What are the two most fundamental properties of the stars we study? If you said mass and chemical composition, you get the prize, at least as determined by the California & Carnegie Planet Search team. Their new paper lays out the discovery of a gas giant orbiting the M-class red dwarf GJ 317. And they first discuss the discovery in the context of the core accretion model for planetary formation, and the correlation between the metallicity of a star and the chances of its harboring detectable planets.
The notion seems sound: The host star inherits its characteristics from the same disk out of which the planets around it form. If you increase the amount of metals in the system (metals being defined as elements higher than hydrogen and helium), you increase the surface density of solid particulates, and that ought to bump up the growth rate for the core materials that become planets. In a gas giant, such a core then becomes massive enough to capture a gas envelope.
But the case around M dwarfs, those dim red stars that may comprise as much as 75 percent of the galaxy, needs a special look. If the mass of a protoplanetary disk scales with the mass of its central star, then larger mass stars should be more likely to produce planets. Greg Laughlin (UC-Santa Cruz) has studied this relationship in low mass stars like red dwarfs. His finding: Because their disks have lower surface densities (and longer orbital time scales), such stars should have problems producing Jupiter-mass planets. That leads to Neptune mass worlds that have exhausted their supply of gases in the disk through which they move.
So far, the analysis squares with observation. Most of the planets detected around M dwarfs are a good deal smaller than Jupiter. Until the California & Carnegie team’s recent work, only two nearby M dwarfs were known to have Jupiter-mass companions: GJ 876 and GJ 849. The upshot is that the frequency of giant planets is two to three times higher among stars of the Sun’s mass as compared to M dwarfs. All this, of course, has to be kept in the context of the relatively small sample within whose confines we work.
But GJ 317 now adds to the total, with the team finding a gas giant in an 1.897 year orbit around the star. We now have six M dwarfs known to harbor at least one Doppler-detected planet, with GJ 317 being just the third out of 300 surveyed to show a Jupiter-class world. A second possible Jovian planet is also being monitored in a 2700 day orbit, but no firm detection is yet being claimed.
But if the second planet is borne out, the result shouldn’t surprise us. Note this from the discovery paper (internal references deleted for brevity):
Multi–planet systems appear to be relatively common among M dwarfs compared to Sun–like stars. All M stars with one Jovian planet show evidence of a second companion. GJ 876 has a pair of Jupiter–mass planets in a 2:1 mean motion resonance, along with an inner “super Earth” GJ 849 has a long–period Jovian planet with a linear trend. Of the 3 M dwarfs with Neptune–mass planets, two have multiple planets or evidence of an additional companion: GJ 581 harbors 3 low–mass planets, and GJ 436 has a linear trend. Only GJ 674 appears to be in a single–planet system. From the ﬁrst 6 planet detections around low–mass stars, it appears as though M dwarfs have an 80% occurrence rate of multi–planet systems, compared to the 30% rate measured for FGK stars.
Interesting, no? One possible cause for the difference is that fact that planets around low mass stars are more readily detectible by radial velocity methods. On the other hand, the team notes that all the Jovian planets detected around M dwarfs so far would have been detectable around Solar-mass stars. So we seem to be looking at a real effect here, one that will demand the accumulation of a lot more data through future surveys before its implications are fully understood.
Another key finding of this work is that A-type stars in the broader stellar sample studied are fully five times more likely than M dwarfs to harbor a gas giant. The team’s conclusion:
This important result establishes stellar mass as an additional sign post for exoplanets, along with metallicity. Just as metallicity informs the target selection of searches for short–period planets, stellar mass will be an important factor in the target selection of future high–contrast direct imaging surveys. While the lower luminosities of M dwarfs provide favorable contrast ratios that facilitate the detection of thermal emission from young giant planets, our results show that A–type stars are far more likely to harbor such planets.
Thus the correlation between stellar mass and the likelihood of finding giant planets seems to be firming up, but as the authors note, we need a larger star sample to really understand the relationship. The California & Carnegie team have added a number of higher mass stars to their earlier samples in hopes of tightening the focus. As we range between the smallest and some of the largest planet host stars, we’ll not only find more and more planets, but also uncover facts that should help in the target choice for future planet-hunter spacecraft.
The paper is Johnson et al., “A New Planet Around an M Dwarf: Revealing a Correlation Between Exoplanets and Stellar Mass,” accepted by the Astrophysical Journal and available online.