Here is a plot of the timings of the transits of WASP-4b, taken from a new paper led by Luke Bouma:
The curve in the plot shows that the transits are occurring progressively earlier as time passes. One possible explanation is that the planet’s orbit is decaying under the influence of the tidal interaction between the star and planet. This is expected to occur in most hot Jupiters, though how quickly is debated.
However, Bouma have also obtained radial-velocity observations of the system, which show that the star is accelerating towards us. This can result from it being in a wide orbit with another object (the authors suggest a wide-orbiting companion of 10-to-300 Jupiter masses at a distance of 10-to-100 AU). Since the system is accelerating towards us, the light-travel time is decreasing, and this (not orbital decay) means that the transits occur earlier.
Wide companions are expected in hot-Jupiter systems, since, in most theories for the occurrence of hot Jupiters, the gravitational perturbation of a distant companion is needed to shrink the hot-Jupiter orbit down to the current values of only a few days.
Bouma et al recommend continued radial-velocity monitoring of hot Jupiters in order to distinguish orbital decay from accelerations caused by orbiting companions.
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.
Here’s a topic we’ll be hearing much more about: how the observed spectrum of a transiting exoplanet is affected by transiting across star-spots. In “transmission spectroscopy” the starlight shines through the planet’s atmosphere during transit, and the easiest thing to do is assume that the star itself is a uniform light source.
But as discussed by papers led by Ben Rackham, if the planet passes over a dark region (star spot) or bright region (faculae), this would change the observed spectrum.
A new paper led by Alex Bixel about WASP-4b is the first to attempt to correct for this effect. The authors’ transit observations show a clear crossing of a starspot (the feature is shown in blue, the spot shows as a upward bump since the planet is then removing less light):
And here is the difference it makes. The blue curve is the observed spectrum, presumed to be of the planet’s atmosphere. The orange curve is then the spectrum corrected for the presence of the star spot.
The details of how to do this are complex, and are discussed at length in the above papers. The central message is that “active FGK host stars can produce such features and care is warranted in interpreting transmission spectra from these systems”.
However, there is good news in that: “stellar contamination in transmission spectra of FGK-hosted exoplanets is generally less problematic than for exoplanets orbiting M dwarfs”, and that such signals “are generally minor at wavelengths of planetary atomic and molecular features”. Overall the authors say that their study “bodes well for high-precision observations of these targets”.