Tuesday, August 11, 2015

A Transit in Our Own Back Yard

Figure 1. Schematic view of the four-planet architecture proposed for HD 219134, a small Sun-like star of spectral class K3 at a distance of only 6.53 parsecs (21 light years). Each planet icon is designated by letter, along with the minimum planet mass in Earth units. A transit-like event, potentially involving planet b, was observed by the Spitzer Space Telescope.
A team led by Fatemeh Motalebi has just reported a remarkable exoplanetary system right in the Sun’s back yard. The host star, HD 219134, is a metal-rich K3 dwarf located only 21 light years away in the direction of Cassiopeia. As such, it has been an object of exobiological interest for some years (Porto de Mello et al. 2006). Using radial velocity data, Motalebi and colleagues present a mixed-mass architecture with three low-mass planets inside a semimajor axis of 0.25 AU and one small gas giant just outside 2 AU (Figure 1, Table 1).

Although such a hierarchical arrangement of masses and orbits is rare in the current extrasolar census, HD 219134 bears a family resemblance to five other mixed-mass systems distributed near and far: HD 10180, GJ 676A, Kepler-48, Kepler-68, and Kepler-90. In each of these, two or more low-mass planets orbit inside 0.5 astronomical units (AU) and one or more gas giants orbit outside 1 AU. If we relax the boundary for the inner planets to 0.7 AU, the same description fits our own Solar System.

Table 1. Characteristics of the planetary system around HD 219134
Column 1 provides the planet name; column 2, the minimum mass in Earth units (Mea); column 3, the semimajor axis (a) in astronomical units (AU; 1 AU = distance of Earth from Sun); column 4, the orbital eccentricity (e); and column 5, the orbital period in days.
But there’s more! Given the short orbital period of the innermost planet (b), its geometric transit probability is significant. To explore this likelihood, the discovery team were granted access to the Spitzer Space Telescope during a predicted transit window. Observations of the host star yielded a light curve consistent with a transit. Therefore, in addition to obtaining radial velocity data indicating a minimum mass of 4 to 5 Earth units (4-5 Mea) for planet b, they were able to calculate a radius in the range of 1.52 to 1.69 Earth radii (Rea), with a preferred value of 1.606 Rea. If the presence of transits can be confirmed by additional observations, HD 219134 b will add to the small but invaluable population of nearby low-mass planets detected both in transit and by radial velocity measurements. Given their proximity, these planets can be studied in fine detail, unlike their Kepler counterparts, which are often several hundred or even a few thousand light years away.

HD 219134 has a mass of 0.78 Msol, a radius of 0.778 Rsol, a metallicity of +0.11, and an effective temperature of 4699 K. Given its radius and spectral type, its luminosity is only 26.5% Solar. According to recent models by James Kasting and colleagues (2014), these characteristics correspond to a habitable zone extending from 0.5 to 0.9 AU. Current data on HD 219134 leave this orbital space unoccupied, whereas in our own system, Venus dominates the same range of semimajor axes.

Since the discovery paper is still a preprint, with formal publication at some unknown future date, we should consider the available results as preliminary. Nevertheless, given the extraordinary nature of this system, those results are worth examining in detail.

