Authors: Venkatesan et al 2025
First Author’s Institution: UC Irvine
Status: Preprint posted on Arxiv
Intro
Kepler’s 1st Law states that the shape of the orbits of planets is not a perfect circle, but rather an ellipse with the host star at one focus. An ellipse deviates from a circle through a parameter called eccentricity (e). For orbits with e = 0, the shape recreates a perfect circle. For larger values of e, up to 1, orbits are still bound but become increasingly elongated or oval-shaped.
For a planet with an orbit of e = 0 (circular), the planet is always at the same distance away from the host star. However, for a highly eccentric orbit, at one portion of the orbit, the planet is very close to the host star and at the other end it is very far, see Figure 1. This change in distance from the host star can have large implications for the climate of the planet. While our first intuition says “the closer the planet is to the star, the hotter the planet will get because it receives more energy from the star”, in the details, it’s not so straightforward.
Another factor is very important: the albedo of the planet. Albedo is a term to describe how reflective the surface of the planet is. The value runs from 0 to 1, and larger values mean the surface is more reflective. Imagine a planet where the surface was entirely entirely made up of a giant mirror. The mirror is entirely reflective, so the albedo would be 1.0. In this scenario, no matter how close or far the planet is to the host star, the planet would reflect back 100% of the light it receives and therefore would not warm up at all. On the contrary, if you painted the whole planet black, the planet would reflect no light and so the albedo would be 0. In this case, the closer the planet is to the star, the more light and energy it would receive, all of which would be absorbed, which would greatly increase the temperature of the planet.
More realistically than a mirror planet is an ice-world planet: one that is completely covered by ice. Sometimes this scenario is called “snowball Earth”, think of a planet like Hoth from Star Wars!. Ice is highly reflective like the mirror. However, not all ice is the same. While we are most familiar with ice as meaning “water ice”, another kind of ice, CO2 ice, or “dry ice”, has different reflective properties. Not only that, but water ice and dry ice are each more or less reflective depending on the wavelength of light you are shining onto it. Water ice reflects highly in the visible light range we are used to, but dry ice reflects highly in the infrared range, where water ice is more absorbing, see Figure 2. With different stellar spectral types emitting different amounts of light in the visible and infrared portions of the spectrum, the host star itself has a big effect on how the energy is balanced on the planet.
Already we have built up a lot of parameters that can be tuned to make a planet totally iced over or totally ice free: eccentricity, reflectivity, and surface composition. How do they all work together to produce a planet’s climate? Today’s paper dives into this idea further which has big implications for our understanding of habitability!
Methods
The authors model an exoplanet climate under various conditions by tuning the various parameters described above. In particular, the authors are interested in “warm” vs. “cold” starts to their simulations where the planet either starts ice-free or iced-over, respectively. Then they simulate how much energy would be needed to be added or removed to either completely ice over the “warm start” planet or completely melt all the ice in the “cold start” planet, based on the tuning of eccentricity, albedo, and stellar type.
The authors find a few interesting results:
- Highly eccentric planets are very resistant to entering a global ice world state, Snowball Earth. They find the increase in stellar flux at the planet’s closest point to the star is sufficient to prevent ice forming at all latitudes of the planet. This is true for planets orbiting any spectral type of star tested, F/G/K/M. Similarly, when an eccentric orbit planet starts in a snowball state, it quickly thaws out. See Figure 3, where the eccentricity of the planet is plotted with the spectral type and the total energy (as a percent of Earth’s energy from the Sun) required to melt the snowball state.
- When accounting for water vs. dry ices, the planets orbiting F/G/K stars require significantly more energy from their stars to thaw out than the planets orbiting M dwarf stars. This is because the M dwarf stars emit more in the infrared, which more easily melts the dry ice. This means that planets orbiting M dwarf stars are more likely to have more temperate climates.
- Larger ice particle sizes reduces the amount of energy needed to thaw out as the larger sizes scatter more light, effectively reducing the albedo of the planet.
In all, this work is the first to incorporate such a wide array of tunable parameters, in particular the difference between including or excluding the impact of dry ice. Overall, eccentric planets and M dwarf planets are the best equipped to thaw out of snowball states and maintain a more temperate climate. This in turn can change the way we think about the habitability of planets. It’s not as simple as just the distance the planet is from its host star: a wide variety of factors play large roles in the average climate of a planet.
Featured image credit: Venkatesan et al. 2025
Edited by: Brandon Pries
