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Title: The Hottest Neptunes Orbit Metal-Rich Stars
Authors: Shreyas Vissapragada and Aida Behmard
Authors’ Institutions: Carnegie Science Observatories; Flatiron Institute and American Museum of Natural History
Status: Published in AJ

Sub-Neptune exoplanets, those between the size of Earth (1.0 R_Earth) and Neptune (4.0 R_Earth), are the most common type of planet discovered to date, with every other star hosting at least one on average. However, the class of “sub-Neptunes” is itself divisible into sub-categories based on the planet’s orbital period: these are called “hot,” “warm,” and “cold” for planets that are very close to their star, at a medium distance from their star, and farthest out, respectively. While the lines dividing these three categories are a little blurry, recently the community has begun roughly defining them as those with orbital periods shorter than 3.2 days (hot), those with periods between 3.2 and 5.7 days (warm), and those with periods longer than 5.7 days (out to 100 days, cold). The orbital period of the planet is directly related to the planet’s distance from its host star, so planets with longer orbital periods are farther out from their stars.

Furthermore, it seems that planets in these three categories are not equally abundant across the galaxy: cold sub-Neptunes are far more common than warm sub-Neptunes, which in turn are more common than hot sub-Neptunes. In fact, hot sub-Neptunes are seemingly so rare that exoplanet scientists have coined the term “the Neptune desert” to convey the emptiness of this region of parameter space (see Figure 1 below). But if sub-Neptunes are so common overall, why are the hottest of this class of planets so rare?

There are a few working hypotheses for how hot sub-Neptunes form, each of which predicts that their formation be very rare. First, perhaps they formed in the same way as the common cold versions, but through unique circumstances avoided having their atmospheres stripped by photo-evaporation from the star. Second, they may have formed via collisions of many small planets in the early days of the planetary system, creating one large planet on a short orbit. Or third, they could be “failed” hot Jupiters, meaning they initially formed as much more massive Jupiter-like planets and then, through one or more mechanisms, lost most of their mass until reaching their present-day size.

Today’s Astrobite reports on an article that attempts to find the most likely of these three formation mechanisms. The authors focus on the metallicity of the host stars of hot Neptunes (defined as those with masses between 10 and 100 Earth masses) and ask the question, “Do the host stars of hot Neptunes have a similar metallicity distribution to host stars of any other class(es) of planets?” The idea being, if another class of planets forms around similar host stars, perhaps the planets themselves formed through similar mechanisms.

The authors set out to test the hot Neptune (A) host-star population against four other populations of planet hosts: warm Neptunes (B), cold Neptunes (C), hot Earths (E), and hot Jupiters (D). See Figure 1 for where these populations lie on the period–mass plane. Metallicity studies are often difficult to perform for a number of reasons: first, it is difficult to measure precise metallicity for a single star, and second, data from different instruments and/or different measurement techniques are often inconsistent with each other. The authors have come up with a way to get around both of these issues by using a single source for their data: the Gaia mission’s radial-velocity spectrometer, through which they were able to collate precise measurements in a homogeneous fashion.

With this data, the authors find that the metallicity distribution of the host stars for hot Neptunes is similar to that of both warm Neptunes and hot Jupiters, but different for hot Earths and cold Neptunes. This means that hot and warm Neptunes likely formed via the same mechanism as hot Jupiters, or are themselves the remnants of hot Jupiters that just couldn’t hold onto most of their mass for one or multiple reasons. This has implications for our understanding of planet formation in general, and better contextualizing of the hot Neptune desert in particular. If hot Neptunes are the remnants of failed hot Jupiters, then perhaps they will exhibit other qualities that are known in the hot Jupiter population. For example, that they are usually found as single-planet systems, or with a distant giant planet or star that could have facilitated high-eccentricity migration. In future studies, it will be interesting to target these hot sub-Neptunes with JWST and compare their transmission spectra with those of hot Jupiters to see if the atmospheres of these two kinds of planets are consistent with one another as well.

Original astrobite edited by Catherine Slaughter.

About the author, Jack Lubin:

Jack received his PhD in astrophysics from UC Irvine and is now a postdoc at UCLA. His research focuses on exoplanet detection and characterization, primarily using the Radial Velocity method. He enjoys communicating science and encourages everyone to be an observer of the world around them.

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Understanding exoplanetary system architectures is crucial to uncovering how planetary systems form and evolve. A recent study homes in on the XO-3 system to explore how planets orbit their host stars and what that may mean for their formation pathways.

Alignment Trends

Observations of exoplanetary systems have unveiled multiple interesting properties and characteristics that have expanded our understanding of planet formation beyond our solar system. One such property is the spin–orbit alignment — the direction a planet orbits its host star with respect to the star’s spin. In our solar system, the planets orbit in the same direction the Sun is spinning, but for many exoplanets, this is not the case.

Over the last few years, researchers have noticed a trend in which systems with high planet-to-star mass ratios tend to be more aligned, while planets with smaller mass ratios tend to exhibit larger spin–orbit misalignment. For example, hot Jupiters orbiting small, cool stars tend to remain aligned, while hot Jupiters orbiting massive, hot stars often are misaligned. 

There’s one exception to this trend: planet XO-3b. Orbiting a hot star and within a high planet-to-star mass ratio system, XO-3b was the first exoplanet observed with a high spin–orbit misalignment and has thus been the subject of multiple studies. However, its orbital angle is still uncertain. If this planet is truly an outlier, this would further complicate our understanding of planet formation mechanisms.

Getting to Know XO-3b 

To confirm whether XO-3b is a true outlier and understand why this planet does not bend to the trend, Jace Rusznak (Indiana University) and collaborators acquired new observations of the XO-3 system with NEID, a radial velocity instrument on the WIYN 3.5-meter telescope. By observing how the star’s radial velocity changes as the planet passes in front of the star, they can precisely determine the spin–orbit alignment of the planet as well as measure the properties of the XO-3 system.

Combining the new radial velocity measurements with previous Transiting Exoplanet Survey Satellite (TESS) data for XO-3b, the authors determine that the planet has a spin–orbit misalignment of 40.2 degrees. To confirm that this planet is truly an outlier, the team constructs a sample of single-star exoplanetary systems using the NASA Exoplanet Archive. They find a statistically significant difference in the alignment of systems with planet-to-star mass ratios above and below 2×10^-3. With a high planet-to-star mass ratio of 9×10^-3, the XO-3 system is unusually misaligned compared to other systems with similar planet-to-star mass ratios. 

Undetected Companion

Now left with the conundrum of how XO-3b wound up misaligned, the authors consider the possibility of an undetected stellar binary companion that could have pulled the planet out of line. Previous observations of XO-3 did not directly observe a physically associated stellar companion, but errors in the Gaia astrometry of the star are atypically high for a single-star system. These errors could be the result of an undetected stellar companion gravitationally interacting with both XO-3 and its planet. 

Further observations of the XO-3 system are required to confirm that the system is actually a stellar binary, but if a second star is found, XO-3b would no longer be an outlier in the spin–orbit alignment versus planet-to-star mass ratio trend. This would increase the significance of the emerging relationship that, for single-star systems, planets with high planet-to-star mass ratios tend to be aligned even around hot stars, suggesting that planets in such systems are born aligned and stay that way.

Citation

“From Misaligned Sub-Saturns to Aligned Brown Dwarfs: The Highest Mp/M* Systems Exhibit Low Obliquities, Even around Hot Stars,” Jace Rusznak et al 2025 ApJL 983 L42. doi:10.3847/2041-8213/adc129

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