“Characterising Exoplanets”, Royal Society

The Royal Society held a two day discussion meeting on “Characterising exoplanets: detection, formation, interiors, atmospheres and habitability” on 11-12 March 2013.  I was only able to attend the end of the second day, though luckily this included two of the speakers I most wanted to hear. Having recently enjoyed Professor Charles Cockell’s astrobiology online Coursera course, I was delighted to meet him briefly afterwards.

Professor Charles Cockell

Dr Ignas Snellen from Leiden University mourned the fact that with the Terrestrial Planet Finder and Darwin missions having been cancelled, it is unlikely there will be a dedicated space telescope to search for biomarker gases in exoplanet atmospheres within his lifetime.  He reviewed the exciting results from ground-based telescopes and extrapolated to what may be obtained with the planned E-ELT in terms of better data in the future.

Dr Giovanna Tinetti, UCL,summarised how it is crucial to have more information about exoplanet atmospheres in order for further progress. The number of discoveries of exoplanets is increasing at a fast pace, yet with only weight, size and orbital information for about one third of them, and atmospheric temperature and compositional information for less than 20, it is not yet possible to have any detailed classification system.

Dr Tinetti, Dr Snellen & Prof Cockell

For gas exoplanets, looking at chemical compositional ratios via spectroscopy such as carbon to oxygen ratio, and the amount of hydrogen escaping will give clues as to planetary formation and evolution, while albedos, thermal emissions and the day/night variation give information about the energy budget and planet-star interaction. For terrestrial planets, finding out if they have an atmosphere, and how evolved the atmosphere is will also help to answer planetary formation and evolution questions.

Some of the current issues facing exoplanets research are:
 – Needing better resolution spectroscopy to improve and constrain our knowlege of exoplanet atmospheres
 – Better absolute calibration between different instruments. Much continuing research is from combining data sets from different instruments but this can only give answers as good as the absolute calibration.
 – Data is sparse, with not enough wavelength coverage and often not simultaneous measurements
 – Very low signal to noise ratio (SNR) observations
 – Stellar activity is largest source of astrophysical noise

Habitable Worlds With No Signs Of Life

Professor Charles Cockell, University of Edinburgh, started off by dividing all habitats into three:
1) Uninhabitable
2) Habitable & inhabited
3) Habitable but uninhabited: these show no surface signatures of life, perhaps a planet two young for an origin of life, or conditions too transient or inappropriate, or perhaps origin of life is very rare

60-80% of biomass on the Earth is cryptic, which means it would not show clear surface signatures although there may be hidden signatures, cryptic habitats, for example organisms living in the interior of surface rocks, in salts, in the deep subsurface or ice sheets as in the slide below.

We need to know a lot more about biotic atmospheres in order to
be able to tell them apart. Better resolution imagery and spectroscopy
will not currently be able to unequivocally determine if a world is
inhabited or not, as most habitated worlds in the cosmos will have no
remotely detectable signs of life.

Professor Cockell took the following hypothesis, saying it needs to be rejected if we are to find life: “Most habitable worlds in the cosmos will have no remotely detectable signs of life”.  For this to be rejected, he suggests the following conditions:
1) When planets are habitable, the origin of life usually occurs
2) Once life originates it will usually evolve metabolisms that produce unequivocal biosignature gases which have spectral properties that clearly distinguish them from abiotic materials
3) Once organisms evolve, they will colonise a planet at high biomass
4) Once the planet is colonised, they will produce enough surface biosignatures at sufficient concentrations to be detectable to alien observers.

He suggested that currently the most reliable biosignature is oxygen at high concentration, and particularly if it is found to be in disequilibrium. 

Professor Steve Miller, UCL, concluded the successful meeting with the following remarks:
“It is 21 years since Aleksander Wolszczan and Dale Frail (21/4/92) found the first exoplanet orbiting a pulsar, and 18 years
since Michel Mayor and Didier Queloz found the second exoplanet (6/10/95) orbiting a main sequence star, so however you measure it, Exoplanetology has “come
of age”!”

He summarised the meeting as having a feeling of impatience,
as more data, more missions, more instruments are wanted and needed to
satisfy the thirst for knowledge held by researchers in the exoplanet community.

Further information:
There is an EChO conference (Exoplanet Characterisation Observatory mission) being held at ESTEC, on 1-3rd July 2013.

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