Case Studies

One of the most famous super-Earths/mini-Neptunes, designated GJ 1214b, was discovered by the ground-based MEarth project (pronounced “mirth”) in 2009, even before Kepler entered the science phase of its mission. GJ 1214b is a small, gassy world 2.6 times the size of Earth but just a third of its density, and at the time it was thought to be a rarity in exoplanets. Given its mass and diameter, this planet could potentially consist of a rocky core with a hydrogen-helium atmosphere hundreds of kilometers deep, or it could be a world covered in a deep ocean and atmosphere of steam.

To find out what this puffy little planet is made of, astronomers have scrutinized it with numerous ground- and space-based telescopes. These measurements show that the planet’s atmosphere is very effective at blocking its star’s light. Observers expected to find strong spectral fingerprints from water vapor in the atmosphere, but instead they saw a complete absence of any such features. The absence of such features doesn’t necessarily mean water is absent, but the question is: what could be blocking starlight so uniformly across all wavelengths? The answer: clouds.

Clouds are prevalent around all bodies in our solar system that have an atmosphere. They probably are just as prevalent on exoplanets and challenge our ability to characterize them. In some cases, clouds can help us study the dynamics and temperature of an atmosphere, but their opacity makes it difficult to accurately determine the abundances of other compounds in a planet’s air.

To really understand a planet’s composition—and thus to determine if it belongs in the “rocky” or “gassy” category—we need to measure the amounts of different materials in its atmosphere. Our best approach is to look for gaseous materials in a planet’s atmosphere—since diffusion would distribute these materials equally everywhere. By identifying a molecule and—where possible—its abundance, we can understand more: the planet’s energy budget (how it deals with incoming and outgoing radiation), its chemistry (what reactions are occurring and whether there’s more or less of something than we’d expect), and its dynamics (how all of these interact with one another). Each of these supplies clues to the formation and evolution of the planet.

Water is the most useful of these molecules. Water is the third-most abundant molecule in the universe, trailing only molecular hydrogen (H 2 ) and carbon monoxide (CO). It is very stable and exists in all phases in a vast array of astrophysical contexts. Of course, it is essential for life as we know it. Water’s abundance remains relatively constant under equilibrium conditions at all temperatures (as long as more oxygen than carbon is present), so it’s likely to be well mixed throughout hot, giant-planet atmospheres. The Hubble Space Telescope makes it relatively easy to detect water vapor in the atmospheres of giant exoplanets, resulting in its identification in 80 percent of the planets that we’ve observed. However, determining water’s abundance is still difficult: only 5 percent of Hubble’s measurements yield definitive constraints on how much water is present.

Here in our own solar system, there’s an interesting trend among the outer planets: an atmosphere’s proportion of heavy elements increases as the planet’s mass decreases. For example, compared to their relative abundance in the Sun, Jupiter has 4 times more heavy elements, Saturn has 10 times, and Neptune and Uranus have about 100 times. If we use water vapor in an exoplanet’s atmosphere to estimate the abundance of oxygen (and thus all heavy elements), then we can determine whether this trend applies in other planetary systems.