Planetary atmospheres in 2010

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An overview of the scientific literature on exoclimes in 2010 (if you notice some glaring omission, give us a shout in the comment section).

 

Detections

Transits

The rate of detection of new transiting planets picked up dramatically in 2010 (48 announcements against 10 in 2009) and kept the overall rise close to Moore’s law of doubling number every 18 months.

The most productive programmes are the small-aperture ground-based WASP and HATNet surveys, with number of detections far into the double-digits (e.g. HAT-P-16, Buchave et al. 2010, HAT-P-20 to P-23, Bakos et al. 2010; WASP-17, Anderson et al. 2010, WASP-19 Hebb et al. 2010). Of particular interest are HAT-P-11, the first Neptune-sized planet found by these surveys (Bakos et al. 2010b) , and WASP-33, a planet confirmed using “line tomography”, a nice trick to get round the fact that the host star rotates too fast for classic radial-velocity confirmation (Collier Cameron et al. 2010).

The Kepler mission announced its first hot Jupiter detections based on the first few weeks of data (Borucki et al. 2010), Kepler-4 (Borucki et al. 2010b), Kepler-5 (Koch et al. 2010), Kepler-6 (Dunham et al. 2010), Kepler-7 (Latham et al. 2010), Kepler-8 (Jenkins et al. 2010) and Kepler-9 (Holman et al. 2010) (Kepler-1 to -3 were planets in the Kepler field already detected from the ground). Of particular interest is Kepler-9, a stunning system with several transiting planets showing the transit timing variations due to gravitational interactions between the planets. This is the first secure detection of transit timing variations. Steffen et al. (2010) show that there are other systems with multiple planet transits in the Kepler data, awaiting confirmation.

The CoRoT mission also announced hot Jupiter detections (e.g. CoRoT-6, Friedlund et al. 2010), and one cool Jupiter (CoRoT-9, Deeg et al. 2010), with a period of 95 days and an equilibrium temperature in the liquid-water range.

Direct imaging

Following last-year epochal discoveries of the first directly imaged planets, a fourth planet was found around HR8799 (Marois et al. 2010), and a planet in the disc of beta Pic (Lagrange et al. 2010).

Radial velocity

Among the usual crop of radial-velocity planet announcements (too numerous to list here and not directly relevant individually to atmosphere studies), the Earth-like planet around GJ581 in the habitable zone was the most striking (Vogt et al. 2010) (***)1. Unfortunately, it takes considerable faith to see the planet signal in the data, a faith that we find hard to share. The predictable media reaction to the news of “Zarmina’s world” was not matched by a proportional enthusiasm in the field. The putative habitable planet is simply invisible in the more precise and more extensive HARPS data for this object.

 

Observations

Emission spectrum

Space

Infrared brightness was measured for at least ten planets with the Spitzer telescope, building up the statistics of day-size atmosphere emission in broad passbands in the 3-8 micron region accessible to the IRAC instrument.

These channels mainly measure two features: the presence or absence of a temperature inversion (the 3.6-4.5 micron channel difference depends on the amplitude and a sign of the water bands), and the overall day-side temperature (indicative of the albedo and the efficiency of day-night heat redistribution). Although the uncertainties on individual measurements are still high and sometimes difficult to control, the present results are striking in showing objects all over the place, with low and high redistribution, and the water feature in absorption, absent or in emission.

Spitzer observations:

Object Channels [microns] Reference [all 2010] Temperature Inversion Redistribution
HAT-P-1 3,4,5,8 Todorov et al. modest low
XO-3 3,4,5,8 Machalek et al. yes
TrES-2 3,4,5,8 O’Donovan et al. probably
HAT-P-7 3,4,5,8+Kepler Christiansen et al. probably probably low
‘209 3,4,5,8 Beaulieu et al. yes
‘189 8 Agol et al. no high
‘606 4 Hébrard et al.
TrES-3 3,4,5,8 Fressin et al. no
GJ436 3,4,5,8,16,24 Stevenson et al.

 

Agol et al. (2010) measured 14 transits and eclipse of ‘189 at 8 microns, in search of time variability (aka. “weather”) of the day-side atmosphere. They placed an upper limit of 10% on such variability.

