If you’re thinking about detecting Earth-like planets around other stars, here’s an item that may set the pulse racing a bit faster. Michael Endl, who is an expert at the planet hunt around red dwarf stars (he’s searched for planets around 100 of them already), notes that the diminutive objects are prime targets for exoplanet hunters. And listen to this: “For the red dwarfs with the lowest masses, like Proxima Centauri, we are sensitive to planets down to two Earth masses using the standard radial velocity technique.”
Endl works at the University of Texas, out of which a study led by graduate student Jacob Bean has focused on planet formation around red dwarfs — we looked at this work not long ago. Few gas giants have been detected around red dwarfs. The study examined the dwarfs known to have planets: Gliese 876, Gliese 436, and Gliese 581. Of the three, Gliese 876 is perhaps the most intriguing, as it’s known to have two Jupiter mass planets and a likely third, lower-mass world orbiting around it.
Bean wanted to find out whether dwarfs like these known planet-bearers show high metallicity values (elements heavier than hydrogen and helium). Using computer modeling coupled with telescope observations, he was able to determine that the three dwarfs under study actually have significantly fewer metals than the 200 or so Sun-like stars known to harbor planets. Thus the possibility that the low number of high-mass planets found in our red dwarf surveys thus far may have something to do with low metallicity values in the stars selected for study.
And that makes sense, because a proto-stellar cloud with higher metallicity would be more likely to ‘grow’ a planet. Here’s co-author Fritz Benedict (University of Texas) on the matter:
“Just as rain drops need a speck of dust in the air around which to form, the formation of planets is thought to be assisted by a similar successful first step. More dust in the protoplanetary disk might increase the chances for planet formation.”
But it’s clear that these findings — on just three planetary systems — are the tip of the iceberg, so much work on red dwarf metallicity lies ahead. It’s significant because we need to know why there are so few gas giants around these stars, or whether there are gas giants in wider orbits that have simply not been discovered yet. We also need to know how best to target our planet searches, and if metallicity values for red dwarfs can be established in that regard, we can focus in on the most likely candidates.
I come back to Endl’s comment that radial velocity techniques are sensitive down to planets of just two Earth masses for some red dwarf studies. Given enough time to gather the RV data, we should begin to learn how often such worlds form around these stars. And Bean’s work on metallicity may help us estimate how often they’re joined by Jupiter-class objects in wider orbits. That’s exciting in its own right, but it also works toward filling in much broader patterns of planet formation that will help us characterize solar systems around a variety of stellar types.
The paper is Bean et al., “Metallicities of M Dwarf Planet Hosts from Spectral Synthesis,” Astrophysical Journal Letters 653 (December 10, 2006), L65-L68. And as referenced in our earlier article on this work, it’s also available at the arXiv site.