PLATO Mission Conference 2017

The 2017 PLATO Mission Conference was held earlier this week at The University of Warwick, UK. This was the first science meeting entirely dedicated to PLATO following the mission's adoption by ESA last June (launch due in 2026). PLATO's main objective is the detection and characterization of habitable zone planets around Sun-like stars. This will be achieved through the detection of transits around host stars that are bright enough for detailed radial-velocity investigation. In particular, many PLATO host stars are expected to be characterized through asteroseismology.

At the Conference, I had the privilege of introducing the synergy between asteroseismology and exoplanet observations. Over the past decade, space-based asteroseismology has played an important role in the characterization of host stars and their planetary systems. The future looks even brigther, with space missions such as TESS and PLATO ready to take on this legacy. In my talk (see synopsis below), I started by reviewing current key synergies between asteroseismology and exoplanetary science, after which I presented an outlook on potential synergies. With the launch of TESS fast approaching, I found it appropriate to conclude by talking about the prospects of conducting asteroseismology of exoplanet-host stars with TESS.

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Solar-like oscillations are excited by turbulent convection in the outer layers of stars. Consequently, all stars cool enough to harbor an outer convective envelope may be expected to exhibit solar-like oscillations. The Kepler mission has led to a revolution in the field of cool-star asteroseismology by allowing the detection of solar-like oscillations in several hundred solar-type stars and in over 15,000 red giants (for a review, see Chaplin & Miglio 2013, ARA&A, 51, 353). Of all these stars having detected solar-like oscillations over a hundred are Kepler Objects of Interest (KOIs; i.e., candidate exoplanet-host stars).

As you might know, transit observations provide a direct estimate of the planet-to-star radius ratio. Therefore, precise stellar radii from asteroseismology allow tight constraints to be placed on the absolute sizes of planets, while also helping to determine the stellar luminosity and thus the location of the habitable zone around the star. For bright enough systems, radial-velocity observations may be combined with the transit data to estimate planetary masses. The inferred planetary mass scales with the stellar mass, which asteroseismology can again provide. Last but not least, stellar ages from asteroseismology can potentially be used to assess the dynamical stability of planetary systems and to establish their chronology with respect to one another.

For a third of the asteroseismic KOIs we have been able to measure stellar radii with 1.2% precision, stellar masses with 3.3% precision and stellar ages with 14% precision, making this the host-star sample with the most precise set of fundamental properties to date (Silva Aguirre et al. 2015, MNRAS, 452, 2127). Notice that such performance in terms of precision is commensurate with that expected from the asteroseismology program of PLATO for stars in the P1 core target sample. This was achieved by matching individual oscillation frequencies and complementary spectroscopic data to grids of evolutionary models. We further assessed the internal systematics from changing the input physics in those models, which showed that they are smaller than the statistical errors obtained. A special mention could perhaps go to the Kepler-444 system. Kepler-444 is a metal-poor, Sun-like star from the old population of the Galactic thick disk and the host to a compact system of five transiting planets with sizes between those of Mercury and Venus. We used asteroseismology to measure a precise age of 11.2 Gyr for the host star, making this the oldest known system of terrestrial-size planets (Campante et al. 2015, ApJ, 799, 170).

The availability of a statistical sample of asteroseismic KOIs soon led to the systematic characterization of Kepler planets. Let me give you an example. Using asteroseismology of over a hundred KOIs, we were able to show that while there is an abundance of super-Earth-size planets with low incident stellar fluxes, none can be found with high incident fluxes. We termed this the hot-super-Earth desert (Lundkvist et al. 2016, Nature Commun., 7, 11201). Simulations had indeed predicted that hot super-Earth-size planets could have their envelopes stripped by photo-evaporation. However, this feature had not yet been unambiguously confirmed observationally.

Asteroseismology is also helping us shed new light on the retired A star controversy. Studies based on Doppler surveys have suggested an increasing occurrence rate of giant planets with stellar mass. These studies rely on evolved stars for a sample of intermediate-mass stars (so-called retired A stars), which are more amenable to Doppler observations than their main-sequence progenitors. However, it has been hypothesized that the masses of evolved stars targeted by these surveys — typically derived from a combination of spectroscopy and isochrone fitting — may be systematically overestimated, thus casting doubt on the stellar mass-planet occurrence relation. We are currently in the process of addressing this issue by deriving accurate and precise asteroseismic masses for a statistical sample of Kepler/K2 host and non-host control stars (e.g., Campante et al. 2017, MNRAS, 469, 1360).

Furthermore, asteroseismology has now become a powerful method to characterize dynamical architectures of planetary systems. Asteroseismic observations of the relative heights of rotationally split modes can be used to measure the stellar inclination angle along the line of sight (e.g., Huber et al. 2013, Science, 342, 331). Importantly, this approach is independent of planet size and hence can be used to constrain obliquities for systems with small planets for which Rossiter–McLaughlin measurements are not feasible. This feature is being used mainly to test the primordial star-disk alignment hypothesis by measuring obliquities of multi-transiting systems, therefore impacting on our current understanding of hot-Jupiter formation. One other domain of application consists in the determination of orbital eccentricities via asterodensity profiling (e.g., Van Eylen & Albrecht 2015, ApJ, 808, 126), whereby the stellar density as measured from the transit light curve under the assumption of a circular orbit is compared to an independently determined value from asteroseismology. This approach thus allows transits to be used to directly constrain eccentricities without the need for radial-velocity observations.

The asteroseismology revolution initiated by Kepler is set to continue over the coming decades with the launches of TESS, PLATO, as well as of WFIRST, these missions being expected to raise the number of solar-like oscillators to a few million stars. Notice that over 90% of all detections are expected to be for evolved stars, with PLATO by far contributing the most detections for dwarfs and subgiants. If we combine this with dedicated ground-based efforts such as the SONG network of 1-meter telescopes, we are then positive that the synergy between asteroseismology and exoplanetary science can only continue to grow. In what follows, I highlight what I believe will be two of the most significant synergies.

Evolved stars are obvious targets for synergetic studies since even moderate cadences can be used to simultaneously detect transits and stellar oscillations. Despite the dearth of close-in giant planets orbiting evolved stars unveiled by radial-velocity surveys, data from Kepler/K2 have led to the discovery of several planets around oscillating low-luminosity red-giant branch (RGB) stars. These planets can be used to address key unsolved questions in exoplanetary science with unprecedented precision, such as the role of the incident stellar flux on hot-Jupiter inflation (Grunblatt et al. 2016, AJ, 152, 185). As we will learn below, TESS will allow us to conduct a populational study of giant planets around oscillating low-luminosity RGB stars. Also, planetary composition models depend sensitively on radius, especially in the regime of sub-Neptune-size planets, with density measurements indicating a threshold between mostly rocky and gaseous planets at 1.6 Earth radii. PLATO and — to a lesser extent — TESS asteroseismology of solar-type stars hosting small planets will provide a unique opportunity to precisely study the composition diversity of sub-Neptunes by constraining host-star radii and masses to a few percent (Gettel et al. 2016, ApJ, 816, 95). I should note that Gaia parallaxes alone will not reach comparable precision due to model-dependent uncertainties such as bolometric corrections and reddening.

And this brings us to TESS. TESS will be performing an all-sky survey for planets transiting bright nearby stars. During its primary mission, TESS will monitor the brightness of several hundred thousand low-mass, main-sequence stars over intervals ranging from one month to one year. Monitoring of these pre-selected target stars will be done at a cadence of 2 min, while full-frame images (FFIs) will be recorded every 30 min. In addition, TESS’s excellent photometric precision will enable asteroseismology of solar-type and red-giant stars. Of particular interest is the asteroseismic yield of exoplanet-host stars. This arises from three separate contributions (Campante et al. 2016, ApJ, 830, 138): (i) TESS target hosts, (ii) TESS FFI hosts and (iii) known exoplanet-host stars. We now look at each one of these contributions in turn.

We start with the asteroseismic yield of TESS target hosts. Based on an all-sky stellar and planetary synthetic population, we predict that asteroseismology will become possible for a few dozen target hosts (mainly subgiant stars but also for a smaller number of F dwarfs). Here, I should stress that the effective collecting area of the individual TESS cameras is 2 orders of magnitude smaller than Kepler's, meaning that stellar shot noise will make detection of solar-like oscillations only possible for the very brightest Sun-like stars. This partly explains the scarcity of predicted G and K TESS target hosts. As mentioned before, a very exciting prospect will be that of conducting asteroseismology of red-giant hosts. Based on the same synthetic population, we predict to be able to detect solar-like oscillations in up to 200 low-luminosity RGB stars hosting close-in giant planets. Finally, we expect to detect solar-like oscillations in nearly 100 solar-type and red-giant known hosts, of which about 50% will be evolved stars. I should note that the vast majority of these systems have been discovered using radial-velocity observations and are thus potential targets for CHEOPS, which will be monitoring bright known hosts in the search for transits. Consequently, TESS could in principle be providing asteroseismic measurements for a significant number of potential CHEOPS targets, a link that we are now starting to explore.