Hébrard et al. (2010) observed ‘606 for 12 hours around transit, “refining the system parameters” (codewords in the field for not finding anything worth writing home about).

Beaulieu et al. (2010) identify the water feature in ‘209, and Stevenson et al. (2010) get incredibly precise abundances of CO, CO2, H2O and CH4 from the day-side brightness of GJ-436b in six Spitzer channels.

Crossfield et al. (2010) obtained new data at 24 micron for the phase curve of ups And. Remember that this was the first phase curve announced (Harrington et al. 2006), reporting a very high day-night temperature contrast. It turns out that the initial result was wrong, and the temperature contrast is low after all. The amplitude of the phase curve is 1.3+-0.9 x10-3 instead of 2.9+-0.7 x10-3. The difference boils down to a calibration issue with Spitzer.

Fossati et al. (2010) obtained near UV transit spectroscopy of WASP-12, which has a very high atmospheric scale height that make it a good target, looking for atomic lines in the 2500-2800 Å range. No single line was found, but collectively there seems to be a sign of a deeper transit in the lines than outside (*). Saying that this is the detection in an exoplanet atmosphere of “neutral sodium, tin and manganese, and singly ionised ytterbium, scandium, manganese, aluminium, vanadium and magnesium” is stretching it a bit.

Ground

Following the OGLE-TR-56 break-through, ground-based detections of thermal emission for transiting planets are getting more confident.

Planet Passband Instrument Reference [all 2010]
WASP-19 K HAWK-I/VLT Gibson et al.
WASP-12 z’ ARC3.5m Lopez-Morales et al.
TrES-2 Ks CFHT Croll et al.
WASP-19 H HAWK-I/VLT Anderson et al.
TrES-3 K,H CFHT Croll et al.
‘189 2.4-5.2micron IRTF Swain et al.
‘209 2.3micron CIRES/VLT Snellen et al.
GJ1214 0.78-1micron FORS2/VLT Bean et al.

The conclusions from these measurements are diverse and often tentative. The flux of WASP-19 in H is a puzzle and seems to imply an unphysical negative heat redistribution, the Croll et al. (2010) values for TrES-3 conflict with de Mooij et al. (2009).

Swain et al. (2010) find a huge spike in the spectrum at 3.25-micron, identified as a non-equilibrium line of methane, but very suspiciously correlated with the size of the error bars. More precise measurements are clearly needed.

Bean et al. (2010) find no feature in the spectrum of GJ-1214. A cloud-free hydrogen atmosphere should show detectable water features at these wavelengths, which implies either that the atmosphere is not hydrogen (very interesting) or cloudy (still interesting). To be followed, since GJ-1214, as the only good super-Earth target for transit and eclipse spectroscopy is under intense scrutiny, including Spizter, HST and ground telescopes.

Snellen et al. (2010), in a remarkable study with CRIRES on the VLT, have seen the radial velocity of a cross-correlation with 56 lines of CO near 2.3 microns drift across the transit, measuring for the first time the actual velocity of the transiting planet (as opposed to the velocity of the star+planet measured by radial velocity and transit length). The Doppler trace even showed a hint of the wind speed in the planet’s atmosphere, of the order of 2 km/s.

Thermo-, exo- -spheres

The Hubble Space Telescope was used to measure Lyman-alpha (1215 A) in ‘189 (Levavelier des Etangs et al. 2010), showing signs of an escaping exosphere like ‘209.

Linsky et al. (2010) detected the lines of CII at 1335 Å and SiIII at 1206 Å in a COS/HST near-UV spectrum of ‘209, confirming the presence of outflowing gas first studied by Vidal-Madjar et al. (2003) and Lecavelier et al. (2004).

Schlawin et al. (2010) report a marginal detection of Si IV at 1394 Å in the original Vidal-Madjar STIS data for ‘209. To reinforce the signal they use the trick that exosphere lines are limb-brightened (like a low-opacity sphere) while the transit is limb-darkened.

Direct spectroscopy

The more direct route to exoplanet atmosphere spectrum is, of course, to simply obtain the spectrum of a directly imaged exoplanet. This is hard because of the faintness of the planets and their proximity to the star.

