Atmospheric Characterisation

As more and more exoplanets continue to be discovered, scientists are becoming increasingly interested in characterisation of these worlds.  Just knowing a name like HD 209458b tells us nothing about what this world is really like.

University College London scientists are very active in this exciting field, with particular emphasis on the use of the “transit method”.  When a planet passes in front of it's parent star it causes the starlight to appear dimmer for short period of time. The magnitude of this dip in light allows us to infer the radius of the planet. By repeating this measurement at different wavelengths of light one can see the planet seemingly change size. This apparent change in planetary radius is actually the effect of the exoplanet atmosphere absorbing and emitting light at the different wavelengths.  By studying these changes very carefully we can infer the temperature, pressure and even the chemical composition of the atmosphere!

As an example, Dr. Giovanna Tinetti used the method to make the first discovery of water vapour in the atmosphere of an exoplanet in 2007.  Since then, further studies have seen methane, carbon dioxide and carbon monoxide discovered.  It is this work which could one day allow astronomers to detect signs of life ("bio-signatures") in the chemical make-up of an exoplanet's atmosphere.

Molecular Linelists

When spectra of exoplanets are retrieved, we must compare the observations to that of models in order to understand what we are seeing.  To do this, we use the Quantemol programme suites (developed here at UCL) to generate the spectra expected from various molecules at different temperatures.

The generation of these molecular line lists is led by Prof. Jonathan Tennyson of the Atomic, Molecular, Optical and Positron Physics (AMOPP) Group.  The recent comprehensive line list of water, BT2, was used in the detection of water in the exoplanet HD 189733b and is being used widely in a number of astronomical and non-astronomical studies.  

The AMOPP group have also calculated a number of other line lists, including the molecules HCN and HNC. These line lists will be used during the present grant to search for these species in exoplanets.  As already demonstrated for cool stars, the HCN/HNC ratio provides a useful temperature diagnostic.  In work supported by the Leverhulme Trust, Jonathan and Bob Barber are currently computing a comprehensive list for ammonia too.  This work was originally aimed at the detection and characterisation of Y-dwarfs “stars” but has become of great significance for exoplanet studies.

Page last modified on 16 jul 10 14:22 by Fabrizio Sidoli