  • The three inner planets of HD 219134 are similar in radius and orbit to those in typical Kepler systems, which feature planets with radii between 1 and 3 Rea, likely masses between 1 and 15 Mea (Weiss & Marcy 2014), and periods shorter than 100 days.
  • In terms of mass, the system’s single gas giant falls in the “sub-Jovian desert” (Colon et al. 2015), as it is only two-thirds as massive as Saturn, another wanderer in the same desert. Radial velocity planets are rare at masses between 30 and 100 Mea (especially at the lower end of the range) while transiting planets are rare at radii between 5 and 10 Rea. At 95 Mea, for example, Saturn has a radius of 9.4 Rea.
  • The eccentricities of adjacent planets d and e – respectively 0.32 and 0.27 – are quite high for such a compact system. At face value they suggest a violent dynamical history involving planet scattering. Among the eight Solar planets, by comparison, the highest eccentricity is Mercury’s, at 0.21. All the other planets have eccentricities smaller than 0.1. Yet even these small values offer evidence of a violent past. Relevant to eccentricity is the question of additional stable orbits around HD 219134, especially in the gap between planets d and e. This is where habitable planets would be, if they exist at all. Perturbations from eccentric planet e, however, might keep this gap clear.
  • A major limitation of the data on HD 219134 is the availability of only one proposed transit for planet b. Repeated measurements over time are needed to confirm the reality of transits and refine our picture of the planet and its home system. An immediate challenge is to assess whether any of the other planets transit. Planet c is a likely candidate, given its short period. Planet d is also possible, as its large minimum mass suggests a radius between 2 and 4 Rea. Nevertheless, the relatively high eccentricity indicated for its orbit opens the possibility that planet d is misaligned with the inner planets.
  • The bulk composition of planet b is a question of supreme interest, since it’s one of the few confirmed exoplanets smaller than 2 Rea for which reliable mass values are available (or will be soon). The discovery team preview their own answer to the question in their title, which names “a transiting rocky planet.” Placing their confidence in the high quality and precision of the radial velocity data from HARPS-N and the photometric observations by Spitzer – as interpreted in light of current theory – they describe the planet’s density as “consistent with the value . . . predicted for a 1.606 Rea planet obeying the Earth-like compositional model presented in Dressing et al. (2015).” In the study cited, however, a planet of 1.6 Rea and rocky composition would have a mass in the vicinity of 5 to 6 Mea, notably higher than the preferred value of 4.46 Mea presented for HD 219134 b. Nevertheless, Courtney Dressing herself is one of the authors of the discovery paper, as is Eric Lopez, known for several compelling studies of planetary structure and mass loss (Lopez et al. 2012; Lopez & Fortney 2013, 2014; Wolfgang & Lopez 2015). Maybe the numbers can be made to fit.
In a postscript to the preprint, the authors mention that Stephen Vogt and colleagues have also detected planetary signals from HD 219134. So we can look forward to new information (possibly complementary, possibly divergent) about this already fascinating system. If future observations confirm the transiting status of planet b, this system could become one of the classic case studies in planetology and orbital architecture, alongside such luminaries as 55 Cancri, Kepler-11, GJ 1214, and HD 189733.

super earths in the solar neighborhood

As interpreted by Motalebi and colleagues, HD 219134 b “provides an exquisite constraining point in the mass-radius diagram.” It is the nearest transiting planet detected to date, as well as the smallest within 50 parsecs. The five smallest transiting planets in this volume of space are illustrated in Figure 2, with Earth for scale. As the only exoplanet in this group smaller than 2 Rea, HD 219134 b offers the most insight into the characteristics of extrasolar terrestrial planets.

Figure 2. The nearest transiting planets under 20 Earth masses
Radii are listed in the top row, masses in the bottom row. All measurements are expressed in Earth units, with Earth shown to provide a visual scale. Since we have no clue to the appearance of any of these exoplanets, and since Earth is often described as a blue marble, all planets are represented by marbles.
In Figure 2, all but Earth and possibly HD 219134 b are too large to be just rock and metal, without ices or hydrogen. By contrast, current consensus indicates that all three planets pictured to the right of 55 Cancri e have hydrogen/helium (H/He) envelopes, consistent with the compositional models of Dressing and Lopez (and for that matter Rogers, Wolfgang, and Fortney). Indeed, Lopez and Fortney (2014) have presented a compelling argument, well supported by much current evidence, that the bulk percentage of H/He increases along with radius for exoplanets in the range of 2-5 Rea.

At least three different models are available to portray 55 Cancri e, a quintessential Hellworld with a dayside temperature in excess of 2000 K and an orbital period of only 18 hours. Despite intense irradiation over a likely system age surpassing 8 billion years, this massive sphere is too large to be purely rocky and metallic: it has evidently retained some kind of envelope. In one model, the envelope is composed of water, possibly in a supercritical state (Gillon et al. 2012, Dragomir et al. 2012). In another, a rocky planet with a composition similar to Earth has an H/He atmosphere accounting for less than half of 1% of its bulk mass (Lopez & Fortney 2014). A third study argues that previous estimates of the planet’s radius were confounded by variations in its thermal emissions and transit depth (Demory et al. 2015). New analyses by this group yield a radius of only 1.92 Rea, implying that 55 Cancri e is a rocky sphere with no significant atmosphere. Instead, it has volcanoes that periodically emit opaque, gaseous plumes, which cause the variability observed over time in its transit silhouette and heat profile. At the moment I’m not sure if any of these models would have majority approval among specialists on 55 Cancri.

