Category Archives: WASP planets

Detection of iron in the ultra-hot-Jupiter WASP-121b

Three papers this week arrived on arXiv about the ultra-hot-Jupiter WASP-121b. All three report similar findings (and the near-simultaneous arrival on arXiv presumably reflects the teams being aware of the competition). Cabot et al analyse spectra of WASP-121b from the ESO 3.6-m/HARPS spectrograph, Bourrier et al also analyse HARPS data, while Gibson et al analyse data from UVES on ESO’s 8-m VLT.

All three teams then apply velocity shifts to correct for the orbital motion of the star, in order to try to detect features from the planet. The result is a plot looking like (this is the one from Bourrier et al):

As in the plot for WASP-107b, just below, this shows the spectra as a function of time, through transit. The extra absorption during transit (delineated by dashed lines) is from the atmosphere of the planet absorbing starlight while it is projected against the star’s face. The faint diagonal feature (marked by the green line) is the signal from the planet’s atmosphere, moving with the planet’s orbital velocity.

A schematic of WASP-121b as it transits its hot star in a near-polar orbit (from Bourrier et al).

The three papers report the detection of lines from neutral iron in the planet’s atmosphere, and discuss the possible role of iron absorption in producing an atmospheric temperature inversion. The papers also report a blue-shift of the iron absorption, of order 5 km/s, which could be produced by strong winds running round the planet. That is expected in phase-locked planets, where heat from the irradiated face must be transported round to the night side.

Helium reveals the extended atmosphere of WASP-107b

Here’s a plot from a new paper on WASP-107b by James Kirk et al. It shows data taken with a near-infra-red spectrograph on the 10-m Keck II telescope on Mauna Kea, and is focused on the Helium line at 10833 Å. The plot shows the spectra as a function of time (y-axis), though a transit. When the planet passes in front of its host star (white horizontal lines are times of ingress and egress) the helium line shows excess absorption. This helium is in the atmosphere of the planet and is absorbing some of the starlight. There is a slight change in the wavelength of the absorption owing to the orbital motion of the planet (denoted by the dashed white lines).

The paper shows, firstly, that ground-based telescopes such as Keck can do a fine job of discerning the compositions of exoplanet atmospheres. Secondly, the fact that the absorption extends beyond transit-egress indicates that the atmosphere is boiling off the surface of WASP-107b, under the fierce irradiation of the star, and is forming a comet-like tail.

The IAU announces names for WASP exoplanets

The IAU have recently announced the outcome of their campaign allowing the people of the world to name recently discovered exoplanets and their host stars. The names chosen for WASP exoplanet systems are:

WASP-6 and WASP-6b: Márohu and Boinayel (Márohu and Boinayel are the god of drought and the god of rain, respectively, from the mythology of the Taino people of the Dominican Republic).

WASP-13 and WASP-13b: Gloas and Cruinlagh (in Manx Gaelic, Gloas means to shine, like a star, while Cruinlagh means to orbit).

WASP-15 and WASP-15b: Nyamien and Asye (Nyamien is the supreme creator deity in the Akan mythology of the Ivory Coast, while Asye is the Earth goddess).

WASP-17 and WASP-17b: Dìwö and Ditsö̀ (from the Bribri language of Costa Rica, Dìwö means the Sun, while Ditsö̀ is the name the god Sibö̀ gave to the Bribri people).

WASP-21 and WASP-21b: Tangra and Bendida (Tangra is the supreme creator god in early Bulgarian mythology, while Bendida is the Great Mother goddess of the Thracians).

WASP-22 and WASP-22b: Tojil and Koyopa’ (Tojil is a Mayan deity related to rain, storms and fire, while Koyopa’ means lightning in the K’iche’ Mayan language).

WASP-34 and WASP-34b: Amansinaya and Haik (Aman Sinaya is the primordial deity of the ocean in the Philippine’s Tagalog mythology while Haik succeeded Aman Sinaya as God of the Sea).

WASP-38 and WASP-38b: Irena and Iztok (Iztok and Irena are characters from a traditional story from Slovenia).

WASP-39 and WASP-39b: Malmok and Bocaprins (Malmok and Boca Prins are scenic, sandy beaches in Aruba).

WASP-50 and WASP-50b: Chaophraya and Maeping (Chao Phraya is the great river of Thailand, while Mae Ping is a tributary).

WASP-52 and WASP-52b: Anadolu and Göktürk (Anadolu is the motherland of the Turkish people while Göktürk was the first Turkish state, established in the 5th century).

WASP-60 and WASP-60b: Morava and Vlasina (Morava is the longest river in Serbia, while Vlasina is a tributary).

WASP-62 and WASP-62b: Naledi and Krotoa (Naledi means “star” in the Sesotho, SeTswana and SePedi languages of South Africa, while Krotoa is considered the Mother of Africa and member of the Khoi people).

