Planetary Habitability Under the Light of a Rapidly Changing Star

TL;DR

This study assesses how stellar variability influences planetary climates and habitability in the Habitable Zone. By updating a sample of 9 HZ exoplanets around variable stars with $A_{var} \ge 100$ ppm using extended TESS data, it quantifies flux variations and equilibrium temperatures, finding that variability typically matters far less than orbital eccentricity. The authors also test an extreme variable-star scenario with VPLanet, showing that water loss for an Earth-analog at the inner HZ edge proceeds similarly for quiet and variable hosts, underscoring the robustness of HZ habitability to moderate stellar variability. The results suggest observational biases may underrepresent environments around highly variable stars and open avenues to study exomoons and more extreme variability regimes in future work.

Abstract

Planetary atmospheric energy budgets primarily depend on stellar incident flux. However, stellar variability can have major consequences for the evolution of planetary climates. In this work, we evaluate how stellar variability influences the equilibrium temperature and water retention of planets within the Habitable Zone (HZ). We present a sample of 9 stars that are known to host at least one planet within the HZ and that were identified to have a variability amplitude exceeding 100 ppm based on photometry from the Transiting Exoplanet Survey Satellite (TESS). We investigate the effect that the variability of these stars have on the insolation flux of their HZ planets and the resulting changes in the induced planetary equilibrium temperature. Our results show that for the stars in our sample, the stellar variability has an insignificant effect on the equilibrium temperature of HZ planets. However, we also emphasize that these stars are not representative of more extreme variable stars, since exoplanets are more difficult to detect and characterize in the presence of extreme variability. We also investigate the equilibrium temperature and long-term evolution of a hypothetical Earth-like planet placed at the inner edge of the HZ around a highly variable star. We found that the water loss rates are comparable between both variable and quiet host stars for Earth-like planets in the inner HZ. Overall, these results broaden our knowledge on the impact of stellar variability on planetary habitability.

Figures (8)

• Figure 1: Orbital period versus planet mass for exoplanets in the Habitable Zone Gallary that spend greater than 50% of their orbit within the OHZ. The points are colored by the time spent in the OHZ. The star-shaped points represent 8 of the 9 planetary systems in our sample. TOI-1227 b does not have a tabulated mass in the NASA Exoplanet Archive and thus does not appear in this plot, but is otherwise reported to have an upper limit mass of $<1.7M_J$Mann22.
• Figure 2: Histogram of the stellar variability amplitudes reported in the TESS-SVC fetherolf2023b, the known exoplanet host stars simpson2023, and the 9 stars in our sample (dark green vertical lines) that all have at least one exoplanet that spends $>$50% of its orbit in the OHZ.
• Figure 3: TESS light curves (left column), Lomb-Scargle periodograms (center column), and phase-folded light curves (right column) for TOI-1227 (TIC 360156606; top row), HD 142415 (TIC 342041655; center row), and HD 147379 (TIC 230073581; bottom row). The red curves represent sinusoidal fits to the observed light curves. The dashed gray vertical lines separate individual TESS sectors, and the solid gray vertical lines indicate the selected period from the periodogram. The gray data points in the left panels are data that were not included in the variability analysis. The right panels show all TESS data in gray, and the binned data as black points.
• Figure 4: Flux received by HD 142415 b from combined orbital and stellar variability effects over the course of one orbital period. The changes in flux are primarily dominated by the effects of the planet's eccentricity, but the additional fluctuations in flux caused by the host star's double-sinusoidal variability (see center row panels of Figure \ref{['fig:var_selected']}) can be seen in the zoomed-in sub-panel.
• Figure 5: Same as Figure \ref{['fig:var_selected']}, but for TIC 264101177, which exhibits extremely high amplitude variations. This type of variable star is not represented in our sample of HZ planets around variable stars---likely due to observational bias.