Where are all the ‘hot Neptune’ exoplanets? Orbital chaos may have booted them out

Astronomers have launched a new program known as ATREIDES to study a mysterious “desert” in space. But unlike the deserts of the planet Arrakis conquered by Paul Atreides in the “Dune” novels by Frank Hebert, this desert describes an absence of planets with masses up to around 20 times the mass of Earth that orbit close to their stars, planets scientists refer to as “hot Neptunes.”

The first planets studied by the ATREIDES program, the two worlds of the TOI-421 system, demonstrate misaligned orbits, hinting that this system experienced a more chaotic evolution than our solar system. Studying it could help astronomers figure out why these “hot Neptunes” appear to be so rare in the cosmos, as well as teach us about how planets form elsewhere in the universe.

“The complexity of the exo-Neptunian landscape provides a unique window onto the processes involved in the formation and evolution of planetary systems,” ATREIDES Principal Investigator and University of Geneva (UNIGE) researcher Vincent Bourrier said in a statement describing the ATREIDES program.

To understand why this class of extrasolar planet, or “exoplanet,” is missing from close orbits around other stars, ATREIDES scientists investigated the TOI-421 planetary system. Located around 244 light-years from Earth, TOI-421 is an orange dwarf or “K-type” star orbited by two exoplanets, TOI-421 b and TOI-421 c. What this investigation revealed is a surprisingly tilted orbital situation in TOI-421 that implies that this system experienced a chaotic history, one which may help explain why hot Neptunes are so rare.

TOI-421 b is a scorching hot sub-Neptune planet with a mass around 7 times that of Earth that orbits its star at a distance equivalent to around 6% of the distance between our planet and the sun. TOI-421 c is larger, with a mass of around 14 times that of Earth, which orbits its star at a distance equivalent to around 12% the distance between Earth and the sun, making it a hot Neptune and putting it in a region adjacent to the Neptunian desert called “the savanna.”

“A thorough understanding of the mechanisms that shape the Neptunian desert, savanna, and ridge will provide a better understanding of planetary formation as a whole … but it’s a safe bet that the universe has other surprises in store for us, which will force us to develop new theories,” Bourrier said.

Mapping the Neptunian desert

Over the last 10 years of exoplanet observations, the Neptunian desert has become increasingly complex. Areas further out from stars than the Neptunian desert have been found to be more generously populated with Neptune-sized worlds. This more temperate realm with more Neptune-like exoplanets has come to be known as the “savanna” of the Neptunian desert.

Astronomers have also defined a region between the Savanna and the Neptunian desert, which they call the “Neptunian ridge.” This region is more densely populated by Neptune-like worlds than both the desert and the savanna. The scientists of the ATREIDES program aim to understand these three distinct regions by identifying the processes that lead to the relative planetary populations.

The team wants to test the hypothesis that the Neptunian landscape is created as a result of the way that planets migrate from their birthplaces to the orbits we observe them in.

Some exiled planets would migrate slowly through the disk of gas and dust that exists in these systems during their infancy. This sedate migration should produce planets in orbits aligned with their star’s equator and the orbits of the other planets in their home system. That is similar to the orbits of the planets in the solar system, which are aligned almost to the equatorial plane of the sun.

However, some other planets would be violently thrown from their site of formation via a chaotic process called “high-eccentricity migration.” That should result in those planets falling into highly misaligned orbits.

That means the alignment between a star’s orbital plane and the orbital plane of its planets is key to investigating this migration hypothesis.

The team can’t yet say anything conclusive yet about the Neptunian desert, its neighboring regions, or planetary evolution in general. Many more observations of more planetary systems with hot Neptunes will be needed for that.

However, this research successfully demonstrates the effectiveness of the ATREIDES program and the techniques it has developed and employed.

The team’s research was published on Tuesday (Sept. 16) in the journal Astronomy & Astrophysics.