Tue, 20. August 2013
Astro-Speaker Watch - August 2013
The August guest speaker was Dr. Jonti Horner, a Liverpool FC supporter (booooo....), whose lecture was entitled: "Exoplanets, and life elsewhere."
Dr Horner beginning his talk as Chris turns the lights off.
After a detailed run down of the various exoplanet detection methods, the talk moved on to describe how astronomers are streamlining the search for exoplanetary life.
The ability to examine exoplanets in greater detail will be developed but we will not have the ability to examine many of them in depth. We will have to improve the odds, which means searching for exoplanets with properties we know are suitable for life. Exoplanets which resemble the only planet we know of which harbours life-forms: Earth.
That's a lot of exeplanets - but most of them are very big!
Whilst most of the exoplanets found so far are bigger than Earth, it is felt that over the next decade we will begin to refine techniques and discover Earth type exoplanets. As we begin to find candidates, new instruments are being developed which will be capable of examining the exoplanets and their host stars - but because of the massive amount of telescope time involved in examining each system, the odds need to be reduced by selecting star systems and exoplanets which most resemble our own. Such stringency means we may be only able to examine one or two exoplanets over a timescale of several years.
What are the properties which will be used to narrow the search?
Stars which are more active than the sun may be excluded, on the basis that high stellar activity would be detrimental to formation and/or development of life.
Variable stars and binary stars also pose problems. The large variations in stellar radiation output over days, weeks, months or years would be less conducive to life. Both these types of stars are very common; and orbital instability in multiple star systems is another big factor.
Orbital variations v climate change.
Habitable zones are much quoted as being the place to look for life, the zone where it's not too hot and not too cold, where liquid water will be found. But the length of time an exoplanet has been in the habitable zone is a very important factor. Has the habitable zone of the target star expanded or contracted over it's evolution due to stellar activity? Has the orbit of the exoplanet always been inside the zone or has its orbit varied due to interactions with other bodies?
The lifetimes of massive stars are too short. At the other end of the scale, red dwarfs have often been promoted as favouriting life formation, possibly because the stellar lifetime is longer and the stars are likely to be more stable. However, exoplanets would need to orbit very close to these small, cool stars. How would the proximity to the star and its radiation affect life formation and development? Exoplanets this close would also have a tendency to be subject to tidal locking, similar to Mercury.
Another representation of orbital variations v climate change.
Computer simulations show that many potential star systems, with planetary types and orbital radii different to ours, can be unstable and result in orbital changes which would be disastrous for life on any exoplanet. Some systems may also be subject to what is called "Late Heavy Bombardment," caused by changes in the orbital dynamics, causing a new wave of asteroid bombardment.
Astronomers will need to discover where water comes from in planetary systems. Lack of water will not suit life-forms as we know them. What would be the effects of an over-supply of water, resulting in water planets with no land?
What effect has the Moon had on development of life here on Earth? Tides are important (and maybe its monthly night light cycle is too). Does the Moon assist in stabilising axial tilt? Axial tilt is probably crucial. Exoplanets with large tilts would not rank high on the astronomers check list, due to the effect on seasonal change.
Other factors such as plate tectonics may be crucial to life. Earth, with its life-forms has them. Mars, an apparently dead planet, does not.
Similarly, Earth has a magnetic field which is just right for protecting the planetary surface from harmful solar radiation. The field is created by electric currents formed by the rotating iron core deep inside the Earth. Life-forms here on Earth are dependent on this vital mechanism.
This was not discussed but clearly, suitable atmospheric properties are crucial for our existence and would be a big consideration in determining where we direct our detailed observations.
We have currently confirmed around eight hundred exo-planet discoveries, with thousands of other potential candidates resulting from data received from the now failed Kepler space mission, which studied a small region in Cygnus for three years.
All of these factors are considerations in determining which exoplanets are best candidates for life as we know it. It is a list of factors which drastically reduces the odds of finding such life and is paramount to selecting the best candidates to study.
These details ram home the 'fine-tuning' that is needed to the properties of a planetary system to result in conditions suitable for life as we understand it; and it tells me that such conditions are unlikely to be particularly common in our galactic neighbourhood.
We are searching for evidence of any life, from microbes and plants, to dinosaurs and humanlike forms. In tandem with this we are also directing our SETI radio signal detection to the same galactic region around us and the signal detection from intelligent lifeforms still remains barren.
Astronomers believe that the best place to look for alien lifeforms will be in planets around Sun-like (G-class) stars which are neither active, variable or multiple star systems, with an Earth sized planet and a Moon sized moon, with Earth like properties such as magnetic field, axial tilt and stable orbital characteristics, the right amount of water, an Earth-like atmosphere
So, when astronomers say they are looking for Earth-like planets, they are doing so to improve the odds of them finding evidence of alien life-forms.