A turbulent migration on the edge of the hot Neptune desert


Turbulent migration on the edge of the hot Neptune desert

Distribution of nearby exoplanets as a function of their radius and orbital period. The green and blue features show the approximate boundaries of the Neptune desert and savanna. The white squares indicate the outer planets with measured rotation angles. The planets in our sample are highlighted by blue stars, whose projections are projected onto the sky plane to obtain the best fit orbital structures. By default, we show a configuration where the stellar rotation axis (shown as a black arrow extending from the north pole) points toward Earth, except for HAT-P-11 and Kepler-63 where the degeneracy in i* is broken and a configuration where the south pole is visible is preferred. The stellar equator, plotted as a solid black line, appears only in systems where the stellar tilt (and thus the 3D orbital angle) is constrained. The stellate disc is colored as a function of its RV surface area. The normal plane relative to the planetary orbital plane is shown as a green arrow extending from the center of the star. The solid green curve represents the orbital trajectory of best fit. The thin lines surrounding it show the obtained orbits for the orbital inclination, semi-major axis, and randomly plotted sky-projected spin orbit angle values ​​within 1 of their probability distributions. The star, planet (black disk), and orbit must fit into a specific system. credit: Astronomy and astrophysics (2023). DOI: 10.1051/0004-6361/202245004

All kinds of exoplanets orbit close to their star. Some are similar to Earth, others are similar to Jupiter. Very few of them look like Neptune. Why this anomaly in the distribution of exoplanets? Researchers from the University of Geneva (UNIGE) and the National Center for Competence in Research (NCCR) observed a sample of planets on the edge of the hot Neptune desert to understand their creation.


Using a technique that combines the two main methods for studying exoplanets (radial velocities and transit), 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 formed. These results are published in the journal Astronomy and astrophysics.

Since the discovery of the first exoplanet in 1995, researchers have discovered more than 5,000 planets in the galactic neighborhood, most of them orbiting 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, incl rocky planets Larger than Earth, Neptune-sized gas planets seem to be missing out. Astronomers call this empty “box” in the distribution of nearby planets the Neptune hot desert.

“The distribution of planets close to their star is shaped by a complex interplay between atmospheric and dynamical processes, i.e. the motions of the planets over time,” comments Vincent Poirier, Assistant Professor in the Department of Astronomy in UNIGE’s Faculty of Science. “Today we have several hypotheses to explain this desert, but nothing is confirmed yet, and the mystery remains.”

Have these planets completely lost their atmospheres, eroded by the intense radiation of their star? Did they migrate from their place of birth to the outer parts of the system by a mechanism different from other types of planets, preventing them from reaching the same close orbits?

disrupted migration

In recent work, a team of scientists from UNIGE has provided some answers by looking at the orbital architecture of planets at the edge of this desert. By surveying fourteen planets around this region, ranging from minor planets to gas giants, astronomers have been interested in the way their orbits are oriented with respect to their star’s rotational axis. This information makes it possible to distinguish between soft migrations (planets move in the equatorial plane of their star where they formed) from disruptive migrations (planets migrate and are pushed out of the plane in which they formed).

The researchers were able to show that most of the planets in their sample have orbits that are oblique with the stellar equator. “We found that three-quarters of these planets have a polar orbit (they orbit above the poles of their star), which is a larger fraction than the planets farther from the desert. This reflects the role of disruptive migration processes in the formation of the desert,” says Vincent Poirier, first author.

Two ways combined

To achieve these results, scientists used radial velocity method and the transit method, which is employed to study exoplanets. Omar Attia explains that “analyzing the radial velocities during the transit of a planet allows us to determine if it is orbiting around the stellar equator, around the poles, or if the system is in an intermediate configuration, because different structures will produce different signatures,” a PhD student in the Department of Astronomy at the Faculty of Unige Sciences and second author of the study.

These two methods were combined with data obtained using the HARPS and HARPS-North spectrometers, built at UNIGE and located on the 3.6-m telescope of ESO (European Southern Observatory) and TNG (Telescopio Nazionale Galileo).

There is still a long way to go to fully understand all the mechanisms involved in the formation of Neptune’s hot desert. In particular, it will be necessary to explore the smallest planets on the edge of the Sahara using this technique, which is difficult to reach even with the latest generation instruments such as the ESPRESSO spectrometer, which was built by UNIGE and installed on the largest European telescopes. It will be necessary to wait for the commissioning of ELT, ESO’s 39-meter Super Telescope planned for 2027.

more information:
Poirier et al., Dream, Astronomy and astrophysics (2023). DOI: 10.1051/0004-6361/202245004

Introduction of
University of Geneva

the quote: Turbulent Migration at the Edge of Hot Neptune Desert (2023, January 18) Retrieved January 19, 2023 from https://phys.org/news/2023-01-tumultuous-migration-edge-hot-neptune.html

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