First Author: Diogo Souto; Second Author: Cayman Unterborn; Co-authors: Verne Smith, Katia Cunha, Johanna Teske, Kevin Covey, Barbara Rojas-Ayala, D. A. Garcia-Hernandez, Keivan Stassun, C. Allende Prieto, Olga Zamora, Thomas Masseron, J. A. Johnson, Steven R. Majewski, Henrik Jonsson, Steven Gilhool, Cullen Blake and Felipe Santana
Published at: Accepted in Astrophysical Journal Letters.
Nowadays, It is still challenging to observe exoplanets directly, even with the help of the most powerful telescopes in the world. For example, we can see an image of an exoplanet if it was detected by the direct imaging method, in which we can spot the exoplanet from a photograph (relatively close to its parent star). With this technique, we can have an initial idea from its size (the exoplanet radius), where giants Jupiter are often observed with this method, precisely because they are large exoplanets, implying in higher contrast in the obtained photographs.
Studying the exoplanet atmospheres' is a complicated task. For that, we usually consider the transit spectroscopy method, where we observe the stellar spectra before, during, and after the eclipse of the exoplanet. This method consumes a huge observational time, and accurate measurements of the orbit and the exoplanet period are crucially needed. Besides, by definition, the exoplanet has to "transit" its parent star.
Another technique, commonly used to study the properties of exoplanets is from the star-planet connection. In this method, we based on the physical-chemical properties of the host star to infer about the exoplanets properties. An important assumption in this treatment is that the entire planetary system has been formed from the same molecular cloud, i.e. with the same primordial ingredients. However, this is a well-accepted assumption in the models of planetary formation in literature.
The Ross 128 System
In the article discussed here, we studied the exoplanetary system of Ross 128, which is formed by an M dwarf star (or red dwarf) and - at least - one exoplanet with 1.35 Mo (Earth mass). The exoplanet Ross 128b was recently discovered by Bonfils et al. (2017), and it is the second closest exoplanet ever observed, at a distance of ~10 light years (3.4 pc). Another important fact is that Ross 128b is located at the star's habitable zone, with real chances of having water in the liquid state. This is fundamental for the formation of life as we know it here on Earth. In addition, we determined that the insolation flux (radiation that exoplanet receives) for Ross 128b is very similar to that on Earth, only 1.79 times higher, and that the exoplanet has an average equilibrium temperature of 21 degrees Celcius (70 F).
In this work, for the first time in the literature, we show that it is possible to determine atmospheric parameters (temperature and gravity) and chemical abundances for 8 elements using near-infrared spectra (obtained with APOGEE) in a dwarf M star with an effective temperature of the order of 3200 K. Cool stars, such as Ross 128, are very difficult to study from the optical spectra because of the strong molecular absorptions, such as those from TiO (titanium oxide) and CO (carbon oxide).
Determining the Ross 128 Stellar Properties
Figure 1 shows the methodology used to determine the spectroscopic effective temperature (Teff) and surface gravity (log g) of Ross 128. The method uses the balance of the chemical abundances of oxygen and iron as a function of the atmospheric parameters mentioned. For this, we determined the abundances using the different abundance indicators, the FeH and Fe I lines for iron and the OH and H2O lines for the oxygen abundances. Each of these spectral lines exhibits a different sensitivity to the atmospheric parameter, and the result obtained for Tef or log g will be the one where the abundances reproduce the same value (where there is an interception of the derived abundances as a function of the atmospheric parameters). We can see that the effective temperature is more sensitive to variations in abundances (left panel) in counterpoint to the surface gravity (right panel). In this work, we have determined the effective temperature and surface gravity of Ross 128 using two model atmospheres in the literature, the MARCS and PHOENIX models, respectively (blue and red in the Figure). In both scenarios, the results are similar, within the uncertainties.
Figure 1: Diagrams illustrating the derivation of atmospheric parameters. The left panels display the Teff–A(O) and Teff–A(Fe) curves and the right panels the logg–A(O) and logg–A(Fe) pairs. In all panels, the derived abundances from Fe I and OH lines are shown as solid lines, while those from FeH and H2O as dashed lines. The different colors (blue and red) indicate results derived from the adopted models, MARCS and PHOENIX respectively.
