Scientists at Arizona State University have combined astronomy and geophysics to assess the possibility of other planets being able to support life. After turning their attention towards Tau Ceti, a star system popular with science fiction writers due to its similarities to our own sun, they have concluded that it is probably not going to be mankind’s next home.
So how did they conclude that a planet is uninhabitable from light-years away?
The first condition for life is whether any planet sits in the habitable zone around Tau Ceti. The habitable zone is the area in which the surface of a planet can support liquid water. Amazingly, there are two planets that sit happily in the habitable zone around Tau Ceti: Planet e and Planet f. (Sure, the names could use some work…)
Planet f looks a little more promising than Planet e, but it has only been in the habitable zone for around 1 billion years. This might seem like a long time for us, but it’s early days for a planet that’s creating an atmosphere suitable for supporting life. For comparison, Earth sat around in the sun’s habitable zone for 2 billion years before life made a noticeable change to the biosphere.
A good predictor for a planet's geological makeup is the properties of its local star. Magnesium and silicon are two of the most important rock-forming minerals on Earth. The levels of magnesium and silicon in the sun give us an indication of the levels on Earth. So it follows that the levels of these elements in Tau Ceti correspond to the levels in Planet f.
The actual ratio of magnesium to silicon in Tau Ceti is about 70% greater than that of the sun. This means that the geology of Planet f could be quite different to that of Earth. One difference could be the abundance of a compound called ferropericlase in the lower mantle, making it hot with low viscosity. The mantle is still a solid, but it's more unpredictable and unstable. A ‘runnier’ mantle could create dynamic tectonic movement, leading to earthquakes, shifting geography and dynamic, volcanic action. Great for dramatic nature photography, but not very welcoming for life.
Of course, all of these conclusions are largely speculative and require a lot of extrapolation (and imagination) from data. Tau Ceti’s solar system probably won’t be home to humankind, but this work emphasizes the importance of understanding our own planet and using the data to assess the living conditions of planets we can’t even see with the naked eye.
Life could already be in existence on one of Tau Ceti’s planets—it just hasn’t flourished to the extent that we can detect it from Earth. Maybe we’ll see some changes in 1 billion years but until then, the search for mankind’s next home continues!