No Water, Please: The Case of the Surprisingly Dry 'Hot Jupiters'

This is an artistic illustration of the gas giant planet HD 209458b in the constellation Pegasus. The planet was recently studied by astronomers alongside two other similar extrasolar worlds, for their abundance in water. To the surprise of astronomers, they have found much less water vapor in the planets' atmospheres than standard planet-formation models predict. Image Credit: NASA, ESA, G. Bacon (STScI) and N. Madhusudhan (UC)

This is an artistic illustration of the gas giant planet HD 209458b in the constellation Pegasus. The planet was recently studied by astronomers alongside two other similar extrasolar worlds, for their abundance in water. To the surprise of astronomers, they have found much less water vapor in the planets’ atmospheres than standard planet-formation models predict. Image Credit: NASA, ESA, G. Bacon (STScI) and N. Madhusudhan (UC)

An international team of astronomers which examined the atmospheres of three “hot Jupiters” for their abundances in water vapor found them to be much drier than previously thought, challenging the established theoretical models of planetary formation and evolution.

As reported in previous AmericaSpace articles, exoplanetary research during the last decade has been steadily advancing from the stage of the simple discovery of exoplanets to that of the detailed observation of their atmospheres with the help of various ground- and space-based observatories, utilising a technique known as transmission spectroscopy. More specifically, when an extrasolar planet happens to cross or transit the face of its star as seen by our line of sight here on Earth, it causes a small dip in the star’s brightness which is proportional to the size of the exoplanet itself. If that planet also happens to have an atmosphere, the latter will absorb some of the star’s light in certain wavelengths as it transits, resulting in a wavelength-dependent transit depth, better known as a transmission spectrum. By studying this spectrum of the combined star-planet light, astronomers can extract detailed information about the planet’s atmosphere, like its chemical composition, temperature, density, and overall dynamics.

Space-based observatories, like NASA’s Spitzer and Hubble Space Telescopes, have greatly contributed to this emerging field of exoplanet study in recent years, by allowing astronomers to obtain the transmission spectrum of dozens of extrasolar planets, ranging from “Super Earths” to “hot Jupiter” gas giants. These observations have helped researchers to put constrains on the elemental abundances of their planetary atmospheres, providing valuable insights into the internal processes and formation conditions of these alien worlds. Hot Jupiters, in particular, being the first class of exoplanets ever to be discovered, have also been the most well-studied. Providing an interesting contrast to the Jovian planets of our Solar System like Jupiter and Saturn, which orbit the Sun in very wide, long-period orbits, hot Jupiters lie very close to their respective stars in such short orbits that are considerably smaller than Mercury’s orbit around the Sun. These orbital properties have provided astronomers with the opportunity to study the physical and chemical properties of Jovian-type planets in such extreme temperatures that are completely different from those of the gas giants in own Solar System.

In a new paper published in the The Astrophysical Journal Letters, a research team, led by Dr. Nikku Madhusudhan of the Institute of Astronomy at the University of Cambridge, England, presents the results of their study of the transmission spectra of three hot Jupiters that had been taken in near-infrared wavelengths with the Wide Field Camera 3, or WFC3, on NASA’s Hubble Space Telescope. The three exoplanets, named HD 189733b, HD 209458b, and WASP-12b, have been studied in great detail during the last decade, representing a number of great milestones in exoplanetary research. For instance, HD 189733b, which is the closest transiting hot Jupiter to Earth, lying 63 light-years away in the constellation Vulpecula, was the first exoplanet to have its color detected and the first on which carbon dioxide was found. HD 209458b, which orbits a Sun-like star located 154 light-years away in the constellation Pegasus, was the first transiting extrasolar world ever to be discovered and the first that was found to have an atmosphere, whereas WASP-12b, located 871 light-years away, has one of the lowest densities among exoplanets.

Although previous searches had already determined the presence of water in the atmospheres of all three exoplanets, the goal of Madhusudhan’s team was to more accurately constrain its overall abundance due to the widely varied temperatures that are exhibited on these worlds, which could prove useful to the study of the formation and evolution of the Jovian planets in our Solar System as well. “Atmospheric elemental abundances of Solar System giant planets have led to important constraints on the origins of the Solar System,” write the researchers in their study. “The observed super-solar enrichments of carbon, sulfur, nitrogen, and inert gases, support the formation of Jupiter by core-accretion. However, the oxygen abundance of Jupiter is yet unknown. The upper atmosphere of Jupiter has a temperature that is lower than 200 K, causing water to condense and to be confined to the deepest layers, requiring dedicated probes to measure it … The oxygen to hydrogen and carbon to oxygen ratios are easier to measure for hot giant exoplanets than they are for Solar System giant planets. The vast majority of extrasolar gas giants known have equilibrium temperatures of 1,000 to 3,000 K, thus hosting gaseous H2O in their atmospheres accessible to spectroscopic observations … The motivation behind choosing these three planets is twofold. Firstly, the planets represent transiting ‘hot Jupiters’ over a wide range of irradiation: HD 189733b (1200 K) is one of the coolest transiting ‘hot Jupiters’ known and WASP 12b (2500 K) is amongst the hottest. Secondly, of all the ‘hot Jupiters’ that have been observed using transmission spectroscopy with HST WFC3, these planets have the best spectroscopic precision. Consequently, the goal is to conduct a homogenous estimation of water abundances in a diverse sample of ‘hot Jupiters’ using the best observations with the same instrument.”

