There are a number of truly fundamental questions in biology which are yet to be answered, and one of them has to be whether there is life on other planets in the universe. Statistically, it seems almost certain, given the vast expanse of the universe, with its trillions of stars. Recent discoveries confirming the presence of planets in many neighbouring solar systems have only bolstered this perspective. However, unless we can actually detect life beyond the pale blue line of the Earth’s atmosphere, it’ll only ever remain a mathematical probability.
So if the Earth isn’t unique, how close are we to discovering life amongst the stars? The answer might surprise you. Read on to learn more about astrobiology and dazzle your classmates with your interstellar knowledge.
Many scientists believe there is still a chance that life might exist on another celestial body within our own solar system. In the early 20th century, people imagined there might be civilisations building vast canals on the surface of Mars, or lurking beneath the impenetrable clouds of Venus. Although later science has revealed this to be a fantasy, there is still the possibility that microbial life could have clung on in Mars, or at least once existed in the distant past.
According to evidence collected by NASA, Mars was home to liquid water – possibly even a large ocean – up to nearly three billion years ago, and could have supported microbial life for hundreds of millions of years. It’s believed Mars was much warmer and wetter than today, with a thick atmosphere. Today, the Martian atmosphere is just 1% as dense as ours, meaning its pressure and temperature is much less than the Earth’s. As Mars’s atmosphere bled away, the water would’ve escaped into space too as temperatures and pressure plummeted, until what was left froze at the planet’s poles.
But why did this happen? Scientists believed that Mars’s atmosphere was stripped away by solar wind. This process is prevented on Earth by the planet’s magnetosphere, created by its liquid and solid iron core. Up until around 4 billion years ago, Mars had a similar mechanism. However, Mars today has a weak magnetosphere. Nobody is completely sure how Mars lost its magnetosphere, but some scientists have proposed it was destroyed by a series of massive asteroid impacts, disrupting the planet’s internal dynamo.
While it’s very unlikely anything survives there today, if life had once existed on Mars, it would have major implications for the search for life. It would prove that life could originate on other planets, or could have survived after being carried there on a meteorite from Earth. However, Mars isn’t the only place in the Solar System where microbial life could exist. Here on Earth, life has been found in places you wouldn’t expect it to survive: a lake of bubbling black asphalt, highly acidic pools, volcanic vents in the depths of the sea, even half a mile under the ice in Antarctica.
The discovery of these ‘extremophiles’ has led scientists to believe life could exist in places we never previously thought possible, even in the inhospitable environments of the outer solar system, beyond Mars and the asteroid belt. Jupiter’s moon Europa is regarded as a particularly promising candidate. There is thought to be a huge ocean of liquid water beneath its icy crust, heated by the tidal forces of Jupiter’s gravity. Microbes have been found in even the deepest parts of Earth’s oceans, so perhaps it survives on Europa too, in the vast sub-surface waters. Scientists hope to sample plumes of water vapour erupting from the moon’s south pole to see if it harbours alien life. Saturn’s moon Enceladus has similar conditions, and, with geysers of ice and water vapour erupting from its surface, spacecraft could sample the ocean on Enceladus in the same way. In fact, in April 2017, Nasa announced that chemical reactions taking place deep below the icy surface of Enceladus may make it the most habitable place in the solar system beyond Earth.
There are, potentially, many much more promising places for life to exist beyond our solar system, although they are more difficult for scientists to study. By measuring tiny dips in light from stars as planets pass in front of them, scientists have discovered planets orbiting many stars beyond our solar system. As our techniques have improved, we have begun to detect many more planets around the same size as our Earth. Scientists believe that many of these could be rocky planets with atmospheres. Planets detected in a star’s ‘habitable zone’ – the area around a star where liquid water could, given the right conditions, exist – are particularly exciting. Based on current observations, there may be as many as 11 billion potentially habitable planets in our galaxy alone.
In February 2017, NASA announced the discovery of the first ever system of seven Earth-sized planets around a single star, three of which lie within the star’s habitable zone. Dubbed the ‘TRAPPIST’ system, these planets are, however, around 40 light years, or 235 trillion miles, from Earth, making visiting them with a space probe impossible, at least for the foreseeable future. However, there is one way scientists might be able to detect whether a planet is habitable – by studying their atmospheres.
NASA is launching a new telescope, the James Webb Space Telescope, in 2018, which will aim to analyse light passing through the atmospheres of exo-planets to determine their chemical composition, looking for water, methane, oxygen; the elements for life as we know it. In April 2017, using a telescope in Chile, scientists demonstrated the viability of this concept by detecting the atmosphere around an Earth-like exo-planet for the first time. Known as GJ 1132b, the world was found to be cloaked in a thick layer of methane, water, or both. Unfortunately, the planet is estimated to be a decidedly hostile 370C.
Nevertheless, this discovery proves scientists will be able to study the atmospheres of planets many light-years away, helping them estimate their habitability for life. Indeed, as life on Earth emits so many gasses, one day scientists could detect direct evidence of life on another planet. So while we may be many centuries away from ever visiting another solar system with either a probe or a manned spacecraft, we could discover life in space this century, finally answering that most profound question – are we alone?
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