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Celestial waltz of WASP-148b and WASP-148c

2 July 2020

An international team of scientists, including Professor Andrew Cameron, have discovered two, giant planets interacting near each other, in a gravitational dance.

The planets, detected by the Wide Area Search for Planets (WASP) project, feel each other's gravity and, as a result, speed up and slow down slightly as the inner, faster-orbiting planet overtakes on the inside.

The first planet, WASP-148b, passes in front of its host star and completes an orbit every 8.8 Earth days. The second planet, WASP-148c, takes about four times longer to complete an orbit.

The planets' interaction with each other causes the transit timing of WASP-148b to vary, so it sometimes occurs early and sometimes late. These variations from the usual orbital clockwork have been measured, for the first time, with telescopes on Earth rather than with space telescopes.

Professor Andrew Collier Cameron, of the University of St Andrews and WASP team member, said: "This is the first time we've found a pair of giant planets interacting so closely, and it's exciting to be able to follow their dance from the ground."

WASP detects planets that pass in front of their host star, which causes a small dip in the star light because the planet blocks a fraction of the star's surface from our view during the "planetary transit".

If the star hosts a second planet the gravitational pull between the two planets will cause small accelerations or decelerations of the planets in their orbits. By measuring when transits happen, the gravitational effect of a second planet can be determined if transits do not show exact regularity. These "transit timing variations" can reveal the second planet, even if that planet does not itself transit across the face of the star.

Artistis impression of the two planets. Dr Mark Garlick.

Dr Guillaume Hébrard, of Institut d'astrophysique de Paris (IAP), who led the discovery, said: "WASP has discovered over a hundred transiting giant planets, but this double discovery has more than double the value because of the extra information the timing variations give us."

Before this discovery a few dozen planetary systems showed transit timing variations had been detected, all with space telescopes.

WASP-148b was identified using the SuperWASP instrument, an array of small camera lenses, installed at the Roque de los Muchachos Observatory in La Palma, Canary Islands. From 2014, the host star was observed with the SOPHIE high-resolution spectrograph at the Observatoire de Haute-Provence, France.

SOPHIE was used to make accurate "radial velocity" measurements, revealing the small variations in the star's velocity due to the presence of exoplanets orbiting around it.

WASP-148b is about the size and mass of Saturn with an orbital period of 8.8 days. SOPHIE's observations also directly revealed the second planet WASP-148c, with a mass half that of Jupiter and an orbital period of 34.5 days.

The orbital period of WASP-148c is approximately four times that of WASP-148b and, as a result, the gravitational interactions produce large timing variations. The gravity acts similarly to pushing a child on a swing: by always adding a force at just the right time, the effect is amplified. This is called resonance.

For WASP-148b the transit timing variations are as much as 15 minutes, which is easy to measure. The timings were detected using small telescopes: Nites, Carlos-Sánchez, and Liverpool telescopes in the Canary Islands and the Hubert-Reeves amateur observatory, in Ardèche, France.

Professor Don Pollacco, of the University of Warwick and principal investigator of the WASP project, said: "Even in this era of very expensive space telescopes, small telescopes on the ground can make vital contributions".

Combining the transit timings and radial velocity information, Hébrard's team determined that the orbits of WASP-148b and WASP-148c are in the same plane, just like the planets in our own Solar System. The mutual tilt between the orbits of WASP-148b and WASP-148c must be less than 35o and so that their configuration is stable.

NASA's TESS space telescope (Transiting Exoplanet Survey Satellite) will observe nine consecutive transits of WASP-148b and perform a sensitive search for transits of WASP-148c and additional smaller planets.

For more on the research findings, read the paper published in the Astronomy & Astrophysics journal. (DOI)