Ross 128b: Analyzing a Planet by the Light of its Star

Red dwarfs have a lot of things going for them when it comes to finding possibly habitable planets. A planet of Earth size in the HZ will produce a substantial transit signal because of the small size of the star (‘transit depth’ refers to the amount of the star’s light that is blocked by the planet), and the tight orbit the planet must follow increases the geometric probability of observing a transit. But planets that do not transit are also more readily detected because of the large size of the planet compared to the star, gravitational interactions producing a strong radial velocity signature, which is what we have in the case of Ross 128b.

About 11 light years from Earth, the planet was culled out of more than a decade of radial velocity data in 2017 using the European Southern Observatory’s HARPS spectrograph (High Accuracy Radial velocity Planet Searcher) at the La Silla Observatory in Chile. The location of the planet near the inner edge of its star’s habitable zone excited interest, as did the fact that Ross 128 is much less subject to flares of ultraviolet and X-ray radiation than our nearest neighbor, Proxima Centauri, which also hosts a planet in a potentially habitable orbit.

Image: Artist’s impression of the exoplanet Ross 128b. Credit: ESO.

What we know about Ross 128b is that it orbits 20 times closer to its star than the Earth orbits the Sun, but receives only 1.38 times more irradiation than the Earth, with an equilibrium temperature estimated anywhere between -60 degrees Celsius and 20°C, the host star being small and relatively cool. But bear in mind that what we get from radial velocity is a minimum mass, because we don’t know at what angle this system presents itself in our sky. Now a team led by Diogo Souto (Observatório Nacional, Brazil) is attempting to deduce more about the planet’s composition using an unusual method: Analyzing the composition of the host star.

If we learn the chemical abundances found in the star Ross 128, the thinking goes, we should be able to make reasonable estimates about the composition of any planets that orbit it. Souto and team are presenting new techniques for making these measurements, using data from the Sloan Digital Sky Survey’s APOGEE spectroscope. Measuring the star’s near-infrared light, where Ross 128 shines the brightest, the researchers have been able to derive abundances for carbon, oxygen, magnesium, aluminum, potassium, calcium, titanium and iron.

“The ability of APOGEE to measure near-infrared light, where Ross 128 is brightest, was key for this study,” says co-author Johanna Teske (Carnegie Institution for Science). “It allowed us to address some fundamental questions about Ross 128 b’s `Earth-like-ness.’”

APOGEE is the Apache Point Galactic Evolution Experiment, an investigation using high-resolution spectroscopy to probe the dust that obscures the inner Milky Way. The project surveyed 100,000 red giant stars across the galactic bulge, but also observed M-dwarfs in the neighborhood of the Sun as a secondary study. Tightening up our knowledge of stellar parameters, the paper notes, offers an indirect route to studying exoplanet composition.

The assumption in this work is that the chemistry of a host star influences the contents of the disk from which planets form around it, which in turn affects the interior structure of any planet. Thus we can hope to tell from the amount of magnesium, iron and silicon available something about the exoplanet. This is the first detailed abundance analysis for Ross 128, and it shows that the star has iron levels similar to the Sun. The silicon level could not be measured, but the ratio of iron to magnesium points to a large core for the planet, larger than Earth’s.

Souto and team believe that knowledge of Ross 128b’s minimum mass (from the radial velocity data), coupled with their data on stellar abundances, can provide a broad estimate of the planet’s radius, a key factor because it would allow a calculation of its density. From the paper:

While both mass and radius are not available for Ross 128b, we can estimate its radius given its observed minimum mass and assuming the stellar composition of the host star is a proxy for that of the planet. We calculate the range of radii possible for Ross 128b using the ExoPlex software package (Unterborn et al. 2018) for all masses above the minimum mass of Ross 128b (1.35M ⊕ ; Bonfils et al. 2017). Models were run assuming a two-layer model with a liquid core and silicate mantle (no atmosphere). We increase the input mass until a likely radius of 1.5R ⊕ was achieved, roughly the point where planets are not expected be gas-rich mini-Neptunes as opposed to rock and iron-dominated super-Earths…

Measurements of the temperature of Ross 128 coupled with the estimated radius of the exoplanet and its inferred composition allow the team to calculate Ross 128b’s albedo, the amount of light reflecting off its surface. These estimates allow the possibility of a temperate climate, taking into account the insolation flux (energy received from the host star) and equilibrium temperature. “Our results,” the authors write, “support the claim of Bonfils et al. (2017) that Ross 128b is a temperate exoplanet in the inner edge of the habitable zone.”

But the paper urges caution in the interpretation:

However, this is not to say that Ross 128b is a “Exo-Earth.” Geologic factors unexplored in Bonfils et al. (2017) such as the planet’s likelihood to produce continental crust, the weathering rates of key nutrients into ocean basins or the presence of a long-term magnetic field could produce a planet decidedly not at all “Earth-like” or habitable due to differences in its composition and thermal history. Furthermore, other aspects of the M-dwarf’s stellar activity and its effect on the retention of any atmosphere and potential habitability should be studied, although we find no evidence of activity in the Ross 128 spectra.

Indeed. The number of variables affecting ‘habitability’ is striking. So let’s say this: We have a planet for which mass-radius modeling based on the composition of its host star indicates a mixture of rock and iron, the relative amounts of each being set by the ratio between iron and magnesium. The derived values for insolation and equilibrium temperature are not inconsistent with previous studies indicating a temperate planet at the inner edge of its star’s habitable zone.

The work hinges on modeling of an exoplanet based on a deeper analysis of its host star than has previously been available for an M-dwarf. Tuning up such modeling will demand further data, in particular applying these methods to the host stars of transiting worlds (think TRAPPIST-1) to test their accuracy and reliability in characterizing planets we cannot see.

The paper is Souto et al., “Stellar and Planetary Characterization of the Ross 128 Exoplanetary System from APOGEE Spectra,” Astrophysical Journal Letters Vol. 860, No. 1 (13 June 2018). Abstract / preprint.