HD 219134: A Nearby System with Multiple Transits

While we’ve all had our eyes fixed on TRAPPIST-1 (amid the still lingering excitement of the discovery of Proxima Centauri b), news about another stellar neighbor has caused only a faint stir. But what’s happening around HD 219134 (Gliese 892) is noteworthy, and it’s interesting to see that Michaël Gillon (University of Liège – Belgium) has had a hand in it. Gillon, after all, led the work on TRAPPIST-1’s two waves of exoplanet discoveries, culminating in the startling assemblage of seven Earth-sized worlds around the dim ultracool dwarf star.

HD 219134 is an orange K-class star (K3V) in the constellation Cassiopeia, and only about half the distance, at 21.25 light years, as TRAPPIST-1 (about 40 light years out). It was known before the recent Gillon et al. paper in Nature Astronomy that we had a super-Earth, HD 219134 b, in orbit here, which was soon joined by two more super-Earths, a gas giant and, a few months later, another two planets, making for a total of six.

This system characterization was radial velocity work using the HIRES Spectrograph at the Keck I telescope which you can track in A Six-Planet System Orbiting HD 219134 — Steven Vogt was lead author on that paper. I notice that Greg Laughlin, also at UC-Santa Cruz and a co-author on the Vogt paper, has singled out the latest work by Gillon and team on his systemic site. That’s because Gillon adds to the one already known transiting world (HD 219134 b) another transiting planet (HD 219134 c), which gives us the closest transiting exoplanet to Earth yet found.

Image: Exoplanet hunter Michaël Gillon (University of Liège, Belgium).

Both transits are Spitzer detections, and both planets have mass and radius estimates that make a rocky composition possible (4.74 ± 0.19 M ⊕ and 1.602 ± 0.055 R ⊕ for HD 219134 b, and 4.36 ± 0.22 M ⊕ and 1.511 ± 0.047 R ⊕ for HD 219134 c). Laughlin adds that this system “…very cleanly typifies the most common class of systems detected by the Kepler Mission — multiple-transiting collections of super-Earth sized worlds with orbital periods ranging from days to weeks. Upscaled versions, that is, of the Jovian planet-satellite members of our own solar system.”

Image: From the systemic site, the HD 219134 system plotted on a mass-period diagram of exoplanets found through the various methods of detection. Credit: Greg Laughlin.

We can expect to hear a good deal more about HD 219134 in upcoming space-based observations. Unfortunately, the K-class star is too bright for some, though not all, James Webb Space Telescope instruments. JWST may be able to detect atmospheric signatures for both transiting planets if they have extended atmospheres dominated by hydrogen. More compact atmospheres dominated by H 2 O or CO 2 would demand precisions out of reach for JWST because of stellar brightness. The paper adds: “…the precisions are limited not by the photon noise but by the instrumental systematics.”

However, we may not be done with transits in this system. The detection of the HD 219134 c transit increases the probability that planets d and f also transit. From the paper:

Using the formalism of previous work, we compute posterior transit probabilities of 13.1% and 8.1% for planets f and d, respectively, significantly greater than their prior transit probabilities of 2.5 and 1.5%… A transit detection for one or both of these planets would increase further the importance of the system for comparative exoplanetology, and a search for their transits is thus highly desirable. Although such a transit search is probably out of reach of ground-based telescopes, it could be performed again by Spitzer, whose operations have been extended to end-2018, or by the space missions TESS [Transiting Exoplanet Survey Satellite] and CHEOPS [CHaracterising ExOPlanet Satellite], which are both due to launch in 2018.

Nearby transiting planets around a small star make HD 219134 a target we’ll be investigating for a long time to come. We can begin to put constraints on possible atmospheres for the two inner worlds in this system, and also tighten up our figures for planetary mass and radius while gaining valuable insights into the formation of super-Earths. You can see how the high-priority target catalog is growing for future observations, and we can expect our space-based assets to continue to add to it as TESS and CHEOPS come online.

The paper is Gillon et al., “Two massive rocky planets transiting a K-dwarf 6.5 parsecs away,” published online by Nature Astronomy 2 March 2017 (full text).