Title: Small and Close-In Planets are Uncommon around A-type Stars
Authors: Steven Giacalone and Courtney Dressing
Authors’ Institutions: Caltech and UC Berkeley
Status: Preprint on Arxiv
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 paper is as fascinating as it is empty of planets: the authors searched ~20,000 stars and found a whopping zero planets. What’s the deal?
While the Kepler mission’s survey of staring at a single patch of sky for 4 straight years in searching for exoplanet transits has dominated exoplanet statistics for over the past decade, Kepler focused primarily on spectral types F, G, K, and M. These stars are all less than ~7000 K and while there is a lot of variation among these types, 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,big planets around small stars . Essentially, the occurrence rate measures how many planets of a specified type (like “super-Earths” or “Hot Jupiters” or other) 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 make a measurement of the occurrence rate of different kinds of planets around hosts of this stellar type. The A stars are hotter than our Sun, they start out at ~7500 K and go up to ~10,000 K. 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 (RV) surveys. They both have far fewer spectral lines and they spin much faster than our Sun which both decrease RV 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 the star.
Then, when 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 paper comes in. The authors used the TESS data set to search A-type stars for small planets (between 1 and 8 Earth radii) on short orbits (orbital period less than10 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. Of these, the pipeline identified 299 transit candidates. Looking more closely at these with a complementary dataset, they 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 find that not one passed and therefore the pipeline found zero planets.
Despite finding no planets, null results in science are still very important and in this study, they were used to place constraints on the occurrence rates of different kinds of planets that orbit A-type stars. In particular, they find that sub-Neptune sized planets occur 6 times less frequently around A-type stars than they do around FGK stars. The authors dive into this result and point to earlier studies which 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, see Figure 1. This is a fascinating result: big hot stars seem to host fewer planets than smaller, cooler stars. Why?
The authors note a few caveats to their result. These A-type stars are known to pulsate more often than cooler FGKM stars and this pulsation is imprinted into the flux measurements of the star, which can make it more difficult to find planets. Accounting for these pulsations is another project in itself but could explain why they didn’t find any planets. Furthermore, they acknowledge that A-type stars spin so fast, they actually flatten out at the poles and bulge out at the equator. This in turn makes the high latitudes of the star brighter and the equator dimmer, thus finding small planets that orbit at higher inclinations becomes even more difficult. This effect, called gravity darkening, is essentially not a problem for FGKM stars.
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 UV, which can strip the atmospheres of Neptune-like planets through a mechanism called photo evaporation. 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 binaries, which is the case for many A-type stars, canmake 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 posit if A-type stars either make fewer planets or are able to retain fewer planets. The gas/dust disk that forms 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 the disk doesn’t survive long enough to produce many planets. On the other hand, the A-type stars are very massive and studies show that the large gas disks they produce when they are born in turn 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/temperature regimes. Sometimes finding nothing is just as meaningful!
Astrobite edited by Maria Vincent
Featured image credit: Giacalone & Dressing 2024
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