Exoplanet surveys are useful for more than just astrobiology or increasing the tally of known planets in other solar systems. They can also help us understand the evolution of planetary systems themselves. That’s what a new paper from researchers led by astronomers at the University of Geneva and published in Astronomy & Astrophysics attempts to do - by looking at a large population of “exo-Neptunes” they are attempting to understand the intricacies of how planetary systems are formed.
Apparently one of the members of the massive team is a huge Dune fan because they somehow managed to come up with the name Ancestry, Traits, and Relations of Exoplanets Inhabiting the Desert Edges and Savanna - ATREIDES - for the observational program. As a fellow Dune fan, I applaud the commitment to that acronym, but the idea of “desert edges” and “savannas” doesn’t make much sense without the context of what the survey was actually doing.
One part of the paper describes three different “areas” around other stars with varying levels of exo-Neptunes - in this case defined as exo-planets about ten times the size of Earth. Close to the star, with an orbital period of about 3-4 days, there is a “desert”, where there is a conspicuous lack of Neptune-sized exoplanets, despite theories suggesting there should be plenty. Farther out is a “savanna”, which are at orbital periods longer than 6 days, where there are some of these exoplanets but still not as many as would be expected. That means there’s also a sweet spot, which the paper calls a “ridge”, in between these two regions at about 3-6 days of an orbital period, where there’s a huge number of Neptune-sized planets.
Fraser discusses one of the other mysteries of exoplanet populations.This pattern holds for dozens of stars, and should hold clues as to how the systems themselves are formed. But another major factor in that formation process is what the exo-Neptunes are actually made out of. The paper posits two different types - “fluffy” Neptunes that have extended atmospheres and “Dense” Neptunes whose atmospheres are much more dense, like our own. These each react very differently to the forces present during the formation of planetary system, which the paper also defines as two different categories.
Disk-drive migration (DDM) is a “gentle” process of dragging a planet toward the star using gravitational pull, and typically happens early in the formation of a system. In this scenario, the planet’s orientation and alignment remain remarkably stable. Fluffy Neptunes seem to be more susceptible to this force, and part of the reason there’s a “desert” is that when they get too close, the solar winds begin to strip off their atmosphere, leaving them a rocky core that no longer fits into the category of “exo-Neptune”.
High-Eccentricity Tidal Migration (HEM) is the other evolutionary process mentioned in the paper. It is much more violent the DDM, and typically happens much later in the life of a system. It results from a gravitational nudge, either from a companion star or another planet in the system, that throws the entire system into gravitational chaos. This method typically affects Dense Neptunes more often, and can result in highly eccentric orbits that are misaligned with the star.
Image of a "hot Neptune" that would be located in the desert and actively having its atmosphere stripped off. Credit - NASA/JPL-Caltech/K. Miller (Caltech/IPAC)
The “ridge” that holes the overabundance of Neptunes, appears to be where many exo-Neptunes end their gravitational journey of both DDM and HEM. Any closer and they would have their atmospheres stripped away, but any further and they might not have settled into a stable orbit yet. To prove this theory, the researchers used the ESPRESSO spectrograph on the ESO’s Very Large Telescope in Chile to track the Rossiter-McLaughlin effect, which measures the distortion in starlight caused by an exoplanet’s transit in front of its host star, with particular attention being paid to the red and blue shifted edges of the star as it rotates from our perspective. Watching closely can determine the exoplanet’s orbit and its eccentricity, lending credence to the theory of how the planets of a system got to their current state.
TOI-421 was the first system the survey looked at in detail. It hosts two known planets, a “sub-Neptune” (TOI-421b) and a “warm Neptune” (TOI-421c) located in the savanna. Close inspection with EPSRESSO noted that the two planets were both extremely misaligned with their host star’s equator, they had extremely eccentric orbits, and might even be misaligned with each other’s orbital plane by as much as 35 degrees. All of these findings point to a highly chaotic origin of the system, with both an early phase of DDM and later phase of HEM the likely culprit. TOI-421c has not yet settled into the ridge after surmounting its gravitational disturbances.
But this wa just the first of many systems to be analyzed using these analytical methods. Exo-Neptunes are a particularly common type of exoplanet, so there should be an abundance of data for the research team, which consists of more than 30 scientists, and is open to all comers, to comb through. As they begin to, maybe they’ll come up with another great acronym for their planetary evolution theory - A Realistic Reckoning About Kool Interplantery System - ARRAKIS - anyone?
Learn More:
University of Geneva - The ATREIDES program in search of lost exo-Neptunes
V. Bourrier et al - ATREIDES I. Embarking on a trek across the exo-Neptunian landscape with the TOI-421 system
UT - An Exo-Neptune Beat the Odds and Kept its Atmosphere
NASA - What Are Neptunian Planets?