The orbital environment of HD 219134 b is less extreme than that of 55 Cancri e, but the planet still receives more than enough irradiation to strip a layer of H/He from a rocky object of sufficiently low mass. However, the radius calculated by Motalebi’s group places HD 219134 b right on the theoretical threshold separating rocky Super Earths from gas dwarfs with H/He envelopes (Rogers 2015). A rocky planet of 1.6 Rea must be relatively massive to bulk so large without a significant contribution from water or hydrogen. Dressing and colleagues (2015) propose 6 Mea as the mass of a planet of 1.6 Rea with an Earth-like composition, while the lower mass limit for a rocky planet at this radius is no less than 4.5 Mea. Again, the values proposed by Motalebi’s group place HD 219134 b right on the edge.

transiting hellworlds in deeper space

Outside our immediate neighborhood, four other transiting planets with radii between 1 and 2 Rea have reasonably well-defined masses based on radial velocity measurements (Table 2), although these are presented with wide uncertainties. One of them, Kepler-78b, orbits a K-type star; the other three orbit G-type stars within 10% of our Sun’s mass. The longest period is under five days; the others are under a single day. All are highly irradiated, with equilibrium temperatures (Teq) far in excess of the mean surface temperature of any planet in the Solar System. Thus it is likely that the objects in this small sample have lost most if not all of their volatile content, including water, and any primordial H/He has blown off. We expect all to be truly terrestrial planets, unlike the puffy gas dwarfs so common in the Kepler catalog.

Table 2. Other transiting planets 1-2 Rea with radial velocity measurements
Radii and masses are expressed in Earth units; density in g/cc3; equilibrium temperature (Teq) in Kelvin; and period in days. Data sources: Kepler-78b (Hatzes 2014), Kepler-10b (Dumusque 2014), Kepler-93b (Dressing 2015), CoRoT-7b (Haywood 2014).
Terrestrial planets are understood as differentiated spheres in which the heaviest constituent – iron – has pooled at the center to form a core. Around the core is a mantle of silicate rocks. Both the overall mass and the mass fraction of the core, as well as the degree of differentiation throughout the mantle, are expected to vary from planet to planet. Our Moon, for example, has a much smaller iron core than Earth, and its bulk composition is relatively deficient in iron – yet the Moon's mantle is richer in iron than the Earth’s. On this evidence we might expect the internal structure and composition of terrestrial exoplanets to range from Moon-like, through Earth-like, to the maximum possible iron fraction (Marcus et al. 2009). What we should not expect is low-density planets composed only of silicates, or high-density planets made of pure iron, since these are simply theoretical constructs, and both are unphysical (Rogers 2015).

As a glance at Table 2 reveals, none of the available mass estimates for these four Hellworlds have high precision (see Table 3 for that). Because calculations of density depend on mass and radius, our understanding of the structure of these exoplanets remains fuzzy. Nevertheless, even though the proposed densities range over a much wider parameter space (2.5-8.1 g/cm–3) than the ones determined for the Solar System’s rocky planets and moons (3.3-5.5 g/cm–3), none are unphysical. With luck we should have much better constraints on HD 219134 b in the near future.

Table 3. Physical data on the six best-constrained rocky planets & moons
Radii and masses are expressed in Earth units (Rea, Mea); mean density in g/cm−3; mean surface temperature in Kelvin; and orbital period in days.  
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Dumusque X, Bonomo AS, Haywood RD, Malavolta L, Segransan D, Buchhave LA, et al. (2014) The Kepler-10 planetary system revisited by HARPS-N: A hot rocky world and a solid Neptune-mass planet. Astrophysical Journal 789, 154.
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