The hot Jupiter WASP-148b is rather unusual, since it has a sibling planet, WASP-148c in a 35-day orbit (Hébrard et al. 2020). The system was recently observed by TESS leading to a new paper by Gracjan Maciejewski et al. (Nicolaus Copernicus University and the Instituto de Astrofísica de Andalucía).
The gravitational tug of the outer planet WASP-148c perturbs the orbit of the hot Jupiter WASP-148b. Here are deviations in the timings of the hot-Jupiter’s transit (the green points are new timings from TESS, the blue points are from observations from the Sierra Nevada Observatory, the red line is a model based on the masses and orbits of the planets):
The great boon of transit-timing information is that it leads to measurements of the masses of the planets, which can be combined with radial-velocity measurements to give a better overall characterisation of the system.
Maciejewski et al. also searched the TESS data for transits of the outer planet. The yellow areas are the predicted time of transit, should the planet’s orbital inclination be sufficiently high (the red line is a model showing the predicted depth of the transit; the black triangle marks a transit of the hot Jupiter WASP-148b). There is no indication that WASP-148c transits.
Even though WASP has found nearly 200 planets we are still announcing systems that are unlike any previous ones. WASP-148 is an example, as described in the discovery paper by Guillaume Hébrard et al.
WASP first detected transits of the hot Jupiter WASP-148b in an 8.8-day orbit. Spectroscopic observations with OHP/SOPHIE, aimed at measuring the planet’s mass, then found that there was also a second massive planet in a longer, 35-day orbit:
The orbits of both planets are eccentric, likely because they are perturbing each other by their gravitational attraction. Further, the gravitational perturbations mean that the transits of the inner planet vary in time by 15 mins.
We don’t yet know whether the outer planet, WASP-148c, also transits (since its longer period means that there are gaps in WASP’s coverage of its orbit), but this patch of sky is currently being observed by the TESS satellite. The space-based photometry from TESS will be good enough to detect any transits of WASP-148c, to map out transit-timing variations, and to look for additional planets in the system that are too low mass to have been detected in the radial-velocity data. WASP-148 is thus an important system for studying an unusual planetary-system architecture, with two massive planets in relatively close orbits in resonance with each other.