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‘Hot Jupiters’: Drier Than Expected

Be aware of Open Source, a radio show on Boston’s WBUR that last week did a show about exoplanets and the possibility of extraterrestrial life. Earth 2.0 is available online, featuring David Latham (Harvard-Smithsonian Center for Astrophysics), Dimitar Sasselov (Harvard University), Jason Wright (Penn State) and Sarah Rugheimer (a PhD student at Harvard studying exoplanet atmospheres). The discussion ranges through the Kepler mission to the Fermi question and recent studies of exoplanet atmospheres, the latter particularly appropriate to today’s post.

For I want to talk today about ‘Hot Jupiters’ and their atmospheres, and what we can learn about planet formation by studying their composition. Hot Jupiters were a surprise when first discovered, but models of planetary migration seemed to explain them. We would expect a gas giant to form at or beyond the ‘snow line,’ where volatiles like water would form ice grains. As we saw in our discussion of Kepler-421b (see Transiting World at the Snow Line), planetary embryos that become gas giants should coalesce in this low temperature regime, with the resulting worlds richer in ice and water than the drier inner Solar System, which relies on volatile delivery by impacting comets or other objects with a formation history in the outer system.

Planetary migration is a way of getting those ‘hot Jupiters’ where they have been observed to be. We assume gravitational interactions with other young worlds that drive some gas giants into the inner system, taking a planet that has formed in the cold regions beyond the snow line into close proximity to the parent star. It would be reasonable to assume high water content in these worlds, but new work led by Nikku Madhusudhan (University of Cambridge, UK) comes up with a surprisingly different result.

Madhusudhan and team used near-infrared spectra of hot Jupiters observed by the Hubble Space Telescope, whose position in space allows accurate measurement of water in an exoplanetary atmosphere because it is far above contaminating water in the Earth’s own atmosphere. The method is transmission spectroscopy, in which some of the star’s light passes through the atmosphere of a planet in transit across its face as seen from Earth. The spectrum that results tells us much about the molecules in the atmosphere, but the researchers are finding only a small fraction of the water predicted by standard planet formation models.

Madhusudhan calls the result ‘astonishing,’ and adds:

“It basically opens a whole can of worms in planet formation. 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.”


Image: 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. Credit: NASA, ESA, N. Madhusudhan (University of Cambridge), and A. Feild and G. Bacon (STScI).

The planets in question are HD 189733b, HD 209458b, and WASP-12b, with temperatures ranging from 800 to 2200 degrees Celsius. The water measurement of HD 209458b is the highest-precision measurement of any chemical compound in an exoplanet, and while it does find water, the low abundance creates problems for core accretion scenarios of planet formation beyond the snow line. Is our Solar System unusual in its high water content?

One thing to remember is that exoplanets are, in certain respects, easier for us to measure than some of the worlds in our own system. We know little about the constituents of the planetesimals that formed our own gas giants. The paper explains this seeming paradox while also pointing to an upcoming space mission that can help (internal references omitted for brevity):

Atmospheric elemental abundances of solar-system giant planets have led to important constraints on the origin of the solar system. The observed super-solar enrichments of C, S, N, 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 (P < 1 bar) has T < 200 K, causing water to condense and to be confined to the deepest layers (> 10 bar), requiring dedicated probes to measure it. The upcoming Juno mission to Jupiter aims to measure its O abundance, which is important to estimate the amount of water ice that was available in the planetesimals forming Jupiter and the rest of the solar system.

So Juno should be able to give us a better read on Jupiter’s oxygen, thus helping us better understand the kind of planetesimals that formed in our system’s earliest days. As to the measurements of exoplanets vs. planets closer to home:

The O/H and C/O 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 ~1000-3000 K, thus hosting gaseous H2O in their atmospheres accessible to spectroscopic observations.

HD 189733b, HD 209458b, and WASP-12b are good choices because they range widely in temperature, with HD 189733b being one of the ‘coolest’ hot Jupiters known, and Wasp 12b one of the hottest. On the matter of equilibrium temperature (Teq), I’m drawing on Sara Seager’s book Exoplanet Atmospheres: Physical Processes (Princeton, 2010), which explains that equilibrium temperature is the temperature attained by an isothermal planet after it has attained complete equilibrium with the radiation from the star it orbits. The Madhusudhan paper adds that these hot Jupiters have the best spectroscopic precision of all the hot Jupiters that have been observed using the transmission spectroscopy technique.

