RAS Gold Medal for Professor Michel Mayor

The Royal Astronomical Society has announced the award of a Gold Medal to Professor Michel Mayor of the University of Geneva. Prof. Mayor was, of course, the co-discoverer of the first extrasolar planet around a solar-like star, with the detection of 51 Pegasi b back in 1995. His Observatoire de Genève group developed a succession of planet-finding spectrographs that have led the way to the discovery of many hundreds of extrasolar planets.

Michel Mayor

Prof. Mayor has been an important collaborator for the WASP project, through the CORALIE spectrograph on the 1.2-m Swiss/Euler telescope at La Silla. The CORALIE spectrograph observes all WASP-South planet candidates, and the detection of the radial-velocity signature of a planet — in about 1 in 8 such candidates — is the crucial step that confirms a new planet discovery. Thus Prof. Mayor was a co-author on many of the early WASP planet papers until his retirement.

The WASP project is hugely indebted to Prof. Mayor and is honoured to have collaborated with him on WASP planet discovery. We congratulate him on the well-deserved award of the RAS Gold Medal.

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WASP: an end-of-2014 round up

WASP planets, papers and citation by year

2014 saw 18 new WASP planets published, our most productive year yet. As the green histogram shows, our success at planet finding continues to increase as we accumulate more and more data.

While Kepler exceeds us in terms of sheer numbers, and in finding small planets, it is important to realise that our planets are usually around much brighter stars, and so are often much better targets for ongoing studies.

Refereed papers related to WASP (either about WASP planets or using WASP data) are also climbing strongly, with 75 new refereed papers in 2014 (blue histogram; and see the listing here). Of course most of these are now by third parties, rather than by the WASP consortium itself, which shows the strong and increasing interest in WASP science from groups worldwide.

2014 was also the best ever year for citations regarding WASP science. There were over 2000 citations in the refereed literature in 2014 to papers that mention WASP either in the paper title or in the abstract. The cumulative number of such citations, shown as the lighter red line, is now over 7600. The darker-red line is the same, but for citations only to papers mentioning WASP in the title (which many papers about WASP do not).

Thus the WASP program is healthy and productive, and we expect that it will continue to dominate the discovery of transiting exoplanets around relatively bright stars until the launch of NASA’s TESS mission in late 2017.

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WASP and Kepler K2

WASP-85 is a binary star, with the hot Jupiter WASP-85Ab orbiting the brighter star of the pair. It was in the Campaign 1 field of the revamped Kepler K2 mission, and thus we have the first extensive Kepler-quality lightcurve of a WASP planetary system.

K2 light curve of WASP-85

The WASP discovery paper by David Brown et al presents an initial look at the long-cadence K2 data. The upper plot shows the entire light curve, with obvious variability of the star (presumably because it is magnetically active) and narrow dips caused by the transits. The lower plot shows the data folded on the transit.

K2 light curve of WASP-85 transit

The higher-time-resolution “short cadence” data will be available soon, and should allow a high-quality analysis of this system. The WASP planets WASP-47b and WASP-75b are being observed in the current K2 Campaign 3, which should lead to more space-quality light curves of WASP systems.

In other news, WASP played a minor role in the discovery of the first K2 planet, a super-Earth-sized planet orbiting the bright K-dwarf star HIP 116454. There is extensive WASP data on this star, and while the transits (only 0.1% deep) are too shallow to see in WASP data, the WASP data contribute by showing a possible 16-day rotation period of the host star. The discovery paper by Andrew Vanderburg et al featured in a NASA press release.

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Scattering in the atmosphere of WASP-6b

WASP-6b was WASP-South’s third planet, announced in 2009 by Gillon et al. It is a good target for studying exoplanet atmospheres since it is a bloated planet, only half a Jupiter mass but 20% larger than Jupiter.

Nikolov et al (2014) have now pointed the Hubble Space Telescope at WASP-6b in transit, using the STIS spectrograph. They find that the transit depth varies with colour; effectively the planet looks slightly larger in blue light, since small particles in the planet’s atmosphere are scattering blue light more than red light.

wasp6_rayleigh

The strong blue slope in the plot is characteristic of Rayleigh scattering, the same effect that causes Earth’s atmosphere to look blue (in the plot the red line is a Rayleigh-scattering model, though other model fits are possible).

Nikolov etal state that: “With a broad-coverage optical transmission spectrum measured from HST and Spitzer broad-band transit spectrophotometry, WASP-6b joins the small but highly valuable family of hot-Jupiter exoplanets with atmospheric constraints.”

The field of exoplanet atmospheres is growing rapidly in importance, and it is good to see WASP planets being chosen as prime targets for such work.

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The atmosphere of hot-Jupiter exoplanet WASP-31b

Characterising the atmospheres of exoplanets is a rapidly growing field that is set to increase in importance even more with the forthcoming launch of JWST. WASP planets are prime targets for such work since they transit relatively bright stars. Comparing spectra in and out of transit then gives a transmission spectrum of the planet’s atmosphere.

A new study by David Sing et al presents a state-of-the-art analysis of WASP-31b’s atmosphere using the STIS instrument on the Hubble Space Telescope.

w31_atmos

Notable features include the presence of potassium absorption (the peak labelled K) and the fact that this is stronger than sodium (Na) absorption. The absence of many of the broad features in the plotted models implies a “cloud deck” that results in few spectral features. Also seen is a “Rayleigh scattering” slope implying small atmospheric particles floating above the cloud layer.

WASP-31b is a planet of 0.5 Jupiter masses that is bloated up to 1.5 Jupiter radii. This gives it a large atmospheric scale height that makes it a good target for transmission spectroscopy, since the fluffier atmosphere covers a larger fraction of the star during transit.

WASP-31b was discovered in 2010 by the WASP-South team led by David Anderson.

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2014: A bumper year for WASP planets

2014 is proving to be the WASP project’s most successful year yet for the publication of transiting exoplanets. With two months to go before the end of the year, there are already 17 new planets published in 2014 in refereed journals. 12 more planets have been announced on the arXiv preprint server, though many of those will likely appear with a 2015 publication date.

We are currently finding transiting exoplanets at a rate of about 30 a year (WASP-117 is the highest number published, though we have currently got as far as WASP-134). This results from improvements in data quality owing to adding multiple years of observation. Further, the combination of WASP-South with the TRAPPIST photometer and the Euler/CORALIE spectrograph is proving to be a highly effective team. The process involves a lot of telescope time and hard work — only 1 in 10 of candidates followed up proves to be a planet — but the reward is the strong worldwide interest in studying WASP planets.

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Hubble maps the atmosphere of WASP-43b

WASP-43b is one of the more extreme hot Jupiters found by WASP-South, orbiting its star in only 19 hours, making it the hot-Jupiter planet closest to its star, where its atmosphere gets blasted by the stellar irradiation. Since the host star is relatively dim, a K7V dwarf smaller and fainter than our Sun, the planet’s light is relatively easy to see and thus the system is a prime target for characterising exoplanet atmospheres.

Now, NASA have put out a press release regarding a Hubble Space Telescope observation of WASP-43b which monitored the planet around three of its orbits.

By recording the changes in the observed light around the orbit, as the irradiated face of the planet swings into view and then faces away again, the team have mapped the temperature and the distribution of water vapour of the planet’s atmosphere.

Exoplanet WASP-43b orbits its parent star

The image (Credit: NASA, ESA, and Z. Levay (STScI)) shows the changing view of WASP-43b around its orbit, illustrating the hot, blasted heated face and the darker atmosphere pointing away from the star.

The planet is phase-locked to the orbit by tidal forces, always pointing the same face to its star, and thus we expect dramatic winds as the planet’s atmosphere redistributes heat from the star-facing side to the cooler side.

The Hubble observations are reported in three papers, one accepted for Science, lead by Kevin Stevenson of the University of Chicago (arXiv link). A second paper, led by Laura Kreidberg, also of the University of Chicago, shows that the abundance of water in WASP-43b’s atmosphere is compatible with that in the Sun (arXiv link). A third paper, led by Tiffany Kataria of the University of Arizona, models the planet’s atmospheric circulation (arXiv link).

WASP-43b was announced in 2011 by the WASP-South team in a paper led by Coel Hellier of Keele University.

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