The tantalising possibility that a ‘super-Earth’-sized planet lying 40 light years away could harbour large amounts of water has come a step closer after a careful study done by scientists using data from the Hubble Space Telescope’s Wide Field Camera 3 (WFC3).  

Work on the planet, labelled GJ1214b, appears to offer solutions to contradictory results in previous studies, conducted using the Spitzer Space Telescope and the Very Large Telescope Array, of its home star’s solar system.

The planet in question, which is several times larger than Earth, orbits GJ1214, a cool dwarf star in the constellation Ophiuchus. Scientists used the transit method to take a transmission spectrum from GJ1214b’s atmosphere, where starlight filters through a planet’s atmosphere and betrays its chemical composition from its absorption profile “fingerprints”. This is the first-ever planetary transit performed using the WFC3 since it was installed on Hubble in 2009 on the last-ever service mission by a space shuttle. Space-based observations are a boon for this sort of work, as ground-based telescopes have to contend with interference from Earth’s own atmospheric composition when gathering spectra.

Although the close proximity of GJ1214 is another advantage for studying its planet, trying to discern the exact make-up of a planetary atmosphere is still extremely challenging. One of the big things the scientists of this latest study had to contend with was the star’s own water content; but not entirely for obvious reasons. Molecules of H₂O in the stellar atmosphere absorb light in the near-infrared region that the scientists based their observations in. This causes a perceived darkening around the edges, called limb-darkening, and serves as a way of measuring a star’s radius, and by inference, the planet’s too.

An inaccurate account of limb-darkening would introduce unwanted H₂O noise into the planet’s transmission spectrum and also give large uncertainties in the radii of both objects. With high-quality spectra, the limb-darkening could be ascertained and would not be an issue. With the data that they had, however, the scientists had to compromise a little and let their limb-darkening calculations vary slightly. As it happened, they saw mysterious flat ‘cut-offs’ in the spectra in certain wavelengths. What could be the cause?

Previous studies have suggested that GJ1214b’s radius (around 2.7 times that of Earth) could be due to the presence of a hydrogen gas envelope surrounding a dense rocky-icy core, ‘bulking up’ the planet’s overall size. Thick, high-altitude clouds could be masking some of the features in the spectra. But these clouds would need to be made of dense particles and be expansive in size. In short, it is difficult to support this scenario from the evidence.

The team conclude then that GJ1214b has a dense atmosphere that clings tightly to the planet, which would give a similar cut-off by scattering certain wavelengths. This would negate a bulky, but low-density, atmosphere and a high-density core. The inference is that a high amount of water by mass (whose density is in-between the two) of between 50—70-per cent is the best fit when compared with established models. The scientist suggest further study, particularly with the James Webb Space Telescope – a project recently saved from a funding cull after a sustained campaign by scientists, students and educators.

Involved in the study were the Harvard-Smithsonian Center for Astrophysics, the University of California, the Space Telescope Science Institute, the Smithsonian Astrophysical Observatory, the SETI Institute/NASA Ames Research Center, the Centre de Recherche Astrophysique de Lyon and the Institut fur Astrophysik, George-August-Universit.