So we have high-quality results that have the researchers looking at various scenarios to explain low water abundances. The paper adds that the Galileo probe reported a low H20 abundance in Jupiter that was explained by saying the probe moved through an unusually dry region. But at least one alternative explanation came in a 2004 paper suggesting that Jupiter may have formed by planetesimals dominated by tar rather than water ice. The Madhusudhan results reawaken such questions and cause us to look anew at formation and migration models for all giant planets.

The paper is Madhusudhan et al., “H2O abundances in the atmospheres of three hot Jupiters,” The Astrophysical Journal Letters Vol. 791, No. 1 (2014) L9 (abstract / preprint). On carbonaceous matter in the formation of Jupiter, see Lodders, “Jupiter Formed with More Tar than Ice,” The Astrophysical Journal Vol. 611, No. 1 (2004), 587 (abstract).


Comments on this entry are closed.

  • Tom Mazanec July 29, 2014, 16:52

    Could the hot Jupiters have been formed in the inner system after all?

  • Ross Turner July 29, 2014, 20:10

    I was wondering the same thing, could the hot Jupiters have formed in place, or closer to the star than expected?

  • Rob Flores July 29, 2014, 23:53

    Have astronomers not considered that the solar nebula in some cases are filled with heavier compounds than H2O . And that if they are dense enough
    (for a short period) to act as buffer they may allow larger planets to form
    closer in, in semi-rare occurences. Maybe the solar nebula composition
    is more important than where the snow line is.
    I guess this is good news as far as terrestrial planets is concerned.
    Fewer Jupiters trapaising through a stars HZ may mean fewer
    torn apart planitesimals that could grow into rocky worlds.
    Once again I lament that we cannot definitively say Earth sized planets
    are common at distances where temperatures would allow liquid water,
    except by extrapolation from Kepler data.

  • Brian Swiderski July 30, 2014, 0:01

    Maybe hot Jupiters are just really weird. People seem to be assuming there’s some deeper significance to them, but the only reason they’re so significant is that they’re so easy to find. Maybe they’ll turn out to be very unusual, and explainable by unlikely events like close encounters between stars perturbing gas giants.

  • Ronald July 30, 2014, 7:53

    Tom: good question. However, that would require a very massive protoplanetary dust disk, unlikely with given metallicities.

    “Galileo probe reported a low H20 abundance in Jupiter (…). But at least one alternative explanation came in a 2004 paper suggesting that Jupiter may have formed by planetesimals dominated by tar rather than water ice”.

    Well, if that is indeed the case, i.e. even our own Jupiter is low in water, whereas we do have plenty of it here on Earth, then this may bode well after all for those other planetary systems as well: a water-poor gas giant does not necessarily mean dry terrestrial planets.

  • kzb July 30, 2014, 8:00

    Surely the water in the upper atmosphere is destroyed by uv photolysis?

  • Ronald July 30, 2014, 9:19

    Rob Flores: “the solar nebula in some cases are filled with heavier compounds than H2O . And that if they are dense enough
    (for a short period) to act as buffer they may allow larger planets to form
    closer in, in semi-rare occurences. Maybe the solar nebula composition
    is more important than where the snow line is.”

    Good point! In our solar neighborhood and probably around solar type stars in general, the most common heavier (than Helium) elements, i.e. making up metallicity, are O, C, Fe, N, Si, Mg, S, and to a lesser extent Ni, CA. Al.

    Maybe, if certain heavier elements, such as Fe, Mg, Si, are more abundant, then this may lead to giant planet accretion closer in.

    Does anybody know more about this?

  • Harry R Ray July 30, 2014, 9:52

    The problem astronomers have had to overcome in getting ANY KIND OF GOOD SPECTRA from Hot Jupiters has been that most of the atmospheres of these planets seem to be COMPLETELY OBSCURED by haze layers! This has APPARENTLY been resolved (at least in the case of HD209458)! But, could the apparent lack of water attributable to noy only obscuration, but also ABSORBTION of H2O by the haze layeres? The extreme DIFFERENCE between the values of these three planets leads me to believe that THIS is the case, instead of a more UNIFORM explanation, like tars instead of ice (or the above mentioned water distruction by UV photalysis).