Patience et al. (2010) obtained a fantastic spectrum of 2M1207 in the J, H and K bands, showing H2O and CO bands in this 5-10 MJ object with T~1600 K.

On the HR 8799 system, Hinz et al. (2010) measured the L’ and M colours of the planets, Janson et al. (2010) measured the 2.4-micron feature of “c” with OSIRIS/Keck, and Bowler et al. (2010b,*) got a spectrum of “b” around 4 microns with NACO/VLT. An intriguing and potentially seminal early indication is that the colours of these planets in the near infrared are completely different from those of brown dwarfs of the same temperature. T dwarfs become bluer at lower temperatures, presumably because of the disappearance of clouds, but the planets of HR8799 remain very red. Are we seeing the influence of gravity on the formation of clouds for cold gas giants and brown dwarfs? Or something?

One of a kind: WASP-12

WASP-12 with a period of about one day around a relatively hot star and a very large radius (1.7 MJ), is an extreme “very hot Jupiter” and has received a lot of attention in 2010. Lopez-Morales et al. (2010) have detected the secondary eclipse from the ground and seemingly confirmed the ~5% eccentricity from the radial velocity orbit. “Seemingly”, because both the orbital analysis of Hebb et al. (2009) and the eclipse analysis blithely ignored the effect of red noise and statistical bias on the eccentricity determination, and Spitzer and new RV measurement clearly showed the orbit to be circular after all (Campo et al. for Spitzer, Husnoo et al. for RV, preprints, see 2011).

Li, Lin et al. (2010) used the 5% eccentricity for a model of WASP-12, showing that the amazing persistence of this high eccentricity on such a close orbit injected a huge amount of energy in the planet, leading to the inflated radius as well as mass loss. Alas, the orbit of WASP-12 is not eccentric. Still, the prediction of a large equator-to-pole asymmetry of the planet, and maybe the mass loss estimate, should not be too dependent on the tidal contribution of the energy budget.

Following-up on this, Lai, Helling et al. (2010) study how the outflow can lead to detectable changes in the shape of the transit in UV. Vidotto et al. (2010) interpret a possible early UV ingress in terms of bow shock, that could measure the magnetic field of the planet. Fossati et al. (2010), using the spectropolarimeter ESPADON on the CFHT, find no structured magnetic field on the host star.

 

New methods, upper limits

France et al. (2010) show that auroral emission could be detected in their 1100-1700 A observations of ‘209, but isn’t, possibly indicating a low magnetic field.

Lazio et al. (2010) do not detect ‘606 at radio wavelengths. Vidal-Madjar et al. (2010) observe the “transit spectrum” of the Earth on the Moon during a lunar

eclipse. They show that narrow lines of Oxygen are the most detectable ones in this configuration.

Several papers examined the phenomenology and possible explanation for the present “top two riddles of transiting planets”: atmospheric temperature inversion and anomalous sizes.

Temperature Inversion

Madhusudhan & Seager (2010) explore the space of possible models for the emission spectra with MCMC chains (instead of simply using the best-fit models) and re-visit the statistics of temperature inversion. They find no correlation between inversion and equilibrium temperature, thus not confirming the picture of Fortney et al. (2008) separating non-inverted (pM) and inverted (pL) hot- Jupiter atmospheres as a function of equilibrium temperature.

Knutson et al. (2010) add another possible explanation, by pointing out a correlation between the temperature inversion and the level of activity on the host star. This could be explained if the UV radiation from active host stars destroys photochemically the visible absorber responsible for the temperature inversion.

Inflated hot Jupiters

With dozens of radii measured for hot Jupiters, it is becoming clear that the main factor in the radius inflation is the incoming irradiation from the host star. This weakens the explanations in terms of increased opacity (Burrows et al.) or inefficient convection (Baraffe et al.), in favour of the initial (Showman et al.) suggestion of transferring part of the energy from the atmospheric circulation to the interior of the planet, through some unknown mechanism.

Another hypothesis enjoying favour is that of tidal heating during the orbital circularisation, but it peaked in 2009 and is running out of steam. Matsumura et al. (2010) fit values for the tidal efficiency factor Q required by the present orbital eccentricities of hot Jupiters (and point out an error in Levrard et al. 2009 that invalidates their results that circular hot Jupiters should quickly spiral towards the star). Ibgui et al. (2010) calculate the link between Q and the tidal heating for five planets, and point out the degeneracy between Q and the heat input needed to inflate those planets. Nevertheless, Leconte et al. (2010) show that tidal heating occurs too early in a planet’s history to explain the inflated planets, and Hansen et al. (2010) calibrate the standard “equilibrium tides” theory with hot Jupiters, Io and other cases available to find that it is not sufficient to explain the inflated radii.

Two actual mechanisms were proposed to explain how the downward heat transfer – required to bring enough irradiation energy inside the planet to inflate it – could operate in hot Jupiter atmospheres. The first is Youdin et al. (2010)’s “mechanical greenhouse”. A reasonable amount of vertical mixing due to the atmospheric circulation could transport enough heat downwards. The second is Batygin et al. (2010,)’s “Ohmic dissipation”. This is Joule dissipation by the motion of ionised sodium atoms in atmospheric currents through the planet’s magnetic field. This heat transfer mechanism provides the right energy flux to explain the observed radius, and has become the leading explanation. There are open questions though. To be followed.

The Neighbours

(not planetary atmospheres directly but good to know)

SPIN-ORBIT ALIGNMENT

After last year’s watershed discovery that misaligned hot-Jupiter systems were not rare, more detection came in to increase the statistical sample: (Kepler-8, Jenkins et al. 2010), CoRoT-1 (Pont et al. 2010), WASP-2, -4, -5, -15, -17, -18 (Triaud et al. 2010), WASP-3 (Tripathi et al. 2010). And the misaligned Neptune HAT-P-11 (Winn et al. 2010) showed that tilted orbits were not only a quirk of hot Jupiters (unless HAT-P-11 is itself the remnant of an evaporated hot Jupiter).

On the interpretation side the breakthough came from Winn et al. (2010b,*) , who pointed out that the aligned systems all had host stars cool enough to have a convective envelope. This would lead to tidal realignment of the star, a process too inefficient to apply to hotter (F-dwarf) host stars. The stunning implication of this hypothesis is that before tidal realignment, hot Jupiter systems were even more misaligned, compatible with almost random angle between stellar spin and planetary orbit at the end of the formation process.

Although Bate et al. (2010) show that tidal interaction with the protoplanetary disc can also cause some misalignment, a very wide range of spin-orbit angle argues very strongly against disc migration and for three-body interaction to explain the presence of close-in planets.

STATISTICS OF PLANETS

Howard et al. (2010) combine the early Kepler results with the statistics of the California planet search and find a smooth power-law distribution for planet masses all the way from Jupiters to super-Earths, with dlog(frequency)/dlogM = -0.48. The frequency of planets keeps increasing with decreasing mass (on a log scale), and about one-third of stars should harbour a planet in the Earth-mass range. Contrary to prediction from planet population synthesis, there is no dearth of close-in Neptune-mass planets.

Johnson et al. (2010) confirms the well-known rise of planet occurrence with metallicity, and the rise of planet occurrence with host star mass, form the California planet search.

Another piece of the puzzle, in a very different quadrant, comes from direct imaging. From an absence of detection in 188 targets, Nielsen et al. (2010) conclude that Jupiters beyond 65 AU are very rare.

THE BROWN FRONTIER (BROWN DWARFS)

Goldman et al. (2010) discover a few new T dwarfs, including Ross458C. Burgasser et al. (2010) show that cloudy models provide a better fit to the spectrum of this object than cloud-free models, which is unexpected for this T8-dwarf at T~700 K. So maybe the simple hunch that cool planets have clouds and cool brown dwarfs don’t needs further consideration.

STARS

Torres et al. (2010) give an update on stellar masses and radii (updating the classic Andersen et al. 1991). This is important since the mass-radius relation of host stars is the dominant uncertainty in the determination of the size of transiting planets.

 

MEANWHILE IN THEORYLAND…

Atmospheric escape

Koskinen et al. (2010) build a semi-empirical model to interpret UV transit observations and apply it to the Roche lobe overflow of H and OI in ‘209.

