Predicting the climate of exoplanets
The weather – that favourite topic for small talk with the British. Most of us must listen to at least one weather forecast a day and moan if it is wrong. Our January speaker, Professor Peter Read of Oxford University, does something which seems much more difficult than forecasting what tomorrow will be like here; he leads studies trying to predict the climate of exoplanets. Before we get into details of his talk, let’s be clear about the difference between weather and climate. “Weather” is what happens on a day-by-day basis and can be very changeable. (Don’t we know it in this country!) “Climate” is a description of the broad patterns of that weather over many years. Professor Read’s talk was about climate.
The key question Professor Read aimed to answer is this : “Is an exoplanet habitable?”. The key condition is well known; the planet’s temperature must allow water to exist in its liquid state. However, if you take a simple view balancing the solar energy input and the energy lost by infra red radiation for the Earth, you predict that our temperature should be below the freezing point of water. Things are obviously more complicated … much more complicated mainly because of the influence of a planet’s atmosphere.
To get a more realistic idea of climate conditions on a planet , you need to use detailed mathematical modelling and we were shown some fearsome looking differential equations, none of which are going to feature in this post for the obvious reasons that I did not understand them at the time and I am none the wiser after trying to research them on the internet.
Modelling a climate
The principles behind the mathematical modelling are relatively straightforward to describe. The first step is to do some actual experiments with fluids, representing the atmosphere and observe the way that circulation cells develop under different conditions. These cells are equivalent to anticyclones and cyclones. Next, run your equations to find out which are the key parameters that control these patterns. By changing the value of these parameters, you can check that the different patterns you have observed are indeed predicted to develop . Now you are ready to model those bodies in the Solar System which have substantial atmospheres. You use the known values for the key parameters such as the planet’s distance from the Sun, the size of the planet, the rotation rate of the atmosphere, the nature of the atmosphere in terms of how much light energy it lets in and how much heat energy it lets out, the angle of the tilt axis and quite a few other factors too.
Results from modellings
We were shown a range of modelling results, projected as if onto a sphere. Using the relevant values for the Earth, our typical wind pattern of easterly winds in the tropics and westerly winds to the north and south developed. The standout result was when the modelling was done using values which are rlevant to Jupiter. The familiar bands of clouds appear. The modelling works!
Choosing an exoplanet
Applying the modelling to an exoplanet is still some way off. A suitable candidate is Kepler 186f, thought to be in the habitable zone of its star. Some of the necessary data is known. For example we know the luminosity of its star and 186f’s distance from it. That is why we can say the surface temperature could allow water to exist as a liquid. So far we know nothing about its atmosphere, or indeed if it has an atmosphere (though I think we can judge the likelihood of this from its size and hence its gravitational field strength). Other factors will also influence its climate. These include the rotational speed of the planet and the angle of tilt of its axis of rotation. I suppose the final message on exoplanets might be “Watch this space”.
Snowball Earth?
During the questions after the main talk, it became even clearer how delicately balanced a climate can be with relatively small changes building up into a large effect. The question was about the long period of the “Snowball Earth”. Professor Read explained that this could have been triggered by a small decrease in temperature which led to the ice caps expanding. This in turn would have meant that more light energy was reflected, cooling the Earth even further. It is thought that this coincided with a period when the Earth was tectonically dormant, with the era being ended by many volcanic eruptions releasing carbon dioxide.
Please note : this has been quite a challenging post to write and I have left out a great deal of detail. I hope that what I have included is correct and gives a reasonably accurate overview of the talk.
Talk given by Professor Peter Read from Oxford University
Post written by Katherine Rusbridge
Jan 2017