  • Alex Tolley July 30, 2014, 11:48

    @kzb – but if the hot Jupiters had plenty of water, wouldn’t that water replace that lost, just like we see on Earth?

    I’m wondering if the water is removed by a chemical reaction? The reaction equilibrium would need to be strongly in favor of the products, and possibly the products would need to be denser, so that the reverse reaction would result in the water being liberated deep in the atmosphere, removing it from detection.

  • kzb July 30, 2014, 13:43

    @Alex Tolley: I guess they would have included photolysis in their model, but I can’t see it in the paper.
    These hot Jupiters are having their atmospheres blasted away at millions of tonnes per year. The Earth loses a few kg of hydrogen per year, originating from photolysis of water in the very top of the atmosphere.

    Here’s an idea: photolysis splits H2O, and the hydrogen escapes. The oxygen radical combines with the metals to form oxides and sinks. These planets are hot enough to have metal vapours in the atmosphere.

  • Michael July 30, 2014, 15:37

    Could the water be tied up in the dust as hydrates and be held lower down or be hiding on the colder sunless side of the tidally locked planets. Also my thoughts on hot Jupiter’s is that they form further out and follow the mass of dust and gas streamers that fall into the sun.

  • Dave Moore July 30, 2014, 17:09

    I’m not surprised at the dryness of these planets. Above 900 deg C , Methane and Water are more thermodynamically stable as CO and H; therefor, most of the planet’s atmospheric oxygen would be tied up in CO. Excess of Oxygen would form water, but the amount would depend on the Carbon to Oxygen ratio of the planet.

    Wasp-12b appears to be a planet with a high Carbon to Oxygen ratio.

    Madhusudhan produced this paper too, and from reading from the summary, it almost contradicts the above paper. I would really like to know how he squares the two of them.

  • kzb July 31, 2014, 7:34

    David Moore: your theory definitely has something. I was a bit worried that free hydrogen would rapidly mop up oxygen and re-form H2O. But then you think, free hydrogen would be lost rapidly from the upper atmosphere, and the loss would be greater than for other elements.

  • Holger July 31, 2014, 8:10

    @kzb: “Here’s an idea: photolysis splits H2O, and the hydrogen escapes. The oxygen radical combines with the metals to form oxides and sinks. These planets are hot enough to have metal vapours in the atmosphere.”

    A Jupiter-sized planet is massive enough that hydrogen doesn’t escape (unless it’s _extremely_ hot), and anyway its atmosphere contains more than enough hydrogen to re-produce all photolysed water. Iron only vaporizes at 2900 deg C at standard pressure, and the boiling point is even higher at the extreme pressures inside a gas giant.

  • Dave Moore July 31, 2014, 12:52

    Photolysis has nothing to do with it. This is a matter of thermodynamic equilibrium. Below 900 deg C, Carbon and Oxygen form Methane and Water in a Hydrogen atm. Above this temperature, they preferentially form CO and Hydrogen.

    If your hot gas giant is from an Oxygen rich nebula, the surplus Oxygen will then combine with the Hydrogen to form water vapor. If your hot gas giant is from a Carbon rich nebula, all the Oxygen will be mopped up as CO.

    Note that these planet’s atmospheres are dynamic systems and as a result are not always in thermodynamic equilibrium.

    Here’s another interesting site on hot gas giant atmospheres showing how their albedo and color change with temperature.


  • kzb July 31, 2014, 13:55

    Holger: I don’t know about the particular planets in question, but at least some of these hot Jupiters are losing mass at a spectacular rate. If they are losing atmosphere, the lighter gases (e.g hydrogen) must be enriched in the lost material.

    I’m sure also I heard that some metals were present in the atmospheres also. However I now think the reaction of water with carbon-containing gases followed by stripping of hydrogen is sufficient.

  • Holger August 1, 2014, 9:53

    @kzb: I think this is only true for the very hottest hot Jupiters, which are about to be swallowed by their star. (Traces of metal may always be present in the atmosphere, but not enough to rival the hydrogen.)
    A hot Jupiter that loses all its hydrogen would probably no longer be Jupiter-sized, but rather a chtonian planet – e.g. Jupiter is 70% hydrogen by mass, and even more by volume.

  • ljk September 12, 2014, 8:30

    ‘Hot Jupiters’ provoke their own host suns to wobble

    September 11, 2014

    Full article here: