Category Archives: WASP planets

The atmosphere of exoplanet WASP-36b

Transits of WASP-36b in multiple colours and from different nights.

A new paper by Luigi Mancini et al reports transits of the hot-Jupiter exoplanet WASP-36b in multiple colours. The point is to record the transit depth as a function of wavelength, and thus deduce how opaque the planet’s atmosphere is at different wavelengths. That, in turn, might tell us what the atmosphere is made of.

To do this Mancini et al have used the GROND instrument on ESO’s 2.2-m telescope, which records light in four different colours simultaneously. They observed four different transits of WASP-36b over 2012 to 2015.

The result is the figure below showing the transit depth in the four different passbands (greater depth implying a larger planet radius, plotted as the ratio of planet to star, R_b/R_A).

The black crosses show the transit depth. The dashed versions are corrected for possible star-spots in the transit light curves. The coloured lines represent different atmospheric models.

The data show a clear and strong trend to greater depth in the blue, steeper than would be explained by any of the models shown. This means that something in the planet’s atmosphere is absorbing strongly at bluer wavelengths. What is causing this is unclear, and will require further investigation.

WASP exoplanet skies in Forbes Magazine

A recent article by Brian Koberlein in Forbes Magazine, on “The Wonder of Exoplanet Skies”, features WASP. The article is based on a recent paper by Jake Turner et al which includes observations of 15 hot Jupiters, of which seven are WASP planets.

The paper is one of the first to compile exoplanet transits in the near-UV “U” band. By comparing transit depths at different wavelengths one can discern facts about the exoplanet’s sky, such as whether is it clear or cloudy.

The most interesting result is apparently anomalous U-band transit depths in WASP-1b and WASP-36b, which appear shallower than in the optical, a finding that is hard to explain. Most likely this will have been caused by some observational bias, especially since there appears to be “red noise” in some of the transit profiles.

The image shows transits in the red (Harris R) and the near-UV (Bessell U), along with the residuals against a fitted model.

This sort of work is hard to do from the ground, but such studies point to a bright future for parameterising exoplanet atmospheres.

Spin-orbit alignments for three more WASP planets

A team led by Brett Addison has been pointing the Anglo-Australian Telescope at WASP planets, trying to discern whether the planet’s orbit is aligned with the star’s spin axis.

The rotation of the star means that one limb is approaching us, and so is blue-shifted, while the other limb is receding, and so is red-shifted. The planet can occult blue-shifted light (making a spectral line redder) and then red-shifted light. This is called the Rossiter–McLaughlin (or R–M) effect, and allows us to deduce the path of a transiting planet across the face of its star.

Brett Addison and colleagues report the R–M effect for three more WASP planets, WASP-66b, WASP-87b and WASP-103b. Here are their data for WASP-87b:

All three planets appear to have orbital axes aligned with the star’s spin axis. The authors discuss the mechanisms and timescales by which orbits get “damped” by tidal effects and so become aligned with their star.

WASP-157b, a transiting Hot Jupiter observed with K2

Hot on to arXiv is our latest discovery paper. WASP-157b marks a jump upwards in WASP numbering, since we’ve somewhat rushed this one out. WASP-157 was flagged as a WASP candidate in 2014 and added to our program for radial-velocity (RV) and photometric follow-up. Meanwhile, being in the field of K2 Campaign 6, it was observed by Kepler from July to September 2015. Since K2 data are public, this meant that other groups would soon be on its trail.

A TRAPPIST observation of the transit, shortly before the K2 data were due to be released, along with prioritising it for CORALIE and HARPS RVs, rapidly accumulated enough observations to prove it was a planet. Keele student Teo Močnik then did a good job of analysing the K2 data and turning all the observations into a paper. A gap of only four days between the last CORALIE radial-velocity observation and the paper appearing on arXiv is efficient work!

Here is the K2 lightcurve showing the transits of WASP-157b:

Orbital-period decay in hot-Jupiter WASP-12b?

Closely orbiting hot-Jupiter exoplanets are likely to be spiralling inwards towards their host star as a result of tidal interactions with the star. A new paper by Maciejewski et al reports a possible detection of this orbital-period decay in WASP-12b.

The authors have acquired 31 new transit light-curves over four years, and detect a trend under which the latest transits occur about a minute early compared to an unchanging ephemeris.

Transits of WASP-12b. O–C is the observed time compared to that calculated from an unchanging orbital period. The time (x-axis) is given in both a count of days (BJD) and a count of transits.

This is the most convincing claim yet of a changing orbital period in a hot Jupiter. Whether it shows the spiral infall, though, is less clear. As the authors explain, other tidal interactions between the star and the planet, such as that causing apsidal precession, could account for the effect. Further, in close binary stars there are known to be similar period changes on decade-long timescales that are not fully understood, but which might be caused by Solar-like magnetic cycles on the star.

One suggestion that this is not spiral infall comes from the deduced value of the tidal quality factor, Q, which the authors calculate as 2.5 x 10⁵. This is lower than other estimates of Q as nearer 10⁷.

The way to settle the issue will be to accumulate more data over a longer timespan until the case for spiral infall becomes overwhelming. It will thus be important to continue monitoring WASP-12b, and the other short-period hot Jupiters, over the coming decades.

Looking forward to WASP planets with JWST

The $6-billion James Webb Space Telescope “will likely revolutionize transiting exoplanet atmospheric science due to a combination of its capability for continuous, long duration observations and its larger collecting area, spectral coverage, and spectral resolution compared to existing space-based facilities”, write Kevin Stevenson et al in a new paper looking forward to Cycle 1 observations of exoplanets with JWST.

Of interest to us is at WASP that, of the “community targets” identified by Stevenson et al as the best targets for characterizing exoplanet atmospheres in Cycle 1, seven of the twelve are WASP planets, and in particular “the most favorable target is WASP-62b because of its large predicted signal size, relatively bright host star, and location in JWST’s continuous viewing zone”.

This independent assessment validates WASP’s program of finding exoplanets transiting relatively bright stars, where they make the best targets for ongoing detailed studies.

JWST is now not that far off, as Stevenson et al remind us with this timeline:

Five more WASP transiting hot Jupiters

The WASP-South camera array, in conjunction with the Euler/CORALIE spectrograph and the TRAPPIST photometer, continues to be the world’s most prolific programme for discovering hot Jupiters transiting relatively bright stars of V < 13.

The lastest batch of five (WASP-119b, WASP-124b, WASP-126b, WASP-129b and WASP-133b) was announced by Maxted et al this month.

The discovery has reported by the Daily Mail, The Times of India, and The Hindu, and has been covered by about twenty news websites including Phys.org, wired.co.uk, scienceworldreport.com, techtimes, I4U News, and siliconrepublic.

Artist’s impression of a ‘hot Jupiter’. Credit: Ricardo Cardoso Reis (CAUP)

This derived from a piece by Tomasz Nowakowski, of Phys.org, which includes:

“WASP-126b is the most interesting because it orbits the brightest star of the five. This means it can be a target for atmospheric characterization, deducing the composition and nature of the atmosphere from detailed study, for example with the Hubble Space Telescope or the forthcoming James Webb Space Telescope,” Coel Hellier, one of the co-authors of the paper, told Phys.org.”

And:

“NASA’s Transiting Exoplanet Survey Satellite … might find smaller transiting exoplanets in these systems, as the Kepler K2 mission did with our previous discovery WASP-47. TESS, however, will do this for nearly all WASP planets, whereas K2 is restricted to an ecliptic strip, and so can only look at a few WASP planets,” Hellier said.”.

Clear skies for cool Saturn WASP-39b

Transmission spectroscopy of exoplanet atmospheres — looking at the atmosphere of a planet in transit, backlit by the light of its star — is one of the major growth areas in studying WASP planets.

The latest such study is by Patrick Fischer and colleagues, who pointed the Hubble Space Telescope with its STIS spectrograph at WASP-39b in transit.

The plot shows the resulting data compared with three models of WASP-39b’s atmosphere (depending on how clear or hazy it is, and on the metal abundance compared to the Sun).

Unlike some hot Jupiters, which have very hazy atmospheres with few spectral features, WASP-39b shows a clear detection of potassium and sodium, as expected in largely clear skies.

Comparing to the hazier planets HD 189733b and WASP-6b, Fischer et al remark: “These observations further emphasize the surprising diversity of cloudy and cloud-free gas giant planets in short-period orbits and the corresponding challenges associated with developing predictive cloud models for these atmospheres”.

Calculations of hot-Jupiter tidal infall

Closely orbiting hot Jupiters raise a tidal bulge on their star, just as our Moon does on Earth. Since the planet is orbiting faster than the star rotates, the tidal bulge will tend to lag behind the planet and so its gravitational attraction will pull back on the planet. The orbit of the planet is thus expected to decay, with the planet gradually spiralling inwards to destruction.

Calculating how long this will take is hard, and depends on the efficiency with which energy is dissipated in the tidal bulge of the star. This is summed up by a number called a quality factor, Q, which is, crudely, the number of orbital cycles required to dissipate energy. The higher this number the slower the decay of the planet’s orbit.

In a new paper, Reed Essick and Nevin Weinberg, of the Massachusetts Institute of Technology, present a detailed calculation of Q for hot Jupiters orbiting solar-like stars. They arrive at values for Q of 10⁵ to 10⁶, assuming a planet above half a Jupiter mass and an orbital period of less than 2 days.

The figure shows the resulting infall timescales of all the hot Jupiters predicted to have remaining lifetimes of less than 1 Gyr. By far the smallest lifetime is that for WASP-19b, which is predicted to spiral into its star within 8 million years. This would mean that shifts in WASP-19b’s transit times would be readily detectable, with a shift accumulating to 1 minute in only 5 years.

The calculations presented here are at odds with deductions that Q must be around 10⁷, based on explaining the current distribution of hot-Jupiter periods (e.g. Penev & Sasselov 2011), which would give a much slower orbital decay. We can determine who is right by monitoring transits of WASP-19b and similar systems over the coming decade, and it will be interesting to discover who is right.

Energy recirculation in the hot Jupiter WASP-19b

A team led by Ian Wong of Caltech have announced observations of the hot Jupiters WASP-19b and HAT-P-7b, looking at infra-red light using the Spitzer Space Telescope. By observing the planets around their entire orbit they detect the transit, caused by the planet passing in front of the host star, the secondary eclipse, when the planet passes behind the star, and the “phase curve” caused by the changing visibility of the heated face of the planet.

The figure shows the infra-red light (“heat”) of the WASP-19 system in two pass bands (3.6 microns and 4.5 microns). The middle panels are expanded to show the phase curve, while the lowest panels show the residuals about a fitted model (the red line).

By fitting all three features, the authors can constrain the temperatures of the “day time” heated face of the planet (which faces towards us near the secondary eclipse) and of the “night time” face of the planet (which faces us near transit). From there they can estimate the “recirculation”, how efficient the planet is at redistributing heat from the day-time face to the night-time face.

Such short-period planets are phase-locked by tidal forces, and so always present the same face to the star. Thus redistribution of heat energy requires powerful winds circling the planet.

An interesting plot by Wong et al shows the recirculation in different hot Jupiters against the albedo (the fraction of energy that is reflected).

There appear to be two groups of hot Jupiters: ones with albedos near 0.4, such as WASP-19b, and ones with much lower albedos, such as WASP-14b and WASP-18b. So far there is no simple explanation for this difference.

Further, the recirculation efficiency also appears to be different in different systems. Wong et al suggest that the hot Jupiters experiencing the highest irradiation, such as WASP-19b, are least efficient at redistributing heat, while
longer-period, less-irradiated hot Jupiters such as HD209458b and HD189733b are better at redistribution.

WASP Planets

Transiting exoplanets from the Wide Angle Search for Planets