as it changes in space and time,” Professor Lattanzio says.
It is a significant step, he points out, because until about 50 years ago little science was involved in the search for extraterrestrial life. “Philosophers, mostly, developed assumptions and arguments, such as the premise that the sheer number of stars made the existence of other life logical. But there was no science to test this; no data.
“It took the development of two fields of science, astrophysics and evolutionary biology,” Professor Lattanzio says. “The revolution began when we started to learn quantitatively about the structure and formation of stars and planets, together with the structure and development of living organisms.”
The Drake equation
An example of the progress being made is the emergence of data to support the Drake equation, a tool for calculating the number of extraterrestrial civilisations in the Milky Way with whom communication might be possible. It was conceived in the early 1960s by Frank Drake, who is now Emeritus Professor of Astronomy and Astrophysics at the University of California.
“It really wasn’t much practical use but now some of the variables in the equation are being quantified,” Professor Lattanzio says.
The equation – a lengthy string of symbols that trails mesmerisingly across a whiteboard – begins with the average rate of star formation per year, multiplied by the fraction of those stars that have planets, multiplied by the average number of planets per star that can potentially support life, multiplied by the fraction of these that go on to actually develop life, multiplied by the fraction of these that develop intelligent life.
This only takes us midway into the equation, but the important point is that there are now actual numbers for these early variables. Astrophysics has meaningful figures for the rate of star formation and the fraction of stars with planets (20 to 30 per cent). With advanced knowledge about the galaxy’s chemical evolution, science has begun to be able to estimate the number of planets, and specifically, Earth-like planets that could support life.
This tantalising development brings science and science fiction closer together, although as Professor Lattanzio likes to remind students, ‘life’ does not necessarily equate to ‘intelligence’.
The Drake equation specifically refers to life that is able to build radio telescopes that can talk to us. That is a far cry from life that is very successful but not ‘intelligent’ as we understand it, such as bacteria. “Even creatures like dolphins that we do consider highly intelligent, for example, will never be able to build radio telescopes; and in fact from their ocean world, have no notion of space or the size of the universe.”
A model of star formation
The research that the Monash–Lancashire collaboration is undertaking is based on Professor Gibson’s groundbreaking 2003 work on galactic evolution (the origin of the ‘maps’) and owes much to Monash University’s expansion of its former Centre for Stellar and Planetary Astrophysics into the new Monash Centre for Astrophysics. The collaboration was cemented in 2012 when Professor Gibson spent nine months at the centre as the first recipient of the Kevin Westfold Distinguished Visitor Program, named after Monash University’s first Professor of Astronomy.
At the heart of the research is computer modelling able to create a dynamic representation of the galaxy’s evolution, using data only recently available to astrophysicists. This comes from telescopes such as the twin Gemini Observatory in Hawaii and Chile, the Very Large Telescope in Chile, and space platforms such as Hubble and Kepler. These instruments can perform high-resolution spectroscopy of faint stars.
Added to this is a new source of information – ‘pre-solar meteorite grains’. These are micron-sized pieces of dust that formed in the outer atmosphere of stars. When these grains end up inside meteorites that fall to Earth, scientists are able to dissolve the meteorite and analyse the grains, gaining crucial data about the composition of the star that created the grains.
“It’s actually a piece of the star that we have in the lab … and it gives us fantastically accurate and detailed information,” Professor Lattanzio says.
Data such as this has allowed the researchers to make models showing how stars were formed from the material expelled from previous generations of stars.
“Our models have to reflect this. They must be dynamic, because we also now know that galaxies are not evolving in isolation but are continually accreting gas from regions between galaxies,” he says.
“When stars die – when they have burned up their hydrogen and helium and collapse and explode – they push matter back into the galaxy, and the next generation of stars forms from this gas and the primordial material made by the Big Bang that is still flowing into the galaxy.
“All this ceaseless chemistry is changing the composition of the next generation of stars and affecting the eventual distribution of metals needed to form terrestrial planets – planets on which conditions can develop to create life. And by this, we mean life that is probably carbon-based, needs water, and is killed by the same things – like supernova radiation.”
This evolutionary chain is why one of the central elements of the research has been about finding where and when metals have formed. The next stage is analysing when and where there will be planets with high percentages of carbon, oxygen, magnesium and silicon.
Ms Henkel explains that these composites allow the researchers to start analysing the likeness of other planets to Earth. The planets themselves are beyond the reach of current technology, but their composition can be deduced by analysing the light from their star. “For example, the Earth has a similar carbon-to-oxygen and magnesium-to-silicon ratio to the sun,” Ms Henkel says.
“The carbon-to-oxygen ratio is particularly important because it determines the type of rocks you get, and can also be an indicator of the likelihood of water. If the carbon-to-oxygen ratio is over 0.8, then you get no silicates – and the Earth’s crust [along with most rocky planets and moons] mostly comprises silicates and oxides.
“Once we have identified, from this, the period and location in the galaxy’s evolution where there is going to be a high concentration of suitable rocky planets, we can begin to consider the possibilities of life and, by extension, intelligent life.”
Professor Lattanzio adds that intelligent life did not begin on Earth until the planet was about five billion years old. But their study of galaxy evolution shows similar conditions to those of our sun began occurring elsewhere about nine billion years ago, which means the oldest intelligent life in our galaxy could be four billion years ahead of us.
This is one reason why Professor Lattanzio is not overly confident that there is intelligent life to be found: if it existed to the extent that it could communicate, it has had a long time to do so. “My guess is there are probably a lot of bacteria in space … but not so many dogs, for example. In other words, there may well be little out there that is larger than a microbe. Hopefully we will learn more in the near future.”