The Effect of Tidal Heating and Volatile Budgets on the Outgassed Atmosphere of 55 Cancri e

Abstract

55 Cancri e is a $\sim$8 Gyr rocky world (1.95 $R_{\oplus }$, 8.8 $M_{\oplus }$) orbiting a K-type star. JWST observations suggest a carbon-dominated atmosphere (CO${}_2$/CO) over a global magma ocean ($>$3000 K). We suggest that any CO${}_2$-dominated atmosphere, with trace H${}_2$O/O${}_2$, likely arises from outgassing of its initial volatile reservoir. As solidification drives the magma ocean and atmosphere away from solution-equilibrium, tidal and greenhouse heating can prolong outgassing. Early atmosphere outgassing reflects rapid degassing of the volatile-saturated melt during post-formation cooling. Without tidal heating, an initial 5 wt% water mass fraction ($F_{{\text{H}}_2\text{O}}$) or 3 wt% ${\text{CO}}_2$ mass fraction ($F_{{\text{CO}}_2}$) can sustain outgassing for at least $\sim$10 Myr. With both at 10 wt%, greenhouse warming alone can prolong outgassing up to $\sim$30 Myr. Our model shows that tidal heating can reduce the volatile threshold required to maintain a high surface temperature ($\sim$3200 K at $e=0.005$) and delay outgassing of additional volatiles to the present-day. However, higher tidal heating presents a tradeoff between prolonging tenuous outgassing and enlarging the overall size of the secondary atmosphere. Tidally-enhanced outgassing may produce minor pressure variations that could contribute to the observed phase-curve variability. Additionally, our model shows that tidal heating strongly controls outgassing in the planet's young-to-midlife stage, then shifts toward a volatile inventory dependence at mature ages. Using 55 Cnc e, we present a framework to prioritize atmosphere detections on rocky ultra short period (USP) magma ocean planets, linking age-dependent tidal heating and volatile inventory to the formation and size of secondary atmospheres.

Accepted for publication in ApJ (The Astrophysical Journal), 2026