UC scientist helps discover two exoplanets with strong interactions

Alexandre Correia, a researcher at the University of Coimbra, is part of an international team that has discovered a particularly interesting planetary system around the star WASP-148. The study will soon be published in the international journal Astronomy & Astrophysics.

The system consists of two Saturn-mass planets with orbital periods of only 9 to 35 days. This configuration causes variations in the orbital periods due to the gravitational interactions between the two planets. These variations were first observed from the Earth's surface.

The first exoplanet (a planet orbiting a star other than the Sun) was discovered in 1995, and since then the number of new planets has continued to increase. The search for systems with several planets orbiting the same host star is particularly interesting because they interact with each other and allow us to learn more about the system.

When a single planet orbits a star, it has a well-defined orbital period that does not vary with time. Whenever the planet passes in front of the star, "we can detect a small dip in its light, a phenomenon called a planetary transit". Transits occur at regular intervals, so the planet's orbital period can be measured very accurately. If the star hosts a second planet, gravitational interactions between the two planets cause small changes in their orbits. As a result, planetary transits occur slightly ahead or behind between two passes in front of the star, a phenomenon called "transit timing variations" (or TTVs)," explain the authors of the study, led by the Institute for Astrophysics in Paris (France).

Although theoretically expected, TTVs remained unobserved for a long time, despite numerous observations with ground-based telescopes. In fact, in most cases, gravitational interactions lead to TTVs of a few seconds or less, which are very difficult to detect. The Kepler space telescope was the first to measure TTVs in a planetary system, in 2010, followed by other measurements, but always by space telescopes such as Kepler. Observing TTVs requires very precise instruments and massive planets in relatively close orbits.

Alexandre Correia, researcher at the Physics Centre of the Faculty of Science and Technology of the University of Coimbra (FCTUC), participated in this discovery by analysing the interactions between the planets. "We used numerical simulations to verify that the system is stable up to inclinations of 35 degrees and that the observed TTVs are in line with what would be expected in this type of system".

The team points out that the WASP-148 planetary system "was discovered using only ground-based telescopes. The first planet, with a Saturn-like mass and an orbital period of only about 9 days, was first observed by the SuperWASP instrument installed at the Roque de los Muchachos Observatory in La Palma, in the Canary Islands, Spain". Since 2014, the star has also been observed by the high-resolution spectrograph SOPHIE, installed at the Haute-Provence Observatory in France, which measures variations in the star's speed. The combination of these observations led to the conclusion that the star WASP-148 hosts a second planet, also with a mass similar to Saturn and an orbital period of about 35 days.

Unlike the first planet (9 days), the second planet (35 days) does not transit in front of the star. "This is because the two orbits are different. It is not yet possible to know the exact inclination between the two orbits, but computer simulations of the WASP-148 system show that, for the system to remain stable, the inclination can be at most 35 degrees", explains Alexandre Correia.

The ratio between the two orbital periods is close to four, so the gravitational interactions between the planets are enhanced by a phenomenon known as resonance. The high masses of the planets make this system an ideal candidate for observing TTVs from Earth. In fact, with the help of small telescopes in the Canary Islands (Nites, Carlos-Sánchez and Liverpool telescopes) and in France (Hubert-Reeves amateur observatory in Ardèche), it was possible to observe transits a quarter of an hour later or earlier.

The team emphasises that the result of this research is "the first detection of TTVs from the Earth's surface, obtained through over ten years of observations. In the coming months and years, the WASP-148 system will be the subject of numerous theoretical studies and additional observations, which will allow better measurements of its properties and a better understanding of its structure and evolution". In particular, the system will soon be observed by NASA's TESS (Transiting Exoplanet Survey Satellite) space telescope. "These observations, which are much more accurate than those performed on Earth, will measure nine successive transits of the 9-day-orbit planet, which will make it possible to determine the inclination between the two orbits," the researchers conclude.

Scientific Paper: "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). - Available here.

^{Translation by Diana Taborda}