LOOKING FOR LIFE IN THE GALAXY
While it’s possible to conceive of life forms that are very different from those on Earth, astrobiologists – the scientists who search for life on other planets – generally focus on Earth-like biology because they know exactly what signs to look for. Comparing our planet with other locations in the Solar System that don’t have life, we know that key requirements include the presence of liquid water, an atmosphere, plenty of carbon-based chemicals and a strong magnetic field to protect the planet from harmful radiation.
Thanks to initiatives such as NASA’S Kepler space telescope and Transiting Exoplanet Survey Satellite (TESS), we now know of thousands of exoplanets beyond our own Solar System. But not all of these planets are suitable habitats for Earth-like life. In order to qualify, a planet needs to have a rocky composition like our own, as opposed to a gas giant like Jupiter. It should also lie within the ‘habitable zone’, sometimes known as the ‘Goldilocks zone’, around its host star at a distance where water can exist in liquid form. If a planet is too close to the central star, it will be so hot that any water will boil, while a planet that’s too far away will be so cold that water will only exist in frozen form as ice. To date, at least two dozen potentially habitable planets have been discovered, in the sense that they orbit within their star’s habitable zone and have similar dimensions and compositions to our own planet. Most of these planets belong to different star systems, but one nearby star called TRAPPIST-1 has no less than four potentially habitable planets orbiting around it.
Unfortunately, simply being a rocky world located at the right distance from a star doesn’t mean a planet is necessarily inhabited. Think of our Moon, for example, that is airless and devoid of both liquid water and life. The same may be true of some of the exoplanets that at first sight appear to be habitable. While planet-hunting telescopes like Kepler and TESS can’t tell us whether an exoplanet has an atmosphere or not, there are other instruments that can. An important resource in this context is NASA’S James Webb Space Telescope. This has already shown, for example, that two of TRAPPIST-1’S closest planets are devoid of atmospheres. But these were too close to the parent star to be habitable anyway, and scientists are still awaiting Webb’s results for the more interesting planets lying inside TRAPPIST-1’S Goldilocks zone.
Once an atmosphere is detected around a rocky, Earth-like planet within a star’s habitable zone, it becomes a prime target in the search for extraterrestrial life. A key concept that enters the picture at this point is that of a ‘biosignature’ – a telltale chemical that, in the case of our own atmosphere, is primarily produced by living processes. Key biosignatures include things like oxygen, ozone and dimethyl sulphide, and one of Webb’s main tasks will be to look for such chemicals from other worlds.