Unveiling the future of ultra-short-period exoplanets

The last decades of space exploration, and in particular “exoplanet hunting”, i.e., the search for planets orbiting stars other than the Sun, gave us insights into the evolutionary paths leading the current architecture of our planetary system as well as of other systems. To date, the discovery of 4715 exoplanets belonging to 3247 planetary systems has been confirmed, and there are approximately 5900 planets awaiting confirmation.

Curiously, the architecture of our Solar System seems to be very different from the configurations found in other systems in the galactic neighborhood; for example, a significantly high proportion of massive Jupiter-like planets have been discovered orbiting in regions very close to the star, where, in theory, they should not be, since the models of planetary formation indicate that they form in the most peripheral regions of the protoplanetary disk.

It should be noted that the apparent over-abundance of such planets is related to an observational bias: the sensitivity of the devices used in the detection of exoplanets is limited, being able to recognize with greater ease -for now- only the most notorious exoplanets; that is, large planets (in mass and size) orbiting very close to their star. These characteristics guarantee that they eclipse a large area of the stellar disk during transits, in addition to giving a good gravitational jolt to the star that we can detect by measuring the stellar radial velocity. In any case, this population raises important questions about how these architectures are achieved, and what will be the fate of these very short-period exoplanets.

Precisely, a recent research led by the PhD candidate Jaime Andrés Alvarado Montes from the Centre for Astronomy, Astrophysics and Astrophotonics, Macquarie University in Australia, and in which Mario Sucerquia participates, a FONDECYT postdoctoral researcher of the Millennium Nucleus of Planetary Formation (attached to the University of Valparaiso in Chile), has studied the fate of a subcategory of these anomalous planets known as “ultra-short period planets” (USP). These planets orbit their star in times shorter than one Earth day in nearly circular orbits, and are possibly tidally locked, i.e., always showing the same face to the star throughout their orbit, just as our Moon does with respect to the Earth.

The study, which has recently been accepted for publication in the prestigious journal Monthly Notices of the Royal Society (MNRAS), updates pre-existing models describing the tidal interaction between the planet and the star. This research studies the force generated by the mutual deformation experienced by the bodies as a result of their gravitational interaction, as they rotate, move and, of course, age. Ultimately, the goal is to constrain the rate at which the orbits of the USPs shrink (the planet is said to be migrating) until they are engulfed by their own stars.

According to Alvarado-Montes, leader of the project, “modeling the rate at which exoplanets migrate provides us with better predictive models to know the fate of these exoplanets. Such models can become increasingly complex as we have to account for several effects related to the star or the planet. These effects can be related to changes in the planet’s rotation rate, the efficiency in dissipating the energy associated with the induced mutual deformations, the magnetic braking of the stellar rotation, as well as the loss of angular momentum due to mass ejection”.

“For some time now we have had powerful instruments that have been monitoring some of these exoplanets for decades, taking measurements of the variations of their orbital periods. These measurements, interpreted with models such as those presented in this research, can indirectly reveal the interior structure of planets and stars, as well as show details informing us on the physical processes responsible for planetary migration””, adds Mario Sucerquia.

In particular, the work presented predicts the progressive shift of the transit periodicity of two massive USP exoplanets, WASP 19b and NGTS 10b, objects that orbit their star once every 20 hours or so. These periods indicate that they are located at very close distances to their star, so their surface temperatures are extremely high. The model applied to these systems predicts a higher orbital migration rate for the former and a lower one for the latter, when compared to previous similar investigations. These predictions could be corroborated during the current decade.

The study in question is part of a large project involving researchers from Australia, France, Colombia, Argentina and Chile, which aims to study the phenomenon of gravitational tides in planetary systems. This phenomenon not only affects the planets and their star, but also concerns their possible moons and ring systems, and that according to the authors of this article: “the research carried out so far shows that tides can significantly or radically modify the architecture and possible fates of planetary systems. Furthermore, this type of studies can help us understand the future of planets like Jupiter in our Solar System, since when the Sun will increase its size in the final stages of its life, tidal interactions with Jupiter will be much more intense, thus affecting its fate and the possible habitability of its moons.”

Scientific publication 

This press release was written by Mario Sucerquia

The image is an artist’s impression showing WASP-19b (M. Kornmesser / ESO)

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