Work on the Centauri Dreams internals continues, with the unwelcome result that the site has been popped offline twice because of a possible security problem. Needless to say, this has to be resolved before I can move forward on other aspects of the rebuild. While I deal with that issue, let me respond to a few backchannel questions about yesterday’s post on gas giants in red dwarf planetary systems. What I’m being asked about is my comment that gas giants like Jupiter, at similar distances and installation, around other classes of stars are common compared to what we see at red dwarfs.
This has been a problematic issue, and the matter is a long way from achieving a consensus among researchers. A moment’s reflection yields the reason: Finding gas giants in outer system orbits around a star like the Sun is no easy matter. Radial velocity is most sensitive when dealing with large planets in tight orbits, which is why the first detections in main sequence stellar systems, beginning back in 1995 with 51 Pegasi b, were of the ‘hot Jupiter’ variety. That in itself offered new insights into planetary formation and dynamics. As physicist Isidor Isaac Rabi cogently asked when the muon was first detected, “Who ordered that?”
We’re making all kinds of advances in radial velocity as we use ever more sophisticated instruments to measure the motion induced by orbiting bodies around distant stars, but if we back out to, say, 5 AU, Jupiter’s distance from the Sun, we’re still dealing with extremely tiny effects. Transits are problematic because a planet on a five-year orbit obviously transits its host on long timeframes. Gravitational microlensing is an interesting prospect, because here we can detect planets at the needed distances, but even so the catalog isn’t large and there is much we don’t know.
Fortunately, resources like the California Legacy Survey (719 stars over three decades) are available and have produced data on what we can call ‘cold giants.’ I made my comment because of a paper in the Astrophysical Journal Supplement Series that I learned about through the Pass et al. paper we looked at in the previous post. This is from Caltech’s Lee Rosenthal and colleagues, and it examines the combination of small rocky planets with outer gas giants using the CLS for the bulk of its data. The result is a look at the occurrence of close-in planets with outer giant companions.
The Rosenthal paper addresses radial velocity work on F-, G-, K- and M-class stars and targets both categories of planets, finding that roughly 41 percent of systems with a close-in small planet also host an outer giant. By close-in small planet, the authors mean planets orbiting from 0.023–1 AU with a mass twice to 30 times that of Earth. And the giant planets examined are from 0.23–10 AU and 30 to 6000 Earth masses.
The implication is that stars hosting small inner planets are more likely to have an outer gas giant, for the number is roughly 17 percent for stars irrespective of small planet presence. There is much to be done with data from the California Legacy Survey (the baseline of RV observations goes back to 1988, and is invaluable), but studies like these lead to the conclusion that planets in Jupiter-like orbits are not uncommon among F-, G- and K-class stars. As to the M-dwarfs, the Pass paper indicates the scarcity of gas giants around them, with all that may imply about inner planet habitability. Note that the CLS is made up mostly of FGK stars, with 98% of stars in the sample having stellar masses above 0.3 solar masses..
I haven’t had time to dig into a previous paper using the California Legacy Survey data, this one from Benjamin Fulton (Caltech) with Rosenthal as a co-author, but do note that the authors find that the occurrence of planets less massive than Jupiter (from 30 Earth masses up to 300 as per RV data) is enhanced near 1–10 AU “in concordance with their more massive counterparts.” The complete citation is below.
We still have much to learn about exoplanet system architectures, but we’re making progress as the inflowing current of high-quality data grows ever more powerful.
The paper is Rosenthal et al., “The California Legacy Survey. III. On the Shoulders of (Some) Giants: The Relationship between Inner Small Planets and Outer Massive Planets,” Astrophysical Journal Supplement Series, Vol. 262, No. 1 (17 August 2022), 262 1 (abstract). The Fulton paper is “California Legacy Survey. II. Occurrence of Giant Planets beyond the Ice Line,” Astrophysical Journal Supplement Series Vol. 255, No. 1 (9 July 2021), 255, 13 (abstract).