2024-08-02 04:00:13
Most stars in our galaxy contain planets. The most abundant are Sub-Neptunes, planets between the size of Earth and Neptune. Calculating their density poses a problem for scientists: depending on the method used to measure their mass, astronomers identified two populations, the dense and the less dense.
© ESO/L. Calçada/spaceengine.org
Is this explained by an observational bias or by the physical existence of two distinct populations of Sub-Neptunes? Recent work by the NCCR PlanetS, the University of Geneva (UNIGE) and the University of Bern (UNIBE) supports the second hypothesis. They can be found in the journal Astronomy & Astrophysics.
Exoplanets are abundant in our Galaxy. The most common are those whose size is between the radius of the Earth (about 6400 km) and that of Neptune (about 25,000 km), called “Sub-Neptunes”. It is estimated that 30% to 50% of stars similar to the Sun host at least one.
Calculating the density of these planets poses a problem for scientists. To estimate it, it is first necessary to measure their mass and radius. The problem: planets whose mass is measured by the TTV method (Transit-Timing Variation) are less dense than the planets whose mass has been measured by the radial velocity methodthe other possible measurement method.
“The TTV method consists of measuring variations in the timing of the transit. The gravitational interactions between the planets in the same system will in fact slightly modify the moment when the planets will pass in front of their star,” explains Jean-Baptiste Delisle, scientific collaborator in the Department of Astronomy of the Faculty of Sciences of UNIGE and co-author of the study. “The radial velocity method consists of measuring the variations in the speed of the star induced by the presence of the planet around it.”
Eliminate any bias
An international team of astronomers led by scientists from the PRN PlanetS, UNIGE and UNIBE has published a study explaining this phenomenon. It would not be due to selection bias orobservation but rather for physical reasons. “The majority of systems measured by the TTV method are in resonance,” explains Adrien Leleu, assistant professor in the Department of Astronomy at the UNIGE Faculty of Science and lead author of the study. Two planets are in resonance when the ratio between their orbital periods is a rational number. For example, when a planet orbits its star twice, another planet orbits exactly once. If several planets are in resonance, we then speak of a Laplace resonance chain. “We therefore wondered if there was an intrinsic connection between the density and the orbital configuration in resonance of a planetary system,” continues the searcher.
In order to establish the link between density and resonance, astronomers first had to rule out any bias in the data by carefully selecting planetary systems for their statistical analysis. For example, a large, low-mass planet detected in transit takes longer to be detected in radial velocities. This increases the risk that observations will be interrupted before the planet is visible in the radial velocity data and therefore before its mass is estimated.
“This selection process would lead to a bias in the literature in favor of higher masses and densities for planets characterized with the radial velocity method. Not having measurements of their masses, the less dense ones would indeed be excluded from our analyses,” explains Adrien Leleu.
Once this cleaning was done, astronomers were able to determine, using statistical tests, that the density of Sub-Neptunes is lower in resonant systems than their counterparts in non-resonant systems, regardless of the method of determining their mass.
A question of “resonance”
Scientists have suggested several possible explanations for this link, including the processes of planetary system formation. The study’s preferred theory is that all planetary systems convergent to a resonance chain state in the first moments of their existence, but only 5% remain stable. The other 95% become unstable. The chain of resonance then breaks up, causing a series of “catastrophes”, such as collisions between planets. The planets merge with each other, increasing their density before stabilizing on non-resonant orbits. This process thus generates two distinct populations of Sub-Neptunes, dense and less dense. “The numerical models of the formation and evolution of planetary systems that we have developed in Bern over the last two decades reproduce this trend exactly: planets in resonance are less dense. This study, moreover, confirms that most planetary systems have been the site of giant collisions, similar or even more violent than the one that gave birth to our Moon”, concludes Yann Alibert, professor at the Space Research and Planetary Sciences Division (WP) and co-director of the Center for Space and Habitability at UNIBE, co-author of the study.
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