Centauri Dreams

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).

Gas giant worlds like Jupiter may be uncommon around red dwarf stars, as a number of recent studies have found. It would be useful to tighten up the data, however, because many of the papers on this matter have used stellar samples at the high end of the mass range of M-dwarfs. At the Center for Astrophysics | Harvard & Smithsonian (CfA), Emily Pass and colleagues have gone to work on the question by looking at lower-mass M-dwarfs and working with a lot of them, some 200 in their sample, all within 15 parsecs.

The question is not purely academic, for some scientists suggest that the presence of a Jupiter-class planet – not uncommon around G-class stars like the Sun – is a factor in the development of life. Migrating inward from a formation in the first few hundred million years of the Solar System’s existence, Jupiter would have stirred up plenty of icy cometary bodies through gravitational interactions. Impacts from this infall into the inner system likely delivered a great deal of water and organic molecules to the young Earth, thus becoming a factor in the development of life.

Thus a system like TRAPPIST-1, with its seven rocky planets orbiting a nearby red dwarf, raises the question of whether such a system would have gone through this kind of mixing. No one knows whether life would have begun on Earth without these effects, but the suggestion that systems without a gas giant are barren is plausible. So just how common are red dwarf systems with gas giants equivalent to Jupiter based on what we know so far? It’s telling that only two of the known gas giants orbiting a red dwarf occur around stars of less than 30 percent of the Sun’s mass: LHS 252 b and GJ 83.1 b.

Image: A gas giant around an M-class dwarf, as visualized by artist Melissa Weiss, CfA.

What Pass and team deliver is a statistical analysis, using spectroscopic surveys and radial velocity data on nearby M-dwarfs in the mass range of 0.10–0.30 stellar masses. The data are presented in a paper now in process at The Astronomical Journal. The results confirm the belief that red dwarfs are seldom the hosts for Jupiter-class worlds. In fact, in the entire sample, not a single Jupiter-equivalent planet occurred, allowing the authors to conclude that Jupiter analogues must be found in fewer than 2 percent of low-mass red dwarf systems:

Planets that are Jupiter-like in mass and instellation are rare around low-mass M dwarfs, consistent with expectations from core accretion theory. Compared with previous radial-velocity and microlensing studies that consider broader distributions of M-dwarfs with higher mean stellar masses, our results are consistent with a decrease in giant planet occurrence with decreasing M-dwarf mass…

The authors note the complications of comparing occurrence rate between the various surveys that have so far attempted it, but add:

…the picture of giant planet occurrence from microlensing is still unclear. If Poleski et al. (2021) are correct in their assertion that every microlensing star has a wide-orbit giant planet, our results imply that the distribution of giant planets around low-mass M dwarfs must differ dramatically from more massive stars, whose giant planets are more prevalent near the water snow line than on wide orbits.

These are interesting findings especially in terms of habitability. Rather than assuming that red dwarf planets are unlikely to have life, they could just as easily point to the differences between these systems and our own as offering other avenues for life to develop. CfA’s David Charbonneau makes the point explicitly: “We don’t think that the absence of Jupiters necessarily means rocky planets around red dwarfs are uninhabitable.”

What we do have are planetary systems different enough from ours to encourage speculation on what factors might produce life in different ways than our own system. Consider that the lack of gas giants also indicates more raw material for planetary formation on the scale of smaller rocky worlds. Given the proximity of red dwarf stars with rocky planets, they’ll be at the forefront of astrobiological investigation as we develop the ability to study their atmospheres. The possibilities remain open, and perhaps exotic, as we continue the hunt for life elsewhere. Adds Pass:

“We have shown that the least massive stars don’t have Jupiters, meaning Jupiter-mass planets that receive similar amounts of starlight as Jupiter receives from our Sun. While this discovery suggests truly Earth-like planets might be in short supply around red dwarfs, there still is so much we don’t yet know about these systems, so we must keep our minds open.”

The paper is Pass et al., “Mid-to-Late M Dwarfs Lack Jupiter Analogs,” in process at The Astronomical Journal (preprint).