Space missions on exoplanet atmospheres: an update


In the previous post I revisited a two-year-old post on exoplanet atmospheres: a before/after view of the Hubble Space telescope spectra of hot Jupiters.

In the same vein, let us re-visit this post from 2 years ago, on future space missions devoted to the study of exoplanet atmospheres.

At that date, three space missions were under consideration, the James Webb Space Telescope, FINESSE, and EChO. JWST is the successor to the Hubble Telescope, scheduled for launch in 2019. The other two were small, targeted missions to observe transiting exoplanets, the first aiming for a very rapid launch in 2018 by NASA, the second in 2024 by ESA.

In the post two years ago I was cautious about the last two, and rightly so as it turned out: neither FINESSE nor EChO made it to the final selection by the space agencies.

They both lost to missions dedicated to finding new exoplanets, the American TESS and European PLATO mission. These will identify transiting planets closer to Earth than the targets of the Kepler mission.

MissionTypeLaunch dateStatus
James Webb STGeneral-purpose Infrared Space Telescope2019On schedule
FINESSEExo-Atmospheres 2017Abandoned
TESSDetection of close exoplanets by transits2017Selected (NASA)
PLATODetection of close exoplanets by transit2024Selected (ESA)

The selection of TESS And PLATO underlies the main problem with FINESSE and EChO: they relied on good targets being identified prior to their launch. The selection committees must have felt that it was more sensible to collect the targets first, then plan the atmosphere characterisation missions. Another problem with the FINESSE and EChO projects is that they did not enjoy the unanimous support of the community, led as they were by teams with a track record of spurious claims in the topic of exoplanetary atmospheres.

JWST is thus likely to be the main workhorse in the study of exoplanet atmosphere for the next decade. Several of its instruments are able to carry our exo-atmosphere observations, and its potential in the study of exo-climates is great – even though most of its time will be devoted to the study of the deep universe.

A recent workshop at Caltech in March 2014 assessed the potential of JWST to for exo-atmosphere studies. On paper, it could detect atmospheric features for Earth-size planets. However, there is a catch: in the case presented in the report for instance, 25 transits have to be added to accumulate enough signal to detect atmospheric molecular features. Relying on the law of averages to sum the observations of 25 different transits relies on an idealized view of statistics that rarely works in practice. Noise in complex instruments does not generally average out so nicely. That’s why we need bigger telescopes to measure fainter objects, rather that accumulating enormous amounts of data from small telescopes.

Sometimes it feels that the study of exoplanetary atmospheres progresses at break-neck speed, with all the new results and ideas. But when lead by space missions, some steps forwards can take a decade, or even a generation. Below is a timeline of the field, marking birth by the discovery of 51 Pegasi, with major landmarks and the lifetimes of the space missions.





About Author

I am a professor of planetary science at the University of Exeter. My specialty is the study of exoplanets, in particular the observation and modelling of exoplanet atmospheres. I have done my PhD a the University of Geneva and worked in Chile, France and Switzerland.