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Title: Small and Close-In Planets are Uncommon Around A-Type Stars
Authors: Steven Giacalone and Courtney D. Dressing
Authors’ Institutions: California Institute of Technology and University of California, Berkeley; University of California, Berkeley
Status: Published in AJ
Some of the most interesting articles in exoplanet science concern the discovery of new exoplanets. After all, most of us get into this field with dreams of discovery. Today’s article is as fascinating as it is empty of planets: the authors searched roughly 20,000 stars and found a whopping zero planets. What’s the deal?
While the Kepler mission’s survey that stared at a single patch of sky for four straight years searching for exoplanet transits has dominated exoplanet statistics for more than a decade, Kepler focused primarily on host stars with spectral types F, G, K, and M (i.e., FGKM stars). These stars all have temperatures less than about 7000K, and while there is a lot of variation among these types of stars, they are considered relatively Sun-like (our Sun is a G-type star). Through observing these stars, we were able to build up robust statistics on the occurrence rates of different kinds of planets. Astrobites has covered occurrence rate studies in the past (see these bites on small planets around M-dwarf stars, occurrence of systems with similar architecture to our own, and big planets around small stars). Essentially, the occurrence rate measures how many planets of a specified type (like “super-Earths” or “hot Jupiters”) are likely to be found if you survey a certain number of stars with a certain spectral type.
However, the Kepler mission did not observe enough A-type stars to measure the occurrence rate of different kinds of planets around hosts of this stellar type. A-type stars are hotter than our Sun, with temperatures between about 7500K and 10000K. They are bigger in both mass and radius than our Sun, and they emit more of their light in the ultraviolet portion of the electromagnetic spectrum. Very little is known about planetary systems around A-type stars, in large part due to Kepler’s blind spot, but also because these stars make for very poor targets in radial-velocity surveys. A-type stars have far fewer spectral lines than Sun-like stars and they spin much faster than our Sun, both factors that decrease radial-velocity sensitivity greatly compared to the more Sun-like stars. Most of what is known of planetary systems around A-type stars comes from direct-imaging surveys, which are only sensitive to massive Jupiter-size planets at very large separations from their stars.
Then, when the Transiting Exoplanet Survey Satellite (TESS) came along with the plan to observe most of the night sky, an opportunity to search for planets around more A-type stars became available. That’s where today’s article comes in. The authors used the TESS dataset to search A-type stars for small planets (between 1 and 8 Earth radii) on short orbits (orbital period less than 10 days). To say that this search was an immense labor is almost an understatement. The authors wrote a custom software pipeline to search through the dataset, identify potential transits, and then apply a few rounds of vetting on each candidate.
The authors first had to identify all the A-type stars in the TESS catalog — about 20,000 stars. Of these, the pipeline identified 299 transit candidates. Looking more closely at these with a complementary dataset, the authors ruled out many as false positives (mostly obvious-by-eye eclipsing binaries), leaving only 88 candidates remaining. Next, they inspected the candidates for secondary eclipses, which would indicate the transit is not planetary but stellar; this effort ruled out another half, leaving only 44 candidates. Next they tested for background eclipsing binaries, which can mimic planet transits, cutting more candidates so that only 10 remained. Lastly, they performed statistical tests on the leftovers by analyzing the way nearby stars get brighter or dimmer during the time of the transits to see if anything is correlated. In the end, they found that not one candidate passed and therefore the pipeline found zero planets.
Despite finding no planets, this null result is still very important. In this study, the null result was used to place constraints on the occurrence rates of different kinds of planets that orbit A-type stars. In particular, the authors found that sub-Neptune-sized planets occur six times less frequently around A-type stars than they do around FGKM stars. The authors dive into this result and point to earlier studies that show a decrease in planet occurrence as the host stars get hotter. In fact, this result is well in line with these earlier studies, but now the trend is finally investigated for the even hotter A-type stars, as shown in Figure 1. This is a fascinating result: big hot stars seem to host fewer planets than smaller, cooler stars. Why?
Figure 1: The occurrence rate of sub-Neptune planets versus host-star temperature. Previous studies show that sub-Neptunes are common around smaller, cooler stars. As host-star temperature increases, the occurrence of sub-Neptunes goes down. This has been noted in earlier works, but this work extends the temperature regime greatly at the hot end and shows that sub-Neptunes are very rare around hotter A-type stars. [Adapted from Giacalone and Dressing 2025]
Next, the authors discuss what their result means for the big picture of our understanding of planet formation. First, they discuss if this could be part of an observing bias. There are two ways that A-type stars make finding transits more difficult. A-type stars emit lots of their light in the ultraviolet, which can strip the atmospheres of Neptune-like planets through a mechanism called photoevaporation. Perhaps all the gaseous planets that orbit A-type stars have been stripped of their atmospheres and all that is left behind are the relatively small rocky cores, which are very hard to detect via transits. Additionally, the existence of binary systems, to which many A-type stars belong, can make planets harder to detect. If what is thought to be a single star is in fact a binary, the second star’s extra light can make the transit depths even smaller and therefore even harder to detect.
Finally, the authors pose the question of whether A-type stars simply make fewer planets or are able to retain fewer planets. The disks of gas and dust that form around all stars when they are born, and from which planets are born, are dissipated by the strong A-type star’s stellar winds much faster than those of FGKM stars. Perhaps these disks don’t survive long enough to produce many planets. On the other hand, A-type stars are very massive and studies show that the large gas disks they produce when they are born should produce many planets and in particular many big planets. Perhaps these systems are born with many large planets that make the system gravitationally unstable and all the inner, small planets get flung out of the system.
In all, the authors provide a rigorous study of the occurrence (or lack thereof) of small planets in close orbits of A-type stars. This work sheds light on a severely understudied population and better rounds out our understanding of planet formation across stellar mass and temperature regimes. Sometimes finding nothing is just as meaningful!
Original astrobite edited by Maria Vincent.
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.