Atmospheric chemistry

Line et al. (2010) make photochemical and equilibrium calculations for the abundances of CO, CO2, CH4 and H2O in ‘189, and show that photochemistry can affect the abundances.

Atmospheric radiation

Iro & Deming (2010) compute the time dependence of the vertical temperature profile for a hot Jupiter on a very eccentric orbit. Kane and Gelin (2010) predict the phase curve for a long-period eccentric Jupiter.

Burrows et al. (2010) calculate the expected transit spectrum and phase curve for ‘209, in particular the dependence of the shift between the hottest point and the substellar point on wavelength. Observing the wavelength dependence of this shift is potentially a powerful probe of the atmospheric circulation and structure.

Spiegel & Burrows (2010) present a 1-D modelling of the atmospheric profiles of HAT-P-7 and TrES-2, two extreme cases on the temperature inversion scale.

Fortney et al. (2010) calculate transmission spectra of hot Jupiters from 3-D atmosphere models, comparable to those of Showman et al. (2009). One of the notable results is that TiO does not produce the temperature inversion (a possible difficulty for the explanation of hot Jupiter temperature inversion by the effect of TiO).

Guillot (2010) gives analytic expressions for the radiative equilibrium of hot Jupiter atmospheres, allowing very fast calculations of the temperature profiles. A key parameter at the first level of complexity is the infrared-to-visible opacity ratio (the “greenhouse factor”).

Dissipation

Perna et al. (2010) find that for atmospheric temperatures in excess of 1000 K, the atmospheres is sufficiently ionised that magnetic drag can limit the wind speeds. Following the introduction of the hypothesis of Ohmic dissipation by Batygin et al. (2010) to explain the radius inflation, Perna et al. (2010b) realise that this also pertains to the inflation problem, and evaluate that the magnetic drag can deposit 1% of the energy from irradiation in the interior of the planet.

Circulation

More and more sophisticated circulation models for hot Jupiters hit the market, exploring whether the simplifications in earlier models missed crucial factors.

Rauscher & Menou (2010) use a 3-D circulation model with implicit (rather than grid-explicit) solving, and a relatively simple radiative transfer. They reproduce the main features of the classic Cooper & Showman models, including the eastward jets, but find a number of interesting differences. First, because their model goes deeper, the “return current” to the eastward jets are deep, and do not induce shearing motions in the atmosphere. Also, the 3-D currents can produce temperature inversions.

Dobbs-Dixon et al. (2010) use a circulation model to study the expected temporal variability, aka. “weather”, for ‘209. They find variations of 20 degrees in the latitude of the hottest points, and temperature variations of up to 15% near the terminator. These are potentially observable effects, although they are close to the present error bars.

Wordsworth et al. (2010) run a full General-Circulation-Model (imported from Solar-System studies) to decide if they deem GJ 581d habitable. It “probably” is, but the peril of such early studies is illustrated by the fact that the period of GJ581 that they use (84 days) turned out to have been an aliasing effect, and the real period is 66 days (hopefully).

Lewis et al. (2010) run a “Showman-type” circulation model for the hot Neptune GJ 436b. Interestingly, they find that the atmosphere’s heavy-element abundance is a key factor in the circulation. Solar-composition models show an efficient heat redistribution and low day-night temperature contrast, but more metals in the atmosphere increase the opacity, moving the photosphere to lower pressure, which lowers the efficiency of the recirculation and results in a high temperature contrast.

REVIEWS:

3 major reviews of the topic in 2010:

Seager & Deming, 2010, Exoplanet atmospheres, Ann. Rev. A&A

Baraffe, Chabrier & Barman, 2010, Physical properties of extrasolar planets, Rep. Prog. Phys.

Showman, Cho & Menou, 2010, Atmospheric circulation of exoplanets, chapter in “Exoplanets”, Seager ed.

Stars (*) indicate our very subjective candidates for key reads this year for exoplanetary atmospheres.

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About Author

I am a professor of planetary science at the University of Exeter. My specialty is the study of exoplanets, in particular the observation and modelling of exoplanet atmospheres. I have done my PhD a the University of Geneva and worked in Chile, France and Switzerland.

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