The enigma of hot Jupiters, or exoplanets that are slightly larger and warmer than the gas giants found in our solar system—yet appear to have less water than they should based known concepts of planetary formation—may have been solved in a newly-published Nature study.
In the paper, astronomers from the US, UK, and France used NASA’s Hubble and Spitzer space telescopes to study 10 different hot Jupiters. They found that they appear to be oddly dry because thick clouds and haze are preventing the water from being detected.
Gas giant exoplanets that only had a faint water signal were covered by such clouds, the study authors explained in a statement. However, the scientists reported that they were able to locate evidence that H2O was present on those hot Jupiters that lacked such cloud cover. In short, the water molecules do exist on these planets, but on some, they are obscured from detection.
“Our results suggest it’s simply clouds hiding the water from prying eyes, and therefore rule out dry hot Jupiters,” explained University of California, Santa Cruz astronomy professor and study co-author Jonathan Fortney. “The alternative theory to this is that planets form in an environment deprived of water, but this would require us to completely rethink our current theories.”
It’s not that the H2O isn’t there; we just couldn’t see it
As the Los Angeles Times and the Washington Post reported, scientists have long believed that gas giants would contain an abundance of water, as the elements needed to form molecules of H2O are among the most abundant in the universe and the temperatures would be conducive to its existence. This made the absence of it around hot Jupiters so puzzling.
In theory, the researchers explained, it should be easier to detect water around these exoplanets than around the actual Jupiter, since their proximity to their host stars cause their atmospheres to be heated and well mixed. Thus, water vapor should be evenly distributed throughout the air. In most cases, however, their atmospheres appeared to be drier than they should be.
Fortney and his colleagues analyzed 10 hot Jupiter exoplanets—all of which had orbits in which they pass in front of their host stars (transit)—and found that those that appeared to be lacking in water were the same ones that had clouds formed out of exotic materials such as liquid droplets of iron. This, the authors noted, solves the mystery as to why these exoplanets appear dry.
The next goal, Fortney told the Times, is to use this information to precisely measure the amount of water in each planets’ atmosphere, and to measure the abundance of oxygen in their host stars to see if there is a correlation between the chemical composition of a star and that of its planet.
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Feature Image: ESA/Hubble & NASA
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