By ANNA BOLUDA
Enric Palléis a researcher at the Institute of Astrophysics of the Canaries (IAC) and the coordinator of the session on the search for life on exoplanets during the European Week of Astronomy and Space Science EWASS 2015. One of the high points of this European conference was the award of the Tycho Brahe prize of the European Astronomical Society to Michel Mayor of the Geneva Observatory for his discovery of the first exoplanet around a normal star, 20 years ago.
Since then the search for exoplanets has become one of the most active, prolific, and exciting fields in astronomy.
How has the search for exoplanets changed in recent years?
Exoplanets are very interesting, because they are a relatively new field, only a little over 20 years ago we didn`t know of any, and today there are over two thousand confirmed planets orbiting other stars, and a further four or five thousand candidates waiting to be confirmed. The field has evolved a great deal from the first detection to the possibility of making statistical predictions about the number of planets and the most common types in the universe. Now we are immersed in the process of charactering them.
We have begun to measure chemical compounds in the atmospheres of some planets, for the time being the biggest and those nearest to their stars, because it is easiest to make the measurements for them. In the near future we would like to make these kinds of measurements for planets which are not gas giants, but for rocky planets like the Earth, because these are candidates for bearing life. In the next few years efforts will be centred on trying to measure the chemical composition of Earth-sized planets whose orbits are at distances from their stars which allow them to have water in liquid form, known as the “zone of habitability”.
This means measuring the atmosphere to see what it is made of, and to see if we can detect thermodynamic disequilibrium. For example if an extraterrestrial measured the chemical composition of our atmosphere he or she would know that the planet was inhabited, not only that it is habitable, at the right distance from the Sun, but that it is inhabited, because if it were not for the biological cycle the chemical composition of the atmosphere would be different. For example, the oxygen would recombine with other elements and would effectively disappear. This is what we intend to measure on the exoplanets: a chemical composition which could not be understood without the presence of life.
What instruments are needed to make these measurements, and when will they be available?
At the present time this is still not possible. With the new generation of instruments on the ground, such as the ELT’s (extremely large telescopes) and the James Webb space telescope we will be able to approach the measurements we need, although maybe we will still fall a bit short. With the following generation of instruments we will surely be able to carry out these studies systematically on a few tens of planets.
It´s difficult to pin down a time frame, but we can estimate that from 2020 or 2022 we will have the extremely large telescopes working, and in a few years the James Webb space telescope will also be working. When we scientists build a telescope we are already thinking of the next one, and we hope that in some 15 to 20 years we will have the instruments which are up to the job of detecting rocky planets and seeing whether we can find signs of life.
We have to combine these estimates with estimates of the frequency of life in the universo. If wherever the conditions are present life does appear, then we won´t take long to find it. If it turns out to be a rarity, and we have to explore a couple of hundred planets which have in principle the capacity to sustain life until we find one where life has in fact developed, we will take longer. The more we have to explore, the further we will have to look, and the more time we will need to invest.
Is the scientific community convinced that there is life in some other part of the universe?
Of course. I am only speaking for myself, but I think that nobody doubts it for a moment. When we observe the Universe we can see that the elements are the same everywhere. The stars are the same everywhere too. There are different types of stars, but a white dwarf nearby is the same as one at the other edge of the Galaxy. The physical and chemical processes are also the same, and with the same elements. It is hard to think that among all the thousands of millions of possibile sites life has developed on only one of them.
“What novelties are being presented in this symposium in EWASS 2015 about exoplanets and the search for life?”
The session deals with the latest advances in the field of exoplanets in general, but we are trying to make all the contributors focus their talks on how we are going to make this first detection of a habitable planet. There are several possible techniques. One would be high dispersion spectroscopy, separating the wavelength a great deal, and trying to see, through our own atmosphere, the Doppler shift of the lines of chemical compounds. Another would be to use coronography, to block the light from the star, and try to see directly the reflected light from a planet. Another could be transit spectroscopy: if we find a planet like the Earth which transits across its star some of the light from the star passes through the atmosphere of the planet, and leaves indicators of its composition, of the chemical compounds in this atmosphere.
All these techniques are now being developed and competing, and we are applying them in different circumstances. We don´t know which will be the first to detect what we are seeking. In this symposium we will try to examine the potential and the expectations of each of them, both in the short term and in the long term. When we do make this detection it will certainly not be a carbon copy of the Earth, so that we will need to interpret the data we receive. For that reason there will be a number of talks dealing with the theoretical effort which is being put into understanding what kind of signals we will see from worlds which although they are similar to the Earth, are not exactly the same. For example what would happen if the Earth were habitable but much nearer to its star, (if the star were much cooler and smaller than the Sun it would need to be nearer so that it would receive the same degree of heating, so that liquid water could exist on its surface). The spectrum of this star would be very different from that of the Sun, its light would be redder, it might have surface explosions which could affect life, and so on. We also need to take into account the different types of life which might evolve and how these could affect the atmosphere in order to understand how to detect them.
What do you expect from this symposium?
To start with, it is going to be a genuine update. This is a field which develops so quickly that at times for those of us working on one of its aspects, it is difficult to survey the whole scene. It is an exciting field because almost every week new lines emerge which make us revise our own methods. For example high resolution spectroscopy was not being considered for exoplanet studies until a couple of years ago, but now it is one of the most revealing methods. This symposium gives us a chance to think deeply for two days, and to carry on with renewed enthusiasm.
