Here’s the latest update on the changes in the orbital period of WASP-12b, from a new paper by Samuel Yee et al.
The times of transit are getting earlier, which means that the period is decreasing slightly. By also considering the times of occultation (when the planet passes behind the star), and also the radial-velocity measurements of the system, the authors deduce that the changes are not the effect of some other planet, but are a real decay in the orbit of WASP-12b. This is expected to occur as a result of tidal interactions between the planet and its host star.
One notable conclusion is that the rate of period decay in WASP-12b is much faster than that in WASP-19b, which shows no detectable period change yet, despite it being an even shorter-period hot Jupiter, which should increase tidal interactions. Yee et al suggest that the difference could arise if the host star WASP-12 is a sub-giant star, whereas WASP-19 is not.
As NASA’s TESS satellite surveys the Southern sky is it observing many of the WASP planets. One interesting piece of analysis is to check how the transit timings compare with predictions, to look for changes in the orbital periods.
Here’s a plot from a new paper by Luke Bouma et al.
The orange Gaussians show the error range within which TESS-observed transits would be expected to occur, based on previous data, if there has been no change in the period. The blue Gaussians are the actual TESS measurements.
For most of the planets the two ranges overlap, which means the transit times are as expected. For WASP-4 (top-left), however, the transits arrived early by 80 secs, too much to be accounted for by the expected error in the ephemeris.
This suggests that the period of WASP-4b might be changing rather rapidly.
Since TESS is likely to re-observe the Southern hemisphere in future years, it will be interesting to see what happens next.