This graph compares observations with modeled infrared spectra of three hot-Jupiter-class exoplanets that were spectroscopically observed with the Hubble Space Telescope. The red curve in each case is the best-fit model spectrum for the detection of water vapor absorption in the planetary atmosphere. The blue circles and error bars show the processed and analyzed data from Hubble's spectroscopic observations. Image Credit: NASA, ESA, N. Madhusudhan (University of Cambridge), and A. Feild and G. Bacon (STScI)

This graph compares observations with modeled infrared spectra of three hot-Jupiter-class exoplanets that were spectroscopically observed with the Hubble Space Telescope. The red curve in each case is the best-fit model spectrum for the detection of water vapor absorption in the planetary atmosphere. The blue circles and error bars show the processed and analyzed data from Hubble’s spectroscopic observations. Image Credit: NASA, ESA, N. Madhusudhan (University of Cambridge), and A. Feild and G. Bacon (STScI)

To that end, Madhusudhan’s team conducted a Bayesian statistical analysis to see how well the Hubble observations fitted with established theoretical models of elemental abundances of planetary atmospheres. According to the leading theory of planetary formation by core accretion, gas giant planets should be composed of the same chemical elements that were part of the primordial stellar nebula that formed them, albeit in more enhanced proportions. In addition, water should be the most abundant molecule at the high temperatures exhibited by hot Jupiters. Yet the researchers’ results showed that the observed water abundances were much lower than those predicted by theory. “Our water measurement in one of the planets, HD 209458b, is the highest-precision measurement of any chemical compound in a planet outside our Solar System, and we can now say with much greater certainty than ever before that we’ve found water in an exoplanet,” says Madhusudhan. “However, the low water abundance we have found so far is quite astonishing.” The spectra of the other two exoplanets in the study showed similar results. “Our estimate of the H2O abundance in HD 189733b is the most stringent for this planet to date, and is lower compared to previous estimates using other transmission datasets,” writes Madhusudhan’s team. “The observational uncertainties [for WASP-12b] are larger compared to those of HD189733b and HD 209458b, because the latter two planets orbit much brighter host stars which lead to higher photon fluxes and hence better precisions. Consequently, our constraint on the H2O abundance ofWASP-12b is much less precise … [Yet], a potentially low H2O abundance in the atmosphere of WASP-12b is consistent with previous studies.”

One possible explanation, according to Madhusudhan’s team, that could account for the observed apparent dryness of hot Jupiters is the presence of global layers of clouds or hazes in their atmospheres that obscure the water signatures from appearing in their spectra. Yet, as the researchers point out, the low concentration of atmospheric water vapor was observed on all three exoplanets, while HD 209458b which has a higher temperature than HD 189733b exhibited even lower amounts of water than the latter. Since the chance for clouds according to the authors should increase with decreasing temperature, then HD 189733b should have been the one displaying a lower abundance in water vapor. Furthermore, based on the planets’ composition, the cloud layers should be positioned at very high altitudes where atmospheric pressure would be too low to sustain them. “If clouds are indeed responsible for our low estimates of H2O abundances, our results stress the need for rigorous theoretical efforts to explain several challenges in the cloud hypothesis,” concludes Madhusudhan’s team.

Vertical structure of Jupiter’s atmosphere and clouds, as predicted by theoretical models and remote observations (right), compared to observations according to NASA's Galileo entry probe’s nephelometry experiment (left). Cloud opacity is denoted by the yellow curve. Were these measurements representative of a local spot, or of the planet's entire atmosphere? Image Credit: Galileo Probe Project website/NASA

Vertical structure of Jupiter’s atmosphere and clouds, as predicted by theoretical models and remote observations (right), compared to observations according to NASA’s Galileo entry probe’s nephelometry experiment (left). Cloud opacity is denoted by the yellow curve. Were these measurements representative of a local spot, or of the planet’s entire atmosphere? Image Credit: Galileo Probe Project website/NASA

An alternative explanation, according to the researchers, is that gas giant planets form differently than what theoretical models predict, with the original planetesimals in the primordial stellar nebula accreting substantially less water than previously thought. As the researchers point out in their study, direct measurements of Jupiter’s atmosphere that were taken with the atmospheric entry probe of NASA’s Galileo spacecraft in the 1990’s similarly revealed lower than expected water abundances. While these results had been explained at the time as being a localized phenomenon of a uniquely dry spot that the entry probe happened to plunge through, Madhusudhan’s team argues that nevertheless the possibility exists that Jupiter might have formed from planetesimals that were dominated by tar instead of water. “It basically opens a whole can of worms in planet formation,” says Madhusudhan. “We expected all these planets to have lots of water in them. We have to revisit planet formation and migration models of giant planets, especially ‘hot Jupiters’, and investigate how they’re formed.”

Despite their puzzling nature, these results showcase the tremendous value of exoplanetary spectroscopy, not only for the understanding of the formation and evolution of these many and widely different and fascinating worlds in those other parts of the galaxy, but also for the understanding of our own corner of the Milky Way and the processes that gave rise to the beautiful Solar System that we are all part of. “There are so many things we still don’t know about exoplanets, so this opens up a new chapter in understanding how planets and Solar Systems form,” says Dr. Leo Drake Deming, a professor at the University of Maryland’s Astronomy Department who led the spectroscopic study on HD 209458b in 2013, on which the results of Madhusudhan’s team were based.

As is always the case in the study of the Universe, the more things we learn about the Cosmos, the more we learn about ourselves.

 

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