Earths environmental conditions are vital to understanding the possibility of life on other planets, writes WILLIAM REVILLE
THE DISCOVERY of life elsewhere in the universe will be a major milestone in the history of science. We are presently travelling along an exciting stretch of the road towards that discovery, with the identification of planets (exoplanets) beyond our solar system. Today I will describe how exoplanets are detected and some of the properties an exoplanet must have in order to be capable of supporting life.
Although astronomers always assumed the existence of exoplanets, surprisingly, the first such planet was not positively identified until 1995. Many more exoplanets have been discovered since and the tally currently stands at 492.
Exoplanets are so far away from Earth that it is extremely difficult to see even the largest of them through our telescopes. Astronomers rely on three methods to indirectly detect exoplanets: the radial velocity method, the transit method and the gravitational microlensing method. Each method detects an exoplanet by noting the effects of the gravitational tug it exerts on the star it orbits. The planet can’t be seen but the star can because it is big and bright.
Most exoplanets have been detected by the radial velocity method. The gravitational pull of the exoplanet on its star causes the star to “wobble” from side to side as the planet orbits around it. This wobble changes the measured speed of the planet and astronomers can use this measurement to detect the exoplanet and to infer its mass.
The second method relies on the fact that when an exoplanet moves across (transits) the face of its star, it blocks some light from the star, slightly dimming it. Detection of the dimming identifies an exoplanet and the extent of the dimming indicates the size of the planet.
Thirdly, a strong gravitational field can noticeably bend light. When the earth and two stars in space are lined up, the image of the most distant star can be magnified by the gravity of the nearer star when seen through a telescope on earth. This image varies in a recognisable way if the nearest star is orbited by an exoplanet. This method is called gravitational microlensing.
The only life we know about is life on earth. Most scientists assume that extraterrestrial life will operate on the same principles as earthly life – a carbon-based biochemistry operating in an aqueous environment. Since earthly life can only live under certain environmental conditions, this allows us to predict which exoplanets might harbour life.
Firstly, the parent star that supplies energy to the exoplanet must fulfil certain criteria if it is to support life. It must burn for at least several billion years to allow life a chance to evolve and must emit enough suitable ultraviolet radiation to initiate important atmospheric interactions like ozone formation. Perhaps five to 10 per cent of stars in the local Milky Way galaxy fulfil these criteria.
Next, the exoplanet must orbit its sun in the “Goldilocks” zone if it is to support life. If it orbits too close to the sun, water will boil away and the surface of the planet will reach a deadly temperature. In our solar system, Mercury and Venus are too close to the sun to support life. On the other hand, if the exoplanet orbits too far away from its sun, water will be permanently frozen and life as we know it will be impossible. Only in the Goldilocks zone can an exoplanet accommodate liquid water – an essential ingredient for life.
If the exoplanet is less than 0.3 times the mass of the earth it will have insufficient gravity to retain an atmosphere. It will also be unable to retain a hot core (core heat will then leak away into space) and consequently will be unable to generate plate tectonic activity such as we have on earth. The outer crust of the Earth is cracked into giant plates that move about on an underlying molten layer. Where adjoining planes move apart, we find strings of volcanoes and hydrothermal vents. When the plates move together, mountain chains are thrown up and plates are also dragged down (subducted) into the molten underlay.
Volcanoes vent carbon dioxide into the atmosphere and carbon dioxide is removed when plates are buried at subduction zones. These activities regulate atmospheric carbon dioxide at a level that maintains temperate conditions – the earth is neither frozen nor sweltering in heat. Tectonic-powered mountain building also counters erosion. If plate tectonics stopped on earth, erosion would eventually wash the mountains into the sea, raising sea-levels so that a global ocean would cover the earth and eliminate land-based life.
Almost all exoplanets discovered so far are unsuitable for life. However, at the accelerating rate they are being discovered, suitable candidates for life will surely turn up soon.
William Reville is associate professor of biochemistry and public awareness of science officer at UCC – see understandingscience.ucc.ie