A deviation from symmetry is always an interesting thing from a physicist's point of view. It's worth taking a closer look. Here it is the light curve of the star WASP-167. WASP-167 is a pulsating star which is larger and hotter than the Sun, and is known
to host a hot Jupiter on a 2-day, retrograde orbit. The system was observed by NASA’s Transiting Exoplanet Survey Satellite (TESS) during three ‘Sectors’, each of 28 days duration, in 2019, 2021, and 2023.
Phase curve
A planet orbiting its star appears to an observer in different phases, similar to the moon. Observing such a system continously over complete phases can reveal importand information, e.g. surface temperature or albedo (back radiation).
In the ideal, symmetrical case, the planet orbits around its star on a circular orbit and - provided its orbital plane lies on the observer's line of sight - changes the starlight in a characteristic way. If the planet is in front of the star, the star is dimmed. As the phase curve progresses, the observer sees an increasingly large part of the planet's surface, which is heated or reflected by the incident radiation from the star. In the case of the planetary system WASP-167, the conditions are more complicated: the light curve in the vicinity of the eclipse is asymmetrical:
Phase curve of WASP-167
Top: Entire light curve with deep transit and shallow eclipse feature. Centre: light curve in the vicinity of the eclipse (orbital phase =0.5), various contributions as the cause of the asymmetry are marked in colour Bottom: deviation of the simulated light curve from the measurements.
Credit:
Kálmán et al, Astronomy & Astrophysics, submitted in March 2024
The data from TESS were modelled using using the Transit and Light Curve Modeller (TLCM) code developed at EPA by Szilárd Csizmadia. By analysing the light curve covering many full orbits of the planet (not just the transits) we were able to measure the mass of the planet to be 0.34 ± 0.22 times the mass of Jupiter, by assessing the degree of tidal deformation, and from Doppler beaming. This is a valuable technique for stars such as WASP-167 whose rapid rotation makes mass determination via the usual radial velocity technique difficult or impossible.Furthermore, by measuring the depth of the occultation (106.8 ± 27.3 ppm), the dayside temperature of the planet was estimated at 2790 ± 100 K, and the geometric albedo at 0.34 ± 0.11. We also find evidence that the heat on this planet is less well-distributed between the dayside and nightside hemispheres than on other similar planets, and find a hint of a possible interaction between the orbit of the planet, and the pulsations of the star.
A paper ‘The phase curve of the ultra-hot Jupiter WASP-167b as seen by TESS’ was published in July 2024 in Astronomy and Astrophysics. The first author is Hungarian PhD student Szilárd Kálmán, who visited EPA last autumn to work with Szilárd Csizmadia and Alexis Smith, who are both co-authors of the paper.
The full paper can be read here: https://arxiv.org/pdf/2403.19468.pdf
