The new planet discovered around the red dwarf GJ 849 isn’t just another footnote in the unfolding story of exoplanet discoveries. This world, a gas giant about 80 percent as massive as Jupiter, promises to teach us more about planet formation around M-class stars, by far the most common stellar objects in the galaxy (with the possible exception of brown dwarfs). And the more we learn about the so-called core accretion model, the more we’ll understand what to expect as we point our telescopes in the direction of other red dwarfs.
Not bad for a planet circling one of the 152 stars within 200 parsecs of the Sun known to have planets. But bear this in mind: most of the stars around which we’ve found these planets have been like our Sun, in the range of 0.7 to 1.3 times its mass. We’ve studied over 200 M-class dwarf stars with planet detection in mind, but until now had discovered planets around only three. GJ 436 shows a Neptune-class world, as does GJ 581, while GJ 876 seems to be a triple system, and is the only one known to contain Jupiter-sized planets. GJ 849 thus makes the fourth M-class planetary system.
Red dwarfs are tiny objects, but that makes planets of a certain mass easier to detect, and the fact that we’ve found so few gas giants around them could be seen as evidence that the disks out of which planets form are less robust around red dwarfs than Sun-like stars. That would make a certain amount of sense, for the core accretion model says that when gas giants form, rocky cores build up through collisions in protoplanetary disks around young stars, reaching a critical mass after which gas from the disk accumulates in runaway fashion.
So if red dwarfs have smaller, less massive protoplanetary disks, the planets forming there may never reach the needed critical mass. That would suggest that red dwarfs are probably more likely to have Neptune-class ‘ice giants’ than planets like Jupiter. But here’s the problem: our data only produce information about planets orbiting around 2 AU and closer to the parent star. In other words, we need longer observation periods to see whether there are not in fact Jupiter-class worlds further out in the systems around such stars.
So the new planet is an exciting find. GJ 849 b is a gas giant some 2.35 AU out, an order of magnitude further than any other M-class planet, and only the second Jupiter-mass world found around a red dwarf. More data are now required to see whether there are more gas giants around these stars than current thinking suggests. If there are, we may have to revise our ideas about core accretion in red dwarf systems, whose protoplanetary disks may turn out to be more massive than previously imagined.
The paper on this work is R. Paul Butler et al., “A Long-period Jupiter-mass Planet Orbiting the Nearby M Dwarf GJ 849,” available here in preprint form. And note this item from the paper: “At 8.8 pc, the orbital separation of GJ 849 b corresponds to a projected separation 0″.27. Thus, the proximity of GJ 849 provides a unique opportunity for high-resolution imaging using adaptive optics and future space-borne astrometric missions such as the Space Interferometry Mission.”
