Megan Whewell explains how other planets are classified,Since the first planet was confirmed orbiting a star other than our own Sun in 1995, this field of exoplanet research has grown hugely. We are now aware of over 700 other planets, in over 500 different systems. Like any new branch of science, its focus is now beginning to shift from discovery to classification and understanding, and this is where a newly proposed scheme of two ‘habitability indices’ comes in.
The scheme has been proposed by scientists from a variety of organisations, including Nasa and the SETI Institute, and involves two ways of classifying exoplanets by the likelihood of life existing there. The first uses a traditional method of comparing the new planet to Earth and giving it a rating accordingly. This is called the Earth Similarity Index (ESI) and ranges from 0.0 (not similar at all) to 1.0 (the highest rating, and unsurprisingly the rating of Earth itself). Each ESI is based on factors such as mass, radius and predicted temperature of the planet, which can be interpreted to give the likelihood of life as we know it on Earth to exist there.
As mentioned, comparing other planets to Earth is quite a traditional approach to searching for life on exoplanets, and is currently done by calculating a so called habitable or ‘goldilocks’ zone around the parent star and seeing if the planet’s orbit is within that area. This ‘goldilocks’ zone covers the distance from the star where the temperature would be just right for liquid water to exist, not too hot and not too cold.
Some scientists have expressed worries that we are unnecessarily narrowing our search by just using this system to determine which planets are our ‘best bet’ to discover alien life, so may be missing out on the exciting possibility of life existing in more extreme environments. Studies in some of these possible environments here on Earth have shown life to be more resilient than previously thought to these extreme conditions, and the existence of these simple extremophile organisms is driving a much wider search for life elsewhere.
That is the reasoning behind the second index within this classification scheme; the wider Planetary Habitability Index (PHI). This rating also ranges from 0.0 to 1.0, with higher numbers indicating a greater possibility for life, and is based on chemical and physical factors that could be favourable to life in general, not just the type of life we know to exist on Earth. This rating more appropriately represents the possibility scientists are considering of life elsewhere in our own Solar System, for example under the Martian surface or within the hydrocarbon lakes on Titan (Saturn’s largest moon).
While the idea of investigating chemical and physical conditions of planets millions of light years away to calculate an accurate PHI rating may sound like science fiction, it is fast becoming reality. The Hubble and Spitzer Space Telescopes have already demonstrated that it is possible to detect molecules in the atmospheres of some large, gaseous exoplanets and future missions such as EChO plan to explore smaller and possibly more habitable planets in the future.
The possibility of finding indications of life on other planes in the near future means this is a very exciting time to be studying exoplanets, and this new classification scheme could be exactly what’s needed to focus scientific attention on the most likely targets.