Title: The Hottest Neptunes Orbit Metal-Rich Stars
Authors: Shreyas Vissapragada and Aida Behmard
Authors’ Institution: Carnegie Science Observatories and AMHM/Flatiron Institute
Status: Preprint posted on Arxiv
Sub-Neptune planets, those with radii 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 furthest out, respectively. While the lines dividing these three categories are a little blurry, recently the community has begun roughly defining them as those interior to 3.2 day orbital periods (hot), those between 3.2 and 5.7 day orbital periods (warm), and those beyond 5.8 day orbital periods (out to 100 days, cold). The orbital period of the planet is directly related to the distance the planet is from the host star, so longer orbital period planets are further out from their star.
Furthermore, it seems that these three categories are not equally abundant across the galaxy: cold sub-Neptunes are far more common than warm, which in turn are more common than hot. In fact, hot sub-Neptunes are seemingly so rare, exoplanet scientists have coined the term “the Neptune desert” to convey the emptiness of this region of parameter space (see Figure 1). But if in general sub-Neptunes are so common, 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 atmosphere stripped by photo-evaporation from the star. Second, they may have formed via collisions of many small planets in their 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 a much more massive Jupiter-like planets and then, through one or more mechanisms, lost most of their mass until reaching their present day size.
Figure 1: The mass-period plane of exoplanets with metallicity color-coded. The lettered boxes denote different classes of planets, with hot sub-Neptunes marked as “A”. The authors compare the host stars of these planets to the host stars of other populations of planets. The orbital period of the planet is on the x axis, the mass (or best mass estimate) is on the y axis, and color indicates metallicity.
Today’s Astrobite reports on a new paper that attempts to find the most likely of these three formation mechanisms. The authors focus on the metallicity of the host stars of hot sub-Neptunes, and ask the question, “Do the host stars of hot sub-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 sub-Neptune (A) host star population against four other populations of planet hosts: warm sub-Neptunes (B), cold sub-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 sub-Neptunes is similar to that of both warm sub-Neptunes and Hot Jupiters, but different for hot Earths and cold sub-Neptunes. This means that hot and warm sub-Neptunes likely formed from 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 sub-Neptune desert in particular. If hot sub-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 Hot Jupiters to see if the atmospheres of these two kinds of planets are consistent with one another as well.
Featured image credit: Vissapragada & Behmard 2024
Edited by: Catherine Slaughter
Author
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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