Most stars in our galaxy have planets. The most common are sub-Neptunes, planets between the size of Earth and Neptune. Calculating their density poses a challenge for scientists: depending on the method used to measure their mass, two populations stand out, dense and less dense. Is this due to observational bias or the physical existence of two different populations of sub-Neptunes? Recent work by NCCR PlanetS, the University of Geneva (UNIGE), and the University of Bern (UNIBE) supports the latter. Learn more in the journal Astronomy & Astrophysics.

Exoplanets are abundant in our galaxy. The most common are those between the radius of Earth (about 6,400 km) and Neptune (about 25,000 km), known as "sub-Neptunes." It is estimated that 30% to 50% of Sun-like stars contain at least one of these planets.

Calculating the density of these planets presents a scientific challenge. To estimate their density, we must first measure their mass and radius. The problem: planets whose mass is measured by the TTV (Transit-Timing Variation) method are less dense than planets whose mass is measured by the radial velocity method, another possible measurement method.

„The TTV method involves measuring variations in transit times. Gravitational interactions between planets in the same system will slightly change the moment when planets pass in front of their star,“ explains Jean-Baptiste Delisle, a research fellow in the Department of Astronomy at UNIGE's Faculty of Science and co-author of the study. „The radial velocity method, on the other hand, involves measuring variations in the star's velocity caused by the presence of planets around it.“

Eliminating Bias
An international team led by scientists from NCCR PlanetS, UNIGE, and UNIBE has published a study explaining this phenomenon. It is not caused by selection or observational bias but by physical reasons. „Most systems measured by the TTV method are in resonance,“ explains Adrien Leleu, an assistant professor in the Department of Astronomy at UNIGE's Faculty of Science and the study's lead author.

Two planets are in resonance when the ratio between their orbital periods is a rational number. For example, when one planet makes two orbits around its star, the other planet makes exactly one. If several planets are in resonance, it forms a chain of Laplace resonance. „Therefore, we wondered if there is an intrinsic link between density and resonant orbital configuration of the planetary system,“ continues the researcher.

To establish a link between density and resonance, astronomers first had to rule out any bias in the data by rigorously selecting planetary systems for statistical analysis. For example, a large low-mass planet detected in transit requires more time to be detected in radial velocities. This increases the risk of observational interruption before the planet becomes visible in radial velocity data, and thus before its mass is estimated.

„This selection process would lead to bias in the literature in favor of higher masses and densities for planets characterized by the radial velocity method. Since we don't have their mass measurements, less dense planets would be excluded from our analyses,“ explains Adrien Leleu.

After this data cleaning was done, astronomers were able to determine through statistical tests that the density of sub-Neptunes is lower in resonant systems than in their non-resonant counterparts, regardless of the method used to determine their mass.

The Question of Resonance
Scientists suggest several possible explanations for this link, including the processes involved in the formation of planetary systems. The main hypothesis of the study is that all planetary systems converge towards a state of resonance chain in the early moments of their existence, but only 5% remain stable. The other 95% become unstable. The resonance chain then breaks, generating a series of "catastrophes," such as collisions between planets. Planets merge, increasing their density, and then stabilize in non-resonant orbits.

This process generates two very different populations of sub-Neptunes: dense and less dense. „Numerical models of planetary system formation and evolution that we have developed in Bern over the past two decades reproduce exactly this trend: planets in resonance are less dense. This study, moreover, confirms that most planetary systems were the site of giant collisions, similar to or even more violent than the one that gave rise to our Moon,“ concludes Yann Alibert, a professor in UNIBE's Department of Space Research and Planetary Sciences (WP) and co-director of the Center for Space and Habitability and co-author of the study.

Source: UNIVERSITY OF GENEVA