The chemical abundances determined for Ross 128 indicate that the star is somewhat similar to our Sun. We obtained similar quantities of iron, carbon, oxygen, aluminum, potassium, and calcium, while the elements magnesium and titanium displays slightly lower abundance compared to the Sun. Using models of planetary formation, we see that the titanium deficiency in the Ross 128 system does not significantly interfere in the composition of the exoplanet Ross 128b; however, the slight depletion of Mg/Fe can affect the relative fraction of core-to-mantle and provide some broad constraints on the possible structures of Ross 128b
Inferring Physical Properties to Ross 128b
The first step in the analysis of Ross 128b was to use a model (ExoPlex; Unterborn et al. 2018) to estimate its radius based on the chemical abundances derived for Ross 128. Given its observed minimum mass (1.35 Mo), we run two-layer models assuming a liquid core and a silicate mantle (with no atmosphere). We increase the input mass until a likely radius of 1.5R⊕ was achieved, roughly the point where planets are not expected to be gas-rich mini-Neptunes as opposed to rock and iron-dominated super-Earths. The model conserves the relative ratios for the dominant rocky planet-building elements assuming all Fe is present in the core with no light elements (e.g., Si, O, H) present. Adopting our abundance results we obtain the molar ratios Fe/Mg=1.12, Ca/Mg=0.09, and Al/Mg=0.08; where the Earth and solar values are 0.90/0.07/0.09 and 0.83/0.06/0.08, respectively as a reference. The estimated radius of Ross 128b is represented in Figure 2 by the solid red curve.
We note that changes in the relative planet’s core size have a large effect on its bulk density (core-mantle). In this scenario, For this model, only if Ross 128b had a super-solar Si/Mg of 1.3 would it match the Earth in its core mass fraction (Si/Mg = 1.3). Unfortunately it was not possible to determine the abundance of silicon (Si) in the Ross spectrum 128; however, we do not expect Ross 128 to be Si-rich due to its slight deficiency in Mg and Ti. This means that if Ross 128b’s composition mimics that of its host star and a sub-Solar Si/Mg, it will have a larger relative core size than the Earth, despite having roughly solar iron abundance. This is due to the relative ratio of Fe to Mg being higher than in the Sun and Earth, combined with the density of liquid Fe being greater than that of magnesium silicates (e.g., rock-dominated)
In Figure 2 we present a density diagram (mass vs. radius as a function of the Earth values) of the exoplanets detected to date. Exoplanets orbiting M dwarf stars, similar to Ross 128, are shown as yellow squares, while yellow circles represent other spectral classes. Rocky exoplanets are highlighted in the Figure. The gray region indicates the limits of radius and mass in which the planet can have more properties more similar to the Earth. The red curve represents the estimated radius for Ross 128b from our results, where all lie below the rock-dominated composition curve of Zeng et al. (2016), i.e., it contains a mixture of rock and Fe, with the relative amounts of each set by Fe/Mg. Another fact that draws our attention in the Figure is that low-density exoplanets are commonly found around M-dwarfs.
Figure 2: The exoplanet mass-radius diagram using the planetary composition based on Zeng et al. (2016) relations. Exoplanets around M dwarfs are shown with orange squares and the other stars hosting exoplanets are shown with orange circles.
In Figure 3, we show the distribution of metallicity ([Fe/H]) distance (pc) distribution of the host stars detected to date in the literature. In the upper panel, we have the insolation flux and in the lower panel the equilibrium temperature of the exoplanets being represented as color bars. We compute of the insolation flux and the exoplanet equilibrium temperature using data from the NASA archive adopting an Earth-like albedo (30%) for all exoplanets. With a green shadow, we highlight the region with d <20 pc (~ 65 light years), calling attention to the opportunity provided by the nearby M dwarf exoplanetary systems. The symbols follow the same notation of Figure 2, with the addition of small dots systems where the exoplanet density is not measured.
Figure 3: The metallicity--distance diagram; in the top panel we plot the computed insolation flux at the planet's distance from the star and in the bottom panel we adopt the planet's equilibrium temperature in the color bar. The symbols follow the same notation from the left panel with the addition of upside down triangles for the M dwarfs that are closer than 20 pc from the Sun and host exoplanets without density measurements. We use dots to indicate the stars hosting exoplanets without density measured.
The M dwarfs closer than 20 pc that host planets without measured densities are shown as upside down triangles. Ross 128b is also presented as blue crosses. We leave the water and gaseous planets as background symbols. From Figure 3, we can see that the exoplanets with the lower degree of insolation flux tend to orbit M dwarfs (yellow squares and triangles). In contrast, the rocky exoplanets around solar-like stars tend to receive much more flux, generally 1000 times more than the Earth. Although, It is worth noting that this can be an observational systematic and more data are needed to whether confirm or not this trend.
In summary, our derived results for the M-dwarf Ross 128 have allowed us to use theoretical models to study the internal composition of Ross 128b. Assuming that Ross 128b has formed with the same composition as its host star, we calculate its mineralogy, structure, and therefore its mass and radius. Our model assumes that there is no atmosphere present in the exoplanet; however, because the addition of this layer (or the addition of light elements to the core) would decrease the density for a given radius, our calculated masses represent the maximum mass of Ross 128b. In this scenario, we calculate Ross 128b would have a relatively larger core than the Earth, regardless of the mantle chemistry, despite having a sub-solar Fe/Mg. The derived planetary parameters, such as insolation flux (S = 1.79 ± 0.26) and equilibrium temperature (Teq = 294 ± 10K) suggest that Ross 128b is a temperate exoplanet in the inner edge of the habitable zone. Possibly, it can be a new home for humanity.