With the TESS satellite observing most of the WASP planets as it surveys the sky, one can use the space-based data quality to look for “transit timing variations” in the WASP planet transits. Such TTVs — slight changes in the time of recurrence of a transit — can be caused by the gravitational perturbation of other planets in the system, and thus can reveal the presence of extra planets even when they themselves do not transit.
A new paper by Kyle Pearson, of the University of Arizona, reports evidence for TTVs in the TESS light-curves of WASP-18 and WASP-126.
Here is the TESS light-curve of WASP-18, showing the transits of the known planet WASP-18b:
And here are possible changes in the transit times, varying systematically with a 2.1-day period (red line):
Here now is the TESS light-curve of WASP-126, showing transits of WASP-126b:
And here are possible TTVs varying systematically with a period of 7.7 days (red line):
The evidence is not yet sufficiently water-tight to be sure of the existence of the extra planets, without adding in further data, but this study points to lots of similar work using TESS data as it continues its multi-year survey.
As TESS continues its all-sky survey it will produce high-quality data containing lots of transits for all the WASP planets. This is especially so for planets near the ecliptic poles, which TESS will observe over many sectors. With TESS Sector 4 data recently released, here are some plots borrowed from David Kipping on Twitter.
The lower plots show the variations in transit timing (O–C is the difference between the observed timing and the timing calculated from an ephemeris).
These plots seem to show something that I’ve suspected for a while, namely that there are correlated deviations in the transit timings, meaning that if one O–C value is slightly early (or late) then the next one is more likely to be the same. Such deviations can also be larger than expected given the errors (the quoted chi-squared value for WASP-100b of 44 for 32 degrees of freedom tells us that the error bars don’t fully account for the variations).
This must be the result of stellar activity, magnetic variations on the surface of the star such as star-spots and faculae. Any deviation from a smooth stellar profile can then alter the transit profile.
Properly accounting for such effects will be important for two sorts of study. The first is looking for “transit-timing variations”, changes in the transit time of a planet caused by variations in its orbit owing to the gravitational perturbations of another planet. The second is looking for long-term changes in the orbital period, such as the inward-spiral decay of the orbit predicted to be caused by tidal interactions of the planet and its host star. The literature contains marginal claims of the latter effect that might be better explained as the effect of magnetic activity of the host star.