Researchers finally decipher density difference of sub-Neptunes

UNIBE and PlanetS have shown the existence of two distinct populations of sub-Neptunes.


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Our galaxy contains a significant number of stars that have accompanying planets. Among these, the most common types are the sub-Neptunes, which range in size between Earth and Neptune. Scientists face challenges in determining the density of these planets due to different measurement methods, which reveal two distinct populations: one dense and the other less dense.

Researchers from NCCR PlanetS, the University of Geneva (UNIGE), and the University of Bern (UNIBE) propose that this dual population of sub-Neptunes may be a physical reality rather than an observational bias.

Exoplanets are plentiful within our galaxy, with the most prevalent falling within the size range of Earth (about 6,400 km) to Neptune (about 25,000 km), categorized as “sub-Neptunes.” It is believed that 30% to 50% of stars similar to the sun harbor at least one of these planets.

Determining the density of these planets poses a scientific challenge, requiring initial measurements of their mass and radius. The issue lies in the fact that planets whose mass is determined using the Transit-Timing Variation (TTV) method are less dense compared to those whose mass has been measured using the radial velocity method, another possible measurement technique.

“The TTV method involves measuring variations in transit timing. Gravitational interactions between planets in the same system will slightly modify the moment at which the planets pass in front of their star,” explains Jean-Baptiste Delisle, scientific collaborator in the Astronomy Department of the UNIGE Faculty of Science and co-author of the study. “The radial velocity method, on the other hand, involves measuring the variations in the star’s velocity induced by the presence of the planet around it.”

A study led by researchers from NCCR PlanetS, UNIGE, and UNIBE has been published, providing an explanation for this phenomenon. According to the study, the phenomenon is not a result of selection or observational biases but rather due to physical causes.

“The majority of systems measured by the TTV method are in resonance,” explains Adrien Leleu, assistant professor in the Astronomy Department of the UNIGE Faculty of Science and principal author of the study.

When the orbital periods of two planets form a rational number ratio, they are said to be in resonance. For instance, if one planet completes two orbits around its star while another completes just one, they are in resonance. Furthermore, if multiple planets are in resonance, they form a chain of Laplace resonances. The researchers thus questioned whether there exists an inherent relationship between a planetary system’s density and its resonant orbital configuration.

In order to establish the relationship between density and resonance, astronomers needed to carefully select planetary systems for statistical analysis to ensure that there was no bias in the data. For instance, it takes longer to detect a large, low-mass planet in transit using radial velocities, which increases the likelihood of interruptions in observations before the planet becomes visible in the data, thus affecting the estimation of its mass.

“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. As we have no measurement of their masses, the less dense planets would be excluded from our analyses,” explains Adrien Leleu.

Once this data cleaning had been carried out, the 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 used to determine their mass.

Several potential explanations for this connection are proposed by the scientists, including the mechanisms involved in the creation of planetary systems. According to the study’s primary hypothesis, all planetary systems move toward a state of resonance chain in their early stages, but only 5% manage to maintain stability, while the remaining 95% become unstable.

As a result, the resonance chain deteriorates, leading to a series of “catastrophic” events, such as planetary collisions, which subsequently fuse the planets together, increasing their density and eventually stabilizing them in non-resonant orbits.

This process leads to the formation of two distinct groups of Sub-Neptunes: those with high density and those with lower density.

“The numerical models of planetary system formation and evolution that we have developed at Bern over the last two decades reproduce exactly this trend: 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 rise to our Moon,” concludes Yann Alibert, professor at UNIBE’s Space Research and Planetary Sciences Division (WP) and co-director of the Center for Space and Habitability and co-author of the study.

Journal reference:

  1. Adrien Leleu, Jean-Baptiste Delisle, Remo Burn, André Izidoro, Stéphane Udry, Xavier Dumusque, Christophe Lovis, Sarah Millholland, Léna Parc, François Bouchy, Vincent Bourrier, Yann Alibert, João Faria, Christoph Mordasini and Damien Ségransan. Resonant sub-Neptunes are puffier. Astronomy & Astrophysics, 2024; DOI: 10.1051/0004-6361/202450587