Is there life elsewhere in the universe? The answer to this most fundamental question is either yes, meaning we are part of a cosmos dotted with other lifeforms and perhaps even other civilisations, or no, in which case we are unique, special and alone. What would it mean to us as species to answer that question?
The Drake Equation was developed by Frank Drake in 1961 as a way to focus on the factors which determine how many intelligent, communicating civilizations there are in our galaxy. [source]
Various organic molecules have been detected in interstellar clouds and surrounding stars, but Earth is the only place we know of in the universe where life has emerged. We used to believe we were the centre of the universe, the hub around which the cosmos revolved. We now know that we are only part of a system of planets, orbiting a run of the mill, main sequence star within a small Galaxy, among many billions of galaxies that make up our universe, and maybe even our universe is not the only one. So is life rare, or inevitable?
There are aspects of our planet that do set it apart from others. Our Sun orbits the Milky Way, far enough from the centre to be protected from too much radiation or gravitational forces but away from the Galaxy’s dangerous, disruptive spiral arms. The Sun is also rich enough in metals (any elements heavier than Hydrogen and helium) to produce planets in the first place, but not so rich in metals that too many Jupiter like planets formed to disrupt orbits within the Solar System. Though Jupiter’s gravity protects earth from excessive impacts that can cause extinction events.
We reside in the habitable zone around the Sun, that maintains liquid water on the planet’s surface. A little closer to the Sun and we could end up like Venus on one side of us, with a surface temperature hot enough to melt zinc. A little further out on the other side is Mars which is, perhaps, too cold to support life. Venus is a good comparison to Earth and is often called our ‘sister planet’ due to our similarities of size, gravity and compositions, yet we are so different. We are a relatively dense planet with a solid inner core and liquid outer core that produces a strong magnetic field to protect us from cosmic radiation.
Venus has only a very weak magnetic field and therefore it lost the water from its surface. Water helps to lubricate plate tectonics on Earth which constantly recycles our crust and limits the carbon dioxide escaping into our atmosphere. On Venus, plate tectonics ceased and the lack of crust recycling led to a build up of carbon dioxide and a runaway greenhouse effect. Our large Moon also helps to stabilise Earth’s wobble as we rotate on our axis which helps to prevent extreme fluctuations in our climate. So maybe we are special and conditions on Earth are just right for life to have emerged and evolved. But there are places even within our own Solar System that could possibly support life. Perhaps microbial life exists on Mars either now or in the past. There are icy moons like Enceladus orbiting Saturn and Jupiter’s moon, Europa that could harbour life in lakes or seas beneath their icy surface. There are current and planned missions to explore and study these exciting possibilities.
However. Water and a rocky planet may be all that is needed for life. For instance, Olivine is a common mineral on Earth and in the universe, it has been found in abundance in interstellar dust. Olivine and water when mixed together produce an exothermic reaction, that releases energy that could be utilised more easily by living organisms than solar energy. So life may not be dependent on a planet or moon orbiting a star, it could be thriving on asteroids in deep space, though these are far beyond our capabilities to find at present.
During the 16th century Giordano Bruno first suggested that the fixed stars we see in the night sky were like our Sun and so could have planets in orbit around them. The Inquisition burned him at the stake in 1600. It was not until 1995 the first extrasolar planet was discovered orbiting around 51 Pegasi, a Sun-like star over 50 light years from Earth. Since then over 700 exoplanets have been discovered. At first these were ‘hot Jupiters’, large gas giants orbiting close to their parent star, but as our detection methods are refined and becoming more sophisticated we are able to discover smaller and smaller planets. We can find out an exoplanet’s orbit, size and mass depending on the detection method used. Ideally we are looking for a rocky planet, of similar mass to the Earth, orbiting within the habitable zone of a Sun-like star. We are closing on that goal with the recent discovery of Kepler-22b which is only 2.4 times the size of Earth and at the right distance from its parent star.
Life elsewhere in the universe may be very different but we have to take the only life we know, which is here on Earth as our starting point. Even here our knowledge of the conditions that life can survive in are being extended all the time, as we find extremophile organisms thriving in habitats that until recently we thought would be barren. Organisms have been found that thrive in temperatures below -60ºC or as high as 105ºC. Others live in extremes of acid or alkaline or in high saline conditions. Some can survive pressures up to 130 MegaPascals and anaerobic organisms that can’t tolerate oxygen and even some that can survive high levels of radiation.
So how do we detect the presence of life on an exoplanet? In 1990 the Galileo probe made a fly-by of Earth and turned its instruments to see if it could detect life on our own planet. Its infrared spectrometer detected ozone, produced when oxygen reacts with ultraviolet radiation from the Sun, and methane. Both together these indicate photosynthesis and therefore the presence of a biosphere. Galileo also detected the so called ‘red edge’ a sharp rise in the near-infrared wavelength reflected, particularly over the continents, which is an indicator of chlorophyll in green vegetation. Thirdly Galileo was easily able to detect our radio and television transmissions. Part of the Venus Express mission currently in orbit around our ‘sister planet’ is to look back at Earth and observe signs of life while our image is only one pixel in size, similar to images we may see of exoplanets.
In the future, with powerful enough telescopes to study an Earth mass exoplanet’s spectra, we could learn the surface temperature of an exoplanet and if liquid water could be present. We could detect the tell tale dips in the spectra that indicate carbon dioxide and oxygen in the form of ozone and possibly the red edge indicating plant life. SETI, the Search for Extraterrestrial Intelligence could then turn its radio antennas towards the planet to look for transmission signals. Then would come the big question. Do we attempt contact? Do we wish to remain alone or to join the cosmic community?