Atmospheres for 'hot Jupiters' drier than expected

Jupiter-class extrasolar planets orbiting close to their host stars are hot. But it's a dry heat – unexpectedly dry.

That could be a problem for current theories of planet formation, which suggest that these planets should host far more water in their atmospheres than they do, according to a team of scientists that has measured the abundance of water in the atmospheres of three so-called hot Jupiters.

The issue has important implications in an era when astronomers no longer are content to merely find new planets but are developing approaches and instruments to study them in greater detail. Theories of planet formation establish expectations that often underpin the design of instruments used to study such planets, notes Nikku Madhusudhan, a Cambridge University astrophysicist who led the team making the observations.

The ability to detect water – a necessary ingredient for hosting organic life – in the atmospheres of planets beyond the solar system is hard enough with hot Jupiters, which are large enough and hot enough for the telltale signs of water to be detected from Earth orbit.

The problem grows more difficult when trying to spot the signs of water on Earth-mass planets in a sunlike star's habitable zone. The problem becomes even more challenging if the amount of water such planets have is far less than planet-formation theories predict.

As for the James Webb Space Telescope, currently set for launch in October 2018, "no one has actually planned its instruments thinking that there would be less water than we expect," Dr. Madhusudhan says.

But for studying hot Jupiters, that's not necessarily a problem.

If the Hubble Space Telescope can make the measurements, the more-capable James Webb certainly will be able to as well, and with greater precision. But Earth-scale planets, especially at Earth-like distances from their host stars, will be difficult even for the James Webb telescope to study even if the abundance of water on such planets were to meet current theoretical expectations, Madhusudhan says.

If the abundances are substantially less than theories predict, as a growing body of evidence suggests for some of these planetary systems, fully characterizing the atmospheres of these planets could await yet another generation of space telescopes.

The general picture of planet formation, based on observations in our solar system and modeling, holds that bits and pieces of dust and debris in the disk of dust and gas surrounding a young star bump and gather until a chunk of material grows large enough to gravitationally capture yet more debris. For gas giants, at some point, this core of planet-to-be grows large enough that accretion goes into overdrive, allowing the burgeoning planet to sweep up yet more debris, gas, and ices.

The theory also predicts that the relative abundance of chemical elements in the gas giant's atmosphere should be higher than the relative abundance of the same elements in the host star.

Previous, less-precise measurements of water in the atmospheres of hot Jupiters hinted that the water present was below the so-called solar abundance.

Madhusudhan and colleagues used the Hubble and Spitzer Space Telescopes to hunt for water in the atmospheres of three planets – WASP-12b, HD 189733b, and HD 209458b, which are in systems that range from 60 to 900 light-years away.

All showed water concentrations well below solar abundances. For HD 209458b, the measurements were the most precise to date – between 5 percent and 0.7 percent of solar abundance. The measurements were only 1 percent of solar abundance for WASP-12b and about 0.7 percent for HD 189733b, although the scientists acknowledge that for these two planets, their measurements were somewhat less precise and so the abundance of water on each could be closer to the solar abundance.

The results have been accepted for publication in The Astrophysical Journal Letters.

The results were surprising on two counts, the researchers say. The abundances were well below theoretical predictions. And water is predicted to be the most abundant molecule in a planetary atmosphere sporting the same abundances as the sun and at the temperatures encountered for these three planets. If these objects are well short of solar abundance for a molecule that should be dominant, are these planets' atmospheres also falling well short of other types of molecules?

The team explored explanations for the dearth of water in these planets' atmospheres – obscuring clouds and haze high in the planets' atmospheres, for instance. But at least for now, they fell short on physical grounds.

As a theoretician himself, Madhusudhan says that the solution to the discrepancy probably rests with ideas about planet formation, based on these data and some he's seen that are yet to be published.

He and his team at Cambridge "are working on several scenarios of planet formation that can make hot Jupiters close to their stars that explains the low abundance that we see."