Tag Archives: WASP-107b

First results on the atmosphere of WASP-107b

Being a Neptune-mass planet (0.12 MJ) bloated to a near-Jupiter radius (0.94 RJ) makes WASP-107b’s atmosphere very fluffy, and that, coupled with it transiting a moderately bright K star (V = 11.6) makes it a superb target for atmospheric characterisation.

Laura Kreidberg et al have pointed the Hubble Space Telescope at WASP-107b to make the first atmospheric study. Here’s the WFC3 spectrum:

Hubble Space Telescope spectrum of WASP-107b

The broad features at 1.15 and 1.4 microns are due to water absorption in WASP-107b’s atmosphere. Kreidberg et al model the features, finding that they are compatible with expectations given solar abundances. They are not deep enough, though, to be produced by fully clear skies, and a layer of high-altitude cloud is also required.

WASP-107b is one of the prime exoplanets already chosen for early observations with the imminent James Webb Space Telescope, so it is exciting to know that its atmosphere does show prominent molecular features.

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Super-Neptune WASP-107b has an oblique orbit

WASP-107b is only twice the mass of Neptune but nearly the radius of Jupiter. It is thus a hugely bloated and fluffy exoplanet and one of the more important of the recent WASP discoveries, being a prime target for atmospheric characterisation (see the discovery paper by Anderson et al 2017).

WASP-107b was also in the Campaign-10 field of the K2 mission, leading to a Kepler-quality photometric lightcurve. Recent papers by two teams, led by Teo Močnik and Fei Dai, have arrived at a similar conclusion: WASP-107b seems to be in an oblique orbit, rather than in an orbit aligned with the rotation axis of the host star.

spot_tran

The conclusion comes from star spots. If the orbit is aligned, consecutive transits will repeatedly cross the same star spot, producing a “bump” in the lightcurve each time, whereas if the orbit is oblique this will not happen.

Thus one can play the game of looking for transit bumps and seeing if they repeat. But spots can change, by growing or shrinking, so is a smaller bump in the next transit the same spot, or a different one? Also, if there is some uncertainty in the rotational period of the star, then we’re not fully sure exactly where in the next transit the spot will recur.

Star spots in transits of exoplanet WASP-107b

In the figure at left (in which the transit itself, between the dashed lines, has been removed, leaving only the starspot bumps), obvious spots are circled in red, while possible spots are marked with a lighter red. The rotational period of the star is nearly three times the orbital period of the planet, and so, if the spots recurred, they would be seen every three transits. (The gap, and thus the missing of transits 3, 4 and 5, arose from a spacecraft malfunction.)

The conclusion is that the star spots do not seem to recur and thus that WASP-107b is in an oblique orbit.