Clouds and Haze Solve Missing Water Mystery of Hot Jupiter Exoplanets

Summary: A Dec. 14 study in Nature finds clouds and haze solve the missing water mystery of hot Jupiter exoplanets, with clouds weakening water signatures.

artist’s impression of the 10 hot Jupiter exoplanets studied with the Hubble and Spitzer space telescopes; planetary colors are for illustration purposes only, although HD 1897336 is known as the “blue planet”; image addition date 2015-12-14; image credit NASA/ESA: May be used for any purpose without prior permission, via NASA Jet Propulsion Laboratory

A study published online Dec. 14, 2015, in Nature finds clouds and haze as clues to the missing water mystery of hot Jupiter exoplanets.

Hot Jupiters are extrasolar gas giants, similarly sized to Jupiter, but differing in close orbits around host stars and high surface temperatures.

All 10 hot Jupiter exoplanets have orbits that favorably place them in front of their host star, as viewed from Earth. Whenever the hot Jupiter travels in front of its host star, starlight passes through the exoplanet’s outer atmosphere.

Co-author Hannah Ruth Wakeford, presently a NASA postdoctoral fellow at Goddard Space Flight Center, explains: “The atmosphere leaves its unique fingerprint on the starlight, which we can study when the light reaches us.”

The starlight-borne, atmospheric fingerprint conveys signatures of various elements and molecules. Signatures, especially of water molecules, allow the team to identify the 10 hot Jupiter exoplanets’ atmospheres as clear or cloudy. Clear hot Jupiters have strong water signals whereas cloudy, hazy hot Jupiters reveal weak water signals.

The study’s abstract summarizes: “Our results reveal a diverse group of hot Jupiters that exhibit a continuum from clear to cloudy atmospheres. We find that the difference between the planetary radius measured at optical and infrared wavelengths is an effective metric for distinguishing different atmosphere types. The difference correlates with the special strength of water, so that strong water absorption lines are seen in clear-atmosphere planets and the weakest features are associated with clouds and hazes.”

The 10 hot Jupiters selected for the study are HAT-P-1b, HAT-P-12b, HD189733b, HD209458b, WASP-6b, WASP-12b, WASP-17b, WASP-19b, WASP-31b, WASP-39b.

An international team of 21 astronomers observe 10 hot Jupiter exoplanets in the largest study ever conducted of this special class of exoplanets. Multiple observations through the NASA/ESA Hubble Space Telescope and NASA’s Spitzer Space Telescope allow the team to study the 10 hot Jupiter exoplanets across a range of wavelengths, from ultraviolet, at 0.3 micrometers, to mid-infrared, at 4.5 micrometers.

Co-author Jonathan Fortney, an astrophysics professor at the University of California, Santa Cruz, explains the solving of the missing water mystery: “The alternative to this is that planets form in an environment deprived of water -- but this would require us to completely rethink our current theories of how planets are born.” Fortney emphasizes that “Our results have ruled out the dry scenario, and strongly suggest that it’s simply clouds hiding the water from prying eyes.”

HST/Spitzer transmission spectral sequence of hot Jupiter survey targets (upper right); Pressure-Temperature profiles and condensation curves (middle right); Transmission spectral index diagram(lower right): Hannah Wakeford @StellarPlanet, via Twitter Dec. 14, 2015

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:

artist’s impression of the 10 hot Jupiter exoplanets studied with the Hubble and Spitzer space telescopes; planetary colors are for illustration purposes only, although HD 1897336 is known as the “blue planet”; image addition date 2015-12-14; image credit NASA/ESA: May be used for any purpose without prior permission, via NASA Jet Propulsion Laboratory @ http://www.jpl.nasa.gov/spaceimages/details.php?id=pia20056;
"PIA20056: Artist's Impression of 'Hot Jupiter' Exoplanets," May be used for any purpose without prior permission, via NASA JPL Photojournal @ https://photojournal.jpl.nasa.gov/catalog/PIA20056

HST/Spitzer transmission spectral sequence of hot Jupiter survey targets (upper right); Pressure-Temperature profiles and condensation curves (middle right); Transmission spectral index diagram(lower right): Hannah Wakeford @StellarPlanet, via Twitter Dec. 14, 2015, @ https://twitter.com/StellarPlanet/status/676451317154455552

For further information:

Clavin, Whitney, Rob Gutro and Ray Villard. "Missing Water Mystery Solved in Comprehensive Survey of Exoplanets." NASA > Features. Dec. 14, 2015.
Available @ https://www.nasa.gov/feature/goddard/missing-water-mystery-solved-in-comprehensive-survey-of-exoplanets

GeoBeats News. "NASA: Water May Be Hiding in Exoplanets' Clouds." YouTube. Dec. 15, 2015.
Available @ https://www.youtube.com/watch?v=OwVkXIT2NNA

Hannah Wakeford‏ @StellarPlanet. "#Exoplanet missing water mystery solved with comprehensive #HJsurvey @HUBBLE_space #Spitzer." Twitter. Dec. 14, 2015.
Available @ https://twitter.com/StellarPlanet/status/676451317154455552

Sing, David K. "A continuum from clear to cloudy hot-Jupiter exoplanets without primordial water depletion." Nature International Weekly Journal of Science, vol. 529 (Jan. 7, 2016): 59-62. DOI: 10.1038/nature16068
Available @ http://www.nature.com/nature/journal/vaop/ncurrent/full/nature16068.html?WT.mc_id=TWT_NatureMagazine