This phrase likely refers to the movement of exoplanets, specifically Hot Neptunes, on the outskirts of their star’s habitable zone. The “Hot Neptune Desert” is a region in the planetary system where it is thought that Hot Neptunes are unlikely to form, due to the high levels of irradiation from their host star. The term “tumultuous migration” may refer to the process by which these exoplanets may have migrated to their current positions, potentially through interactions with other planets or through interactions with the disk of gas and dust from which they formed.
A team reveals the eventful migration history of planets bordering the Hot Neptune Desert, these extrasolar planets that orbit very close to their star. Exoplanets of various types orbit very close to their star. Some resemble the Earth, while others resemble Jupiter. However, only a few are comparable to Neptune. Why is there a discrepancy in the distribution of exoplanets?
To better understand the formation of this Hot Neptune Desert, researchers from the University of Geneva (UNIGE) and the National Centre of Competence in Research (NCCR) PlanetS observed a sample of planets near the edge of it. They were able to establish that a portion of these exoplanets migrated in a turbulent manner near their star, pushing them out of the orbital plane where they were formed, using a technique that combined the two main methods of studying exoplanets (radial velocities and transits). I have published these findings in the specialized journal Astronomy & Astrophysics.
The distribution of planets close to their star is shaped by a complex interaction between atmospheric and dynamical processes, i.e. the motions of the planets over time.
Vincent Bourrier
Since the discovery of the first exoplanet, researchers have detected more than 5,000 planets in our galactic neighborhood, most of them orbiting very close to their star. If the diversity of these new worlds ranges from gas giants the size of Jupiter or Saturn to smaller planets the size of Mercury, including rocky planets larger than the Earth, gas planets the size of Neptune seem to be missing. Astronomers call this empty ”box” in the distribution of close-in planets the Hot Neptune Desert.
“The distribution of planets close to their star is shaped by a complex interaction between atmospheric and dynamical processes, i.e. the motions of the planets over time,” says Vincent Bourrier, assistant professor at the UNIGE Faculty of Science. “Today, we have several hypotheses to explain this desert, but nothing is certain, and the mystery remains,” says one researcher. Were these planets’ atmospheres completely destroyed by their star’s intense radiation? Did they migrate from their birthplace to the system’s outskirts via a different mechanism than other types of planets, preventing them from reaching the same close orbits as other types of planets?
Disrupted migration
In a recent paper, a team of UNIGE scientists provides some answers by studying the orbital architecture of planets near the desert’s edge. The astronomers were interested in the way the orbits of fourteen planets around this area, ranging from small planets to gas giants, are oriented with respect to the axis of rotation of their star. This information allows us to distinguish between soft migration processes (in which planets move in the equatorial plane of the star where they formed) and disruptive migration processes (the planets migrate and are pushed out of the plane where they were formed).
The researchers were able to show that most of the planets in their sample have an orbit misaligned with the stellar equator. ”We found that three-quarters of these planets have a polar orbit (they rotate above the poles of their star), which is a larger fraction than for planets further away from the desert. This reflects the role of disruptive migration processes in the formation of the desert,” summarizes Vincent Bourrier, first author.
Two methods combined
The scientists used the radial velocity method and the transit method, which are both used to study exoplanets, to achieve these results. “Analyzing radial velocities during a planet’s transit allows us to determine if it orbits around the stellar equator, around the poles, or if the system is in an intermediate configuration, because different architectures will produce different signatures,” explains Omar Attia, a doctoral student in the Department of Astronomy at the UNIGE Faculty of Science and the study’s second author. These two methods were combined with data from the UNIGE-developed HARPS and HARPS-North spectrographs, which were installed on ESO’s 3.6m telescope and TNG (Telescopio Nazionale Galileo).
Understanding all of the mechanisms involved in the formation of the Hot Neptune Desert is still a long way off. It will be especially important to use this technique to investigate the smallest planets on the outskirts of the desert, which are currently difficult to reach even with instruments of last generation, such as the spectrograph ESPRESSO, built by UNIGE and installed on the largest European telescopes. It will be necessary to wait until the ELT, ESO’s 39-meter super telescope, is completed in 2027.
