Water in the atmosphere of extra-solar planets

How many generations of humans have looked up at the night sky and wondered how many of stars had planets around them? Perhaps it is only a few, since early humans would have considered our planet and our star to be unique, and would have looked at other stars without comprehending that they were the same as our sun, only at a vast distance. But ours is the first generation to know the answer, that most stars do indeed have planets around them.

My research involves looking for such extra-solar planets. We operate the WASP-South transit survey, which is an array of cameras out in the South African desert which photographs the night sky repeatedly, every clear night, building up light-curves of millions of stars, watching for small dips in their light caused by a planet passing in front of the star, once per orbit.

The depth of the transit dip tells you the fraction of the star that is occulted by the planet, and thus, if you know the size of the star, you obtain the size of the planet. You can then find the mass of the planet by the gravity it exerts on the host star, which you measure using the Doppler shift of the star’s light as it is rhythmically tugged to-and-fro as the planet orbits. The combination of size and mass tells you that planet’s density, and from that you have a fair idea of what it is made of.

Further, we are beginning to be able to detect the atmospheres of extra-solar planets, despite them being hundreds of light-years away. If we can detect molecules in the atmospheres of exoplanets then, in principle, we might detect “biomarker” molecules that indicate organic activity (such as free oxygen). Thus it is realistic that, within a couple of decades, we will have found other Earth-like planets that we know to bear life.

We look for molecules by observing exoplanet host stars both in and out of transit and then comparing the spectra. The body of the planet blocks some of the star’s light, but the thin smear of atmosphere around the planet is only partially opaque, and is back-lit by the bright star. Molecules in the atmosphere can thus cause absorption features in the recorded spectrum. By subtracting the in-transit and out-of-transit stellar spectra you have the transmission spectrum of the exoplanet atmosphere.

Another way of thinking about it is that the molecules cause absorption at particular wavelengths, making the atmosphere more opaque, which means that the planet is effectively marginally bigger at that wavelength, and thus the transit depth is marginally greater.

Even the largest planets are small compared to the stars, blocking typically only 1% of their light, and the atmosphere is only a thin smear around the planet, and hence the change in transit depth caused by such absorption features is only one part in ten thousand. Detecting this is right at the limit of current technology, and involves pushing instruments to their limit, beyond what they were designed to do. For example the Hubble Space Telescope was designed long before we had found any exoplanets, before this work was envisaged, though that doesn’t stop astronomers trying.

To stand the best chance of detecting molecules you need planets around a bright star (hence lots of photons), and you want a large planet with a puffed-up and fluffy atmosphere (maximising the fraction of the star covered by the planet’s atmosphere). A short-period orbit, thus producing lots of transits to observe, is also helpful.

The WASP project is well suited to finding such systems, since it is tuned to finding large “hot Jupiter” planets close to their stars. Our planets are also around relatively bright stars, typically a hundred times brighter than host stars of planets found by the NASA planet-hunting mission Kepler, which looks at fainter stars in a smaller patch of sky.

Hence a team led by Avi Mandell at NASA’s Goddard Space Flight Center chose three WASP planets to observe with the WFC3 instrument on the Hubble Space Telescope, looking for molecules in the atmospheres of WASP-12b, WASP-17b and WASP-19b (the “b” refers to the second object in each system, namely the planet).

The NASA team claim that they have indeed detected a broad absorption feature in all three systems, at a wavelength of 1.4 microns, which they identify as being produced by water molecules.

The transit depth as a function of wavelength in the infra-red for three WASP exoplanets. The broad bump in the middle of the region is possibly an absorption feature (higher transit depth) caused by water molecules. The coloured lines are different models with different compositions. (Figure by Mandell etal, 2013, Astrophysical Journal, 779, 128)

This is important since water is, of course, an essential part of life on Earth. These planets are totally uninhabitable (being large gas giants and far too hot for life), but finding water in an exoplanet atmosphere is an important demonstration of the method.

Can we believe it? Well, the exoplanet literature is currently full of debate on how reliable such results are, and whether they result from instrumental systematics in detectors being pushed to the edge of their capabilities. Further, many such studies are finding “flat” transmission spectra, which indicate opaque clouds blocking all wavelengths equally, rather than the absorption features expected in a clear sky. If many exoplanets have cloudy skies then this work is going to be hard.

There is little doubt, though, that the next generation of facilities, designed for this work, will routinely map out the molecular compositions of exoplanet atmospheres. This should start with the successor to Hubble, the James Webb Space Telescope, scheduled for launch in 2018.

James Webb Space Telescope

To look for life on Earth-like planets in the habitable zone, however, we first need to find such planets. That means finding much smaller planets at much greater distances from their star. That is much harder, since small planets produce much smaller transit dips, and long orbits mean very few transits. Thus we are still some decades away from the investigation of the atmosphere of an Earth-like exoplanet. But we’re on that track, and heading down it faster than many thought possible until very recently.

NASA press release on this work and similar work on two other planets.