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).