Can the Same Planet Be Frozen, Mild or Scorching?
Put an Earth-like planet at Earth's distance from a Sun-like star. This climate model finds three stable climates: frozen, mild or scorching.
In one paragraph
Can one planet have more than one possible climate? This preprint introduces Gen-EBM, a fast 1-D climate model that can follow a planet into a post-runaway greenhouse. For an Earth-like planet at 1.00 AU from a Sun-like star it finds three stable states: a snowball at 203 K, a temperate climate at 288 K and a post-runaway state at 888 K. Temperate climates exist only between 0.93 and 1.00 AU, faster rotation narrows that range, and ocean planets handle eccentric orbits better than land planets.
How did researchers model multiple planetary climates?
Gen-EBM is a one-dimensional energy balance model written in Rust. It divides a planet into latitude bands and, in each, balances absorbed starlight against heat radiated to space, with heat spread between bands by diffusion. It was tuned to reproduce modern Earth.
Its new ingredient is a temperature-dependent outgoing radiation curve taken from 3-D climate model results for water-rich planets, covering Earth-like, runaway-onset and post-runaway climates. This lets it follow a planet into very hot states that most fast models leave out.
The authors ran 464 simulations from 16 starting temperatures at 29 orbital distances, then repeated the analysis for rotation periods of 72, 24 and 8 hr, and for eccentric orbits on land, Earth-like and ocean planets.
What climates can an Earth-like planet have?
At 1.00 AU, the same Earth-like planet has three stable climates: a snowball (203 K), a temperate climate (288 K) and a post-runaway state (888 K). Temperate climates exist only between 0.93 and 1.00 AU, with an inner edge near 390 W m−2, close to the 375 W m−2 from 3-D models. Several climates coexist between 0.78 and 1.07 AU.
Rotation matters. A slow rotator (72 hr) keeps temperate climates over