WASP-64 and WASP-64b: Atakoraka and Agouto (Atakoraka is a mountain range in Togo, while Agouto is the highest peak).

WASP-71 and WASP-71b: Mpingo and Tanzanite (Mpingo is a tree that grows in southern Tanzania producing ebony wood for musical instruments, while Tanzanite is a precious stone).

WASP-72 and WASP-72b: Diya and Cuptor (Diya is an oil lamp used in the festival of Diwali in Mauritius, while Cuptor is a traditional clay oven).

WASP-79 and WASP-79b: Montuno and Pollera (the names are the traditional costumes worn by the man and the woman, respectively, in the El Punto folk dance of Panama).

WASP-80 and WASP-80b: Petra and Wadirum (Wadi Rum is a valley in southern Jordan while Petra is an ancient city).

WASP-161 and WASP-161b: Tislit and Isli (both are lakes in the Atlas Mountains of Morocco, and also mean “bride” and “groom” in the Amazigh language).

The tidal shape of the exoplanet WASP-121b

The moon’s gravity causes a tidal bulge in Earth’s oceans, so that the water facing the moon is raised several metres. Similarly, close-orbiting exoplanets will have a tidally distorted shape, with a tidal bulge facing the host star. The amount of distortion can be quantified by the “Love number” h (named after the mathematician Augustus Love)

Specifically, h2 tells us the relative height of the tidal bulge, and would be zero for a perfectly rigid body that did not distort at all, and would be 2.5 for a perfectly fluid body that adapted fully to the tidal potential. Gas-giant planets have large envelopes of gaseous fluid, so would be expected to have fairly high values of h2. However, they also might have rocky or metallic cores, and so would have values less than 2.5. For example Jupiter has h2 = 1.6 while Saturn has h2 = 1.4.

Transit of WASP-121b observed by HST with a model fit by Hellard et al.

A new paper by Hugo Hellard et al discusses whether h2 for a hot-Jupiter exoplanet can be measured from the shape of the transit lightcurve, given good-enough photometry such as that from the Hubble Space Telescope.

The main problem is that the transit profile is heavily affected by variations in the brightness of the stellar disc, in particular the limb darkening (a star’s limbs appear a bit dimmer, because a tangential line-of-sight into a gas cloud skims only the cooler, upper layers). Thus the Hellard et al paper discusses at length different ways to model the limb darkening.

A star’s disk is dimmer at the edges, so a transiting exoplanet removes less light (here Venus, top right, is transiting the Sun).

The end-result, however, is a claim to have measured h2 for WASP-121b, with a value of h2 = 1.4 ± 0.8. This is not (yet) a strong constraint, but points to the potential in the future, and also flags up the need to understand and properly parametrise limb darkening.

TESS phase curve of WASP-19b

The space-based photometry from the TESS satellite is producing high-quality light curves of many of the WASP exoplanets. Here is the lightcurve of WASP-19b, from a new paper by Ian Wong et al:

In addition to the transit (phase zero), the lightcurve shows a shallower eclipse of the planet (phase 0.5) and a broad variation caused by the changing aspect of the heated face of the planet. Unlike in some planets, the hottest part of the planet directly faces the star, so there is no offset in the phase of the broad modulation.

Wong et al deduce that the dayside face of the planet is heated to 2240 ± 40 K, that there is no flux detected from the colder night side, and that the planet reflects 16 ± 4 percent of the light that falls on it. The last value is relatively high compared to other planets.

Water in exoplanet atmospheres

The Cambridge Institute of Astronomy have put out a press release based on a new paper analysing the water abundance in the atmospheres of 19 exoplanets, 11 of them being WASP planets.

The plot shows the measured water abundance versus the planet’s mass. Welbanks et al state that: “We find a mass–metallicity trend of increasing H2O abundances with decreasing mass”, and also that: “The H2O abundances in hot gas giants are likely due to low oxygen abundances relative to other elements rather than low overall metallicities, and provide new constraints on their formation mechanisms”.

The press release explains that: “The researchers found that while water vapour is common in the atmospheres of many exoplanets, the amounts were surprisingly lower than expected, while the amounts of other elements found in some planets were consistent with expectations”.

The press release has led to coverage in the Daily Express, Astronomy Now, and Science News, among other sites, accompanied by this graphic:

The atmosphere of the inflated hot Jupiter WASP-6b

Atmospheric characterisation of hot Jupiters continues apace, using both ground-based telescopes such as ESO’s Very Large Telescope and satellites such as Hubble.

Aarynn Carter et al have just produced a new analysis of WASP-6b:

The spectrum shows absorption due to sodium (Na), potassium (K) and water vapour, while the modelling implies that the atmosphere is partially hazy. Carter et al state that: “despite this presence of haze, WASP-6b remains a favourable object for future atmospheric characterisation with upcoming missions such as the James Webb Space Telescope.