Discovery of an extrasolar system with two strongly interacting planets

An international team led by Guillaume Hébrard, researcher at Institut d’astrophysique de Paris, and involving researchers from IMCCE has detected and characterized a particularly interesting exoplanetary system. It consists of a first planet, WASP-148b, which passes in front of its host star with a period of 8.8 days, and a second one, WASP-148c, whose period is about four times longer. This configuration causes small variations of the WASP-148b period due to gravitational interactions between both planets. These variations have been observed, for the first time from the ground for this type of phenomenon. Observations and studies of this system will continue in order to better understand its structure and evolution.

Since the discovery in 1995 of the first exoplanet in orbit around a Solar-type star, planets around stars other than the Sun are the topic of many studies. In this context, the search for systems including several planets orbiting the same star is particularly interesting. Such planets attract each other because of their gravity, and this produces changes in their movement that can be detected in some cases.

When a single exoplanet orbits a star, it turns around with a well-defined period that does not vary. If the orbit is oriented so that the planet passes just in front of its star, seen from the Earth we can observe a small dip of the stellar light during what is called a « planetary transit ». The transits reproduce at regular intervals of time allowing the orbital period of the planet to be accurately measured. If the star hosts a second planet, the gravitational interactions between both planets will cause small accelerations or decelerations of the planets on their orbits. One will observe planetary transits a little in advance or late from one passage to another, a phenomenon called « transit timing variation » (or TTVs).

Predicted from a theoretical point of view, TTVs have long remained unobserved despite numerous searches with ground-based telescopes. Indeed, in most cases gravitational interactions lead to TTVs of a few seconds or less, which are difficult to detect. The Kepler space telescope was first to detect TTVs in an exoplanetary system in 2010. This case was particularly favorable to detection because the two main planets of this system have orbital are in resonance, which has the effect of increasing the amplitude of TTVs. Now a few dozen resonant planetary systems with TTVs have been detected, all with space telescopes.

The planetary system WASP-148, whose discovery is announced in spring 2020 by an international team led by Institut d’astrophysique de Paris, and involving researchers from Paris Observatory, was detected from ground-based telescopes. A transiting planetary candidate had first been identified by the SuperWASP instrument, installed at the Roque de los Muchachos Observatory in La Palma, Canary Islands. From 2014, the host star was observed with the high-resolution spectroscope SOPHIE installed at the Observatoire de Haute-Provence, France. SOPHIE accurately measures the variations in the velocity of stars due to the presence of exoplanets. All these observations have led to the conclusion that the star WASP-148 hosts a first planet, WASP-148b, having about the size and mass of Saturn and an orbital period of 8.8 days. SOPHIE’s observations also revealed that a second planet is in orbit around this star, WASP-148c, with a mass half that of Jupiter and an orbital period of 34.5 days.

The system is close to resonance, the orbital period of the planet WASP-148c being approximately four times longer than that of WASP-148b. TTVs were detected, using small telescopes located in the Canary Islands (Nites, Carlos-Sánchez, and Liverpool telescopes) and in France (Hubert-Reeves amateur observatory, in Ardèche): transits of WASP-148b in front of its star were observed and sometimes occurred a quarter of an hour ahead of or behind the prediction by considering a constant orbital period. We thus have a good understanding of the structure of this planetary system and the interactions that govern it, the TTVs of the planet WASP-148b being explained by the presence of the planet WASP-148c. Theoretical studies of gravitational interactions between both planets also indicate that their orbits are approximately coplanar (located in planes with an angle of less than 35 degrees between them), and that this configuration is stable.

This result is the first ground-based detection of TTVs for a resonant planetary system, obtained through ten years of observation. In the coming months and years, the WASP-148 multiplanetary system will be the subject of numerous theoretical studies and additional observations, which will allow the measurements of its properties to be refined and its structure and evolution to be better understood. In particular, the system is currently observed by the NASA’s TESS space telescope (Transiting Exoplanet Survey Satellite). These observations will allow nine consecutive transits of WASP-148b to be observed, and the possible existence of a transit for WASP-148c to be tested, which would be due to the passage of this second planet in front of its host star.

Related scientific publication: "Discovery and characterization of the exoplanets WASP-148b and c. A transiting system with two interacting giant planets", par G. Hébrard, R.F. Díaz, A.C.M. Correia, A. Collier Cameron, J. Laskar, D. Pollacco, J.-M. Almenara, et al., 2020, Astronomy & Astrophysics, in press (arXiv:2004.14645).

Contact IMCCE

  • Jacques Laskar
  • Institut de Mécanique Céleste et de Calcul des Ephémérides (IMCCE)

Contact IAP

  • Guillaume Hébrard
  • Institut d’astrophysique de Paris (IAP), CNRS, Sorbonne Université
  • Tél. : 01 44 32 80 78
Figure 1: Observatoire de Haute-Provence (© OSU Pytheas, CNRS, AMU). The 193-cm telescope is the one using the SOPHIE spectroscope, which contributed to the detection and characterization of the exoplanetary system WASP-148.>
Figure 2: Example of transits from the planet WASP-148b (© G. Hébrard et al.). Each point indicates the amount of light measured from the star. The black line represents the theoretical model. A small dip in this light curve is observed when the planet WASP-148b passes in front of its star. The precise measurement of the transit center allows TTVs caused by the gravitational effects of the planet WASP-148c to be detected.

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