As astronomers found more and more exoplanets in recent years, they've observed an unusual gap in the population. It's called the Neptunian Desert, a curious scarcity of Neptune-sized exoplanets orbiting close to their stars. Researchers just discovered an exoplanet in the Neptunian Desert around a Sun-like star. Can it help explain the Desert?
Finding exoplanets close to their stars is easier than finding them further from their stars. Astronomers are more likely to detect close-in planets when using the transit method because they transit more often. Detecting close-in planets is also easier when using the radial velocity method because they tug on their stars more. With more than 5,000 confirmed exoplanets, scientists are in a position to describe the exoplanet population with some confidence. This is how they arrived at the Neptune Desert, also called the Radius Gap.
Despite finding massive gas giants and small rocky planets close to their stars, there's a notable lack of Neptune-sized planets in this parameter space. We have pattern-seeking brains, and when we detect patterns like this, scientists are driven to understand their causes. The first part of that process is determining if an observational bias is at play.
In the case of the Neptune Desert, observational bias isn't responsible. NASA's Kepler spacecraft would've easily detected exoplanets in the desert using the transit method, and so would TESS. Radial velocity would detect them too. Multiple exoplanet surveys with different telescopes, methods, and criteria all confirm the Neptune Desert. After discarding observational bias, scientists got down to figuring out why the Desert exists.
This graphic illustrates the Neptune Desert. It plots exoplanets based on their size and distance from their stars. Planets the size of Jupiter (located at the top of the graphic), planets the size of Earth, and so-called super-Earths (at the bottom) are found both close to and far from their stars. But planets the size of Neptune (in the middle of the plot) are scarcely found close to their stars. The exoplanet labelled in the image, GJ3470b, is not part of this study. Image Credit: NASA, ESA and A. Feild (STScI)
An international team of astronomers has found a Neptune-sized planet in the Neptune Desert orbiting a Sun-like star about 550 light-years away. Three exoplanets orbit the star TOI-1117. Following convention, the planets are named TOI-1117 b, TOI-1117 c, and TOI-1117 d. The Neptune-sized planet in the Desert is TOI-1117 b. "This is a rare "hot Neptune" that falls within the parameter spaces known as the "Neptunian Desert" and the "Radius Valley"," the paper's authors explain. The Radius Valley is related to the Neptune Desert. It's another exoplanet parameter space that's strangely unpopulated. Together, they describe the puzzling region where very few Neptune-sized planets exist.
The Radius Gap is an exoplanet parameter space where very few planets reside. It's also called the Fulton Gap. Image Credit: Fulton et al. 2017
The discovery is presented in research titled "The TOI-1117 Multi-planetary System: 3 sub-Neptunes, 1 in both the Neptunian Desert and Radius Valley." The lead author is Isobel Lockley from the Department of Physics at the University of Warwick in the UK. The paper will be published in the Monthly Notices of the Royal Astronomical Society. ""This paper presents TOI-1117 b, a transiting short-period sub-Neptune residing in the Neptunian desert, along with two outer sub-Neptunes, TOI-1117 c and TOI-1117 d, on longer, near-resonant orbits," the authors write.
"The Neptunian Desert is an area of period-radius and period-mass parameter space where significantly fewer exoplanets have been discovered," the authors write. Astronomers have found a handful of planets in this space and are keen to understand why there are so few. "Current theories explaining the lack of exoplanets in the Neptunian Desert include photoevaporation and tidal disruption, which induces strong tidal forces and extreme temperature variations, stripping the planets of their gas envelopes and leaving Earth-sized rocky cores," the authors explain. Another possible explanation is that the conditions that allow gas giant formation prevent Neptune-mass planets from settling into close-in orbits during the planet formation process.
These plots show all NASA Exoplanet Archive planets by mass in radius-period space. Left shows TOI-1117 b (red star) falling within the Neptunian Desert (pale blue region). Middle is a Mass-period plot showing TOI-1117 b (red), c (green), and d (blue) and the Neptunian Desert. Right plots radius against insolation flux and shows TOI-1117 b to be within the Radius Valley (pale pink region). Image Credit: Lockley et al. 2025. MNRAS.
TOI-1117 b is puzzling for more than just its position in the Neptune Desert. Its composition is also baffling. The exoplanet is close to the edge of the Neptune Desert and is very close to its star, taking only 2.23 days to complete an orbit. It has a low density, which suggests that it has an atmosphere rich in volatiles. "This is puzzling given the strong X-ray and extreme ultraviolet (EUV) flux the planet is expected to receive, which should have stripped the planet of any gaseous atmosphere via photoevaporation," the authors explain.
To understand the planet, the researchers simulated its evaporation history under various irradiation and atmospheric Hydrogen-Helium fractions. Hydrogen and Helium are the two most abundant gases and provide clues about how planets form in protoplanetary disks. They're trying to understand if the planet could have formed in its current location and held onto its atmosphere. They also considered two scenarios about its internal structure. One where the planet has a rocky core surrounded by a H/He envelope, and one with a rocky core with a water-rich layer and no H/He envelope.
This figure from the research shows the evolution of TOI-1117 b's radius with a range of starting envelope mass fractions, assuming rocky-H/He internal structure with no water (left panel), and a rock-water composition with no gaseous envelope (right panel). The red circle shows the planet's current radius. Each coloured line represents different starting H/He fractions, and the coloured areas around each line represent different X-ray irradiation histories from a solar-mass star. Image Credit: Lockley et al. 2025. MNRAS.
"The inner planet, TOI-1117 b, is on the edge of the Neptunian Desert and in the Radius Valley," the authors write in their conclusion. "The planetary parameters of TOI-1117 b are consistent with a range of internal structure models, from an Earth-like composition to an ocean world with no atmosphere."
The exoplanet's low density means it should have a gaseous atmosphere, but its proximity to its star suggests that atmosphere should be stripped away by photoevaporation. "This contradiction could be explained by the presence of a water-rich layer," the authors explain.
The fact that TOI-1117 b is in a multi-planet environment with two other sub-Neptunes confounds the issue. "TOI-1117’s multi-planet system does not conform with current formation theories," the authors explain. "For TOI-1117 b to be water-rich, it likely formed beyond the snow line and migrated inwards."
As it stands, the authors can't explain why TOI-1117 b is in the Neptune Desert. If the JWST can examine its atmosphere, that would provide more clues. "Atmospheric characterization should be performed, ideally with JWST, to provide information on the evolution of TOI-1117 b," Lockley and her co-authors write. If the JWST can confirm a steam atmosphere or an ocean, that could explain a lot.
Astronomers may not have found all of the planets in this distant system, either. Giant planets could explain how migration worked in the system and if TOI-1117 b formed further from the star and migrated inward. "Giant planets on longer orbits could be present and need to be investigated with future radial velocity measurements," the authors conclude.
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