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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WASP-94: “cousin” planets around twin stars

Press release regarding our discovery of planets around binary stars WASP-94A and WASP-94B:

European astronomers have found two new Jupiter-sized extra-solar planets, each orbiting one star of a binary-star system. Most known extra-solar planets orbit stars that are alone, like our Sun. Yet many stars are part of binary systems, twin stars formed from the same gas cloud. Now, for the first time, two stars of a binary system are both found to host a “hot Jupiter” exoplanet.

The discoveries, around the stars WASP-94A and WASP-94B, were made by a team of British, Swiss and Belgian astronomers. The British WASP-South survey, operated by Keele University, found tiny dips in the light of WASP-94A, suggesting that a Jupiter-like planet was transiting the star; Swiss astronomers then showed the existence of planets around both WASP-94A and then its twin WASP-94B.

Marion Neveu-VanMalle (Geneva Observatory), who wrote the announcement paper, explains: “We observed the other star by accident, and then found a planet around that one also!”.

Hot Jupiter planets are much closer to their stars than our own Jupiter, with a “year” lasting only a few days. They are rare, so it would be unlikely to find two Hot Jupiters in the same star system by chance. Perhaps WASP-94 has just the right conditions for producing Hot Jupiters? If so WASP-94 could be an important system for understanding why Hot Jupiters are so close to the star they orbit.

The existence of huge, Jupiter-size planets so near to their stars is a long-standing puzzle, since they cannot form near to the star where it is far too hot. They must form much further out, where it is cool enough for ices to freeze out of the proto-planetary disk circling the young star, hence forming the core of a new planet. Something must then move the planet into a close orbit, and one likely mechanism is an interaction with another planet or star. Finding Hot-Jupiter planets in two stars of a binary pair might allow us to study the processes that move the planets inward.

Professor Coel Hellier, of Keele University, remarks: “WASP-94 could turn into one of the most important discoveries from WASP-South. The two stars are relatively bright, making it easy to study their planets, so WASP-94 could be used to discover the compositions of the atmospheres of exoplanets”.

The WASP survey is the world’s most successful search for hot-Jupiter planets that pass in front of (transit) their star. The WASP-South survey instrument scans the sky every clear night, searching hundreds of thousands of stars for transits. The Belgian team selects the best WASP candidates by obtaining high-quality data of transit lightcurves. Geneva Observatory astronomers then show that the transiting body is a planet by measuring its mass, which they do by detecting the planet’s gravitational tug on the host star.

The collaboration has now found over 100 hot-Jupiter planets, many of them around relatively bright stars that are easy to study, leading to strong interest in WASP planets from astronomers worldwide.

 An illustration of a planet orbiting one star of a binary system.  In WASP-94, the planet would transit the brighter star, causing a dip in the light that can be detected from Earth.  Another planet orbits the second star at lower-left. It does not transit and is not directly visible, but it can be detected by its gravitational tug on the second star.

An illustration of a planet orbiting one star of a binary system. In WASP-94, the planet would transit the brighter star, causing a dip in the light that can be detected from Earth. Another planet orbits the second star at lower-left. It does not transit and is not directly visible, but it can be detected by its gravitational tug on the second star. [ Image Credit: ESO/L. Calçada/Nick Risinger]

Update: This press release has resulted in articles in phys.org, sciencedaily.com, world-science.net, yahoo.news, Science World Report, and breakingnews.ie.

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NASA’s Chandra X-ray Observatory observes WASP-18

NASA have put out a press release about an observation of WASP-18 by the Chandra X-ray observatory.

WASP-18 was the first planet discovered to have an orbital period of less than 1 day and has the highest tidal interaction between a planet and a star of any known planetary system.

“We think the planet is ageing the star by wreaking havoc on its innards”, report the authors of the paper, “The planet’s gravity may cause motions of gas in the interior of the star that weaken the convection”, which “results in the magnetic field becoming weaker and the star to age prematurely”.

NASA have also produced a nice graphic of WASP-18 and its planet (image credit: NASA/CXC/M.Weiss):

wasp18_chandra

The press release has been picked up by several media outlets including Astronomy magazine, Phys.org, the Daily Mail online, and the International Business Times. (Spot the error in the last outlet’s sentence that: “Scientists have used data provided by NASA’s Chandra X-ray Observatory to find a planet that causes the star it orbits to act much older than it actually is, according to a new study” (added emphasis)!)

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WASP planets will get names in a public vote

Written by Tom Wagg

WASP planets, like all exoplanets, get catalogue numbers but, so far, have not been actually named. The International Astronomical Union policy is now about to change, with the announcement of a contest in which astronomy clubs and non-profit organisations can submit names for exoplanets.

The worldwide public will then be able to vote on their favourite name for an exoplanet and the winning names will be officially sanctioned by the IAU.

IAU

Among the 305 exoplanets which have been selected for the first round of naming are 10 of the earliest discovered WASP exoplanets. These include WASP-12b, which has recently been found to contain water, and WASP-10b, which is thought to have a massive outer companion.

The host stars of WASP-7b and WASP-14b are both bright enough to be visible in a pair of binoculars, one in the Northern Hemisphere and the other in the Southern Hemisphere, which means that it will be possible to name a WASP planetary system that you can readily point to at a star party.

Members of the public can propose names for just one exoplanet, or for a whole planetary system such as 55 Cancri which includes five exoplanets.

Once the naming process is over we will post the new names of our WASP planets and the creators of these names on this blog. To get involved simply follow this link and submit your proposed exoplanet names for your chance to be credited with naming your own exoplanet!

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The grazing transit of WASP-67b

The hot-Jupiter exoplanet WASP-67b is a curiosity, being the only known exoplanet with a grazing transit, such that not all of the planet transits the host-star’s disc. This means that the characteristic “second contact” and “third contact” points are missing from the transit lightcurves. These are the points where, usually, the whole planet is now in front of the star, and the transit is then flat-bottomed, apart from the relatively small effects of stellar “limb darkening”.

The four "contact" points of a planetary transit, illustrated for Mercury transiting our Sun.

The four “contact” points of a planetary transit, illustrated for Mercury transiting our Sun.

In WASP-67b’s grazing transit the planet is never completely in transit and thus the transit lightcurve has a continuously varying V shape. The grazing nature of WASP-67b was confirmed by a detailed study of new transit lightcurves by Mancini et al (2014), who used the GROND instrument on ESO’s 2.2-m telescope at the La Silla observatory.

The transit of WASP-67b from Mancini et al. (2014)

The transit of WASP-67b from Mancini et al. (2014)

The lack of second and third contact makes the system parameters hard to tie down, and thus obtaining a secure estimate of the planet’s radius and density requires Mancini et al’s high-quality lightcurves. WASP-67 is also notable for being in one of the target fields of the revamped Kepler spacecraft’s K2 mission, and thus we can expect ongoing detailed study of this system.

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