Very few, however, are similar to Neptune. Why this anomaly in the distribution of exoplanets? Researchers from the University of Geneva (UNIGE) and the National Center of Competence in Research (NCCR) Planet have observed a sample of planets located at the edge of this Hot Neptune Desert to understand its creation. Using a technique combining the two main methods of studying exoplanets (radial velocities and transits), they were able to establish that a part of these exoplanets has migrated in a turbulent way near their star, which pushed them out of the orbital plane where they were formed. These results are published in the specialized journal Astronomy & Astrophysics. 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.
We test this hypothesis by carrying out a 12-night photometric survey of the metastable helium feature with Palomar, targeting seven gas-giant planets orbiting K-type host stars. We strongly detect helium absorption signals for three planets, tentatively detect signals for two planets, and do not detect signals for the remaining two planets. We interpret these measured excess absorption signals using grids of one-dimensional Parker wind models to derive mass-loss rates, and using these rates we empirically benchmark 1D hydrodynamical outflow models for the first time. The resulting outflow efficiencies are too small to carve the upper boundary of the Neptune desert. We conclude that this feature of the exoplanet population is a pristine tracer of giant planet formation and migration mechanisms. The path to understand of the mechanisms involved in the formation of the Hot Neptune Desert is still long. It will be necessary in particular to explore with this technique the smallest planets at the edge of the desert, today difficult to access even with instruments of last generation such as the spectrograph ESPRESSO, built by the UNIGE and installed on the largest European telescopes. It will be necessary to wait for the commissioning of the ELT, the 39-meter super telescope of ESO, planned for 2027. Transit surveys indicate that there is a deficit of Neptune-sized planets on close-in orbits. If the entirety of this “Neptune desert” is cleared out by atmospheric mass loss, then planets at its upper edge should only be marginally stable against photo evaporation, exhibiting strong outflow signatures in tracers like the metastable helium triplet Neptune exoplanets are similar in size to Neptune or Uranus in our solar system. (Neptune is about four times the size, or radius, of Earth and almost 17 times its mass, or weight.) Neptune exoplanets may though all would be rocky with heavier metals at their cores. Neptune typically have hydrogen- and helium-dominated atmospheres. We’re also discovering Neptune, planets smaller than Neptune and bigger than Earth. No planets like these exist in our solar system.
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