New astronomical programme launched to find lost Neptunes
A collaboration of astronomers, led by University of Warwick and University of Geneva, has launched a vast research program on Neptune-like planets to find those lost near the Neptunian Desert
When looking at planetary systems, there is a region of space very close to the orbiting star known as the ‘Neptunian Desert’, because medium-sized Neptune-like planets are surprisingly rare to find in it. It is thought that the Desert receives strong irradiation from the system’s star, making it challenging for medium-sized planets to hold onto a gaseous atmosphere.
Recently two further regions have been identified; the Savanna (further out from the star with sparse exo-Neptunes) and the Ridge (between the two zones with a surprising number of exo-Neptunes). These varied regions provide an ideal playground for understanding the history of planetary systems and why regions such as the Desert, Ridge and Savanna exist in the first place.
Taking up this mission to map exoplanets located around the Neptunian Desert is an ambitious new international program – ATREIDES - including the Astronomy and Astrophysics Group at the University of Warwick, the University of Geneva Astronomy Department and the Instituto de Astrofísica de Canarias. As of today, a research paper published in the journal Astronomy & Astrophysics of the first exo-Neptune system characterized by ATREIDES (TOI-421) marks the beginning of the collaboration.
ATREIDES – Conquering the Desert
The principle observations of the exo-Neptunes from the ATREIDES project are taking place using the ESO's Very Large Telescope (VLT) with the world's most accurate spectrograph, ESPRESSO. The University of Warwick is contributing with its NGTS telescopes, an exoplanet observation program using the transit method—when a planet passes in front of its star.
“We use NGTS to observe the transit of these Neptunes and thus optimize our use of ESPRESSO/VLT. We can then obtain much more accurate measurements or identify processes, such as stellar eruptions, that could influence the ESPRESSO data,” explains Daniel Bayliss, Associate Professor in the Department of Physics at the University of Warwick.
Scientists plan to observe many Neptunes with ESPRESSO and analyse and model data from all planets in a comprehensive, consistent, and coherent framework. This systematic approach should enable a real comparison between different planetary systems and a better understanding of the mechanisms that shape this complex Neptunian landscape.
"The complexity of the exo-Neptunian landscape provides a real window into the processes involved in the formation and evolution of planetary systems. This is what motivated this ambitious scientific collaboration, ATREIDES, which is focused in particular on a large-scale observation program that we are conducting using the largest European telescopes, the ESO's VLTs with ESPRESSO," explains Vincent Bourrier, Senior Lecturer and Researcher in the Astronomy Department of the Faculty of Science at UNIGE, principal investigator of the ATREIDES program and lead author of the study published today in the journal Astronomy & Astrophysics.
The first pinch of spice: TOI-421
The TOI-421 system is the first Spectro Photometric Inquiry of a Close- in Exoplanet (SPICE) to be published from ATREIDES. It has two Neptune-like planets: a warm Neptune TOI-421 c located in the Savannah far from the star, and a smaller Neptune-like planet closer to the star, TOI-421 b, sitting in the Ridge, close to the Desert.
The existence of hot Neptunes in the ridge is still not fully understood. One of the popular hypotheses that the ATREIDES program aims to address is that these Neptunes migrate from their birthplace to their current orbits in the Ridge in a chaotic process.
Planets, such as those in our Solar System, migrate slowly and early through the gas disk in which they formed producing nicely aligned orbits. Others are violently propelled into their orbits much later through a chaotic process called high-eccentricity migration, which results in highly misaligned orbits. For TOI-421, the two planets in the system are highly misaligned, pointing to a turbulent history in the evolution of the TOI-421 system after its formation.
“A thorough understanding of the mechanisms that shape the Neptunian Desert, the Savannah, and the 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 that will force us to develop new theories,” concludes Vincent Bourrier
The analysis of TOI-421 is the first piece of evidence that a violent origin explains the existence of close orbiting exo-Neptunes. Many planetary systems with exo-Neptunes will need to be observed and analysed with the same rigor as this one before we can outline the evolution and formation of planetary systems. To do this, the ATREIDES collaboration invites all interested astronomers to join the scientific effort.
ENDS
Notes to Editors
The paper ‘ATREIDES I. Embarking on a trek across the exo-Neptunian landscape with the TOI-421 system’ has published in Astronomy & Astrophysics. DOI: 10.1051/0004-6361/202554856
University of Warwick Contact:
Matt Higgs, PhD | Media & Communications Officer (Press Office)
Email: Matt.Higgs@warwick.ac.uk | Phone: +44(0)7880 175403
Image Credits:
Cover Image - Artist’s concept of “hot Neptune” Credit: NASA/JPL-Caltech/K. Miller (Caltech/IPAC)
Graph - Planet radius as a function of orbital period for all known exoplanets, showing the location of the Neptunian desert, ridge, and savanna – Credit: A. Castro-González et al. A&A, 689 (2024) A250
Artist Illustration of finding the lost Neptunes – Credit: Elsa Bersier
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16 September 2025