Ruizhi Zhan (詹睿知)

Ruizhi Zhan (詹睿知)

she/her

Ph.D Candidate @ PKU

Research

My Ph.D project focuses on atmosphere modeling of rocky exoplanets with ultra-short orbital periods, utilizing 3D general circulation models, Isca and ExoCAM.

Atmosphere on the Close-in Rocky Planet 55 Cancri e

  • Atmosphere on the Close-in Rocky Planet 55 Cancri e
    The ultra-short period super Earth 55 Cancri e offers a unique opportunity to study planets in extreme environment. Despite extensive observation, the nature of 55 Cancri e, however, is still poorly understood. Few model with realistic radiative transfer has been employed for hot (non-habitable) rocky exoplanets, mainly due to the incompatibility of standard GCM radiative transfer codes with currently observed hot exoplanets. Here we calculate the absorption cross section from ExoMol line lists. We develop custom correlated-k coefficients from the absorption cross section and validate them against line-by-line radiative transfer.

    Absorption cross section of CO2 at Earth Temperature (300 K) and typical temperature on substellar point of 55 Cnc e (3500K) assuming 1 bar pressure. The two lines are clearly distinguished. So the standard GCM radiative transfer for Earth cannot be applied to 55 cnc e.


    We then perform GCM simulations with non-grey radiative transfer, Isca coupled with SOCRATES, to model the atmospheres on 55 Cancri e.

    The gas absorption in our model includes molecular spectrum (including UV absorption) and collision induced absorption (CIA).


    Our simulations suggest the secondary atmosphere on 55 Cancri e should be thick and carbon dioxide rich.
    Observations from Spitzer and JWST reported significant time variability in the secondary eclipse depth of 55 Cancri e. However, our result suggests that clearsky atmosphere variability is much weaker than observed.


Habitable Zone Around “Dead Stars”?

White dwarfs are compact stars with a size comparable to Earth and a mass comparable to Sun. Planets in the habitable zone (read more about the habitable zone edges here) around white dwarfs have ultra-short orbital periods ranging from hours to days. Due to the small size, white dwarfs offer a unique opportunity to search nearby stellar systems for signs of life. The habitable zone around white dwarfs, however, is still poorly understood.


A schematic of transit timing variations of a G dwarf, M dwarf and a white dwarf planet system.

Here we use the ExoCAM Global Climate Model to investigate the inner edge of the habitable zone around white dwarfs. Our simulations show habitable planets with ultrashort orbital periods (P < 1 day) enter a “bat rotation” regime, which differs from typical atmospheric circulation regimes around M dwarfs. Bat rotators feature mean equatorial subrotation and a displacement of the surface’s hottest regions from the equator toward the midlatitudes.


Surface temperature and zonal mean zonal wind as a function of rotation period. From left to right: bat rotator (P = 0.5 days; this work), compared to a rapid rotator (P = 2 days), Rhines rotator (P = 10 days), and slow rotator (P = 20 days). Dynamical regimes for the slower rotating planets are defined as in Haqq Misra et al. (2018). In (a), “SP” corresponds to the substellar point. In (b), red vs. blue shading represent eastward vs. westward winds. The simulations assume a 10,000 K white dwarf and 2.12 times Earth’s instellation.

The “bat rotation” regime expands the white dwarf habitable zone by ∼50% compared to estimates based on 1D models.


Estimated rotation regimes inside the habitable zone of white dwarfs with different stellar temperatures, as a function of relative stellar flux.

The James Webb Space Telescope should be able to quickly characterize bat rotators around nearby white dwarfs thanks to their distinct JWST via thermal phase curves.

We also qualitatively explain the onset of bat rotation using shallow water theory. Read more here: [arxiv]/ [journal].