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Abstract:

To date, only 18 exoplanets with radial velocity (RV) semi-amplitude <2 �� s⁻¹ have had their masses directly constrained. The biggest obstacle to RV detection of such exoplanets is variability intrinsic to stars themselves, e.g. nuisance signals arising from surface magnetic activity such as rotating spots and plages, which can drown out or even mimic planetary RV signals. We use Kepler-37 – known to host three transiting planets, one of which, Kepler-37d, should be on the cusp of RV detectability with modern spectrographs – as a case study in disentangling planetary and stellar activity signals. We show how two different statistical techniques – one seeking to identify activity signals in stellar spectra, and another to model activity signals in extracted RVs and activity indicators – can each enable a detection of the hitherto elusive Kepler-37d. Moreover, we show that these two approaches can be complementary, and in combination, facilitate a definitive detection and precise characterization of Kepler-37d. Its RV semi-amplitude of 1.22 ± 0.31 �� s⁻¹ (mass 5.4 ± 1.4 M_⊕) is formally consistent with TOI-178b’s 1.05^+0.25_−0.30 �� s⁻¹, the latter being the smallest detected RV signal of any transiting planet to date, though dynamical simulations suggest Kepler-37d’s mass may be on the lower end of our 1σ credible interval. Its consequent density is consistent with either a water-world or that of a gaseous envelope (⁠∼0.4 per cent by mass) surrounding a rocky core. Based on RV modelling and a re-analysis of Kepler-37 TTVs, we also suggest that the putative (non-transiting) planet Kepler-37e should be stripped of its ‘confirmed’ status.

Abstract:

In this study, we report the results of an active-source seismology and ground-penetrating radar survey performed in northwestern Greenland at a site where the presence of a subglacial lake beneath the accumulation area has previously been proposed. Both seismic and radar results show a flat reflector approximately 830-845ĝ€¯m below the surface, with a seismic reflection coefficient of -0.43ĝ€¯±ĝ€¯0.17, which is consistent with the acoustic impedance contrast between a layer of water and glacial ice. Additionally, in the seismic data we observe an intermittent lake bottom reflection arriving between 14-20ĝ€¯ms after the lake top reflection, corresponding to a lake depth of approximately 10-15ĝ€¯m. A strong coda following the lake top and lake bottom reflections is consistent with a package of lake bottom sediments although its thickness and material properties are uncertain. Finally, we use these results to conduct a first-order assessment of the lake origins using a one-dimensional thermal model and hydropotential modeling based on published surface and bed topography. Using these analyses, we narrow the lake origin hypotheses to either anomalously high geothermal flux or hypersalinity due to local ancient evaporite. Because the origins are still unclear, this site provides an intriguing opportunity for the first in situ sampling of a subglacial lake in Greenland, which could better constrain mechanisms of subglacial lake formation, evolution, and relative importance to glacial hydrology.