The challenge of working with a small sample of exoplanetary systems — and one tilted toward those detectible through radial-velocity methods — is that building up solid models of planet formation is tricky. I’m thinking about this in terms of the recent planetary conference at Santorini, and also recalling work performed at the University of Texas, where Michael Endl and team have looked into the relationship between planets around red dwarfs and the metallicity of their stars.
It’s an intriguing question and one that only continuing observations can nail down. Metallicity refers to the presence of elements higher than hydrogen and helium in a star’s composition, something we can determine through spectroscopic analysis. Endl and co-author Fritz Benedict, as originally noted in this post, worked with graduate student Jacob bean on a study of three dwarfs known to have planets: Gliese 876, Gliese 436 and Gliese 581, noting their lower values of metallicity compared to stars of spectral types F, G and K (our Sun is a G-type star).
Most red dwarfs studied in our surveys thus far show low metallicity, and the number of high-mass planets found around them is small. Are higher levels of metallicity necessary for gas giants to form? It sounds perfectly logical: More dust in the protoplanetary disk should encourage planetary formation, so we should expect few gas giants around red dwarfs, as seems to fit current observation. This backs the core accretion model of planet formation, in which planets build up rocky cores as they make their way through crowded protoplanetary disks, eventually becoming massive enough to begin the process of accumulating dense atmospheres.
But every exception to the rule helps us understand the rule better, and draws the comparison between it and alternatives like the disk instability model. We already have two gas giants around Gliese 876. And now the California & Carnegie team have come up with two more gas giants, both around the star GJ 317. Steinn Sigurðsson (Pennsylvania State) noted this find at the Santorini conference. GJ 317 is an M3 red dwarf some 10 parsecs from here with about a fourth the Sun’s mass. The planetary masses are 1.2 and 0.8 Jupiter masses respectively, with orbital periods of 673 and 2700 days. “Bit of a theory buster,” muses Sigurðsson, “high mass planets in wide orbits around a low mass metal poor star…”
Maybe not so much a theory buster as a theory tweaker. But that’s how we learn things, finding the anomalies and figuring out how to account for them. The result is a sounder theory that can encompass oddball worlds where we do run into them. We need a larger sample of M dwarfs to develop broader patterns for mass and metallicity, and we need time — perhaps we’re going to begin finding more gas giants in wide orbits around M dwarfs as our data accumulate. Ultimately, we’ll use such observational data to help tune up our target list for planet-finder missions looking for terrestrial worlds around such stars.
The Extrasolar Planets Encyclopedia offers what is known about GJ 317b and GJ 317c. Note that the latter is still a work in progress as radial-velocity data accumulate. The Texas work is Bean et al., “Metallicities of M Dwarf Planet Hosts from Spectral Synthesis,” Astrophysical Journal Letters 653 (December 10, 2006), L65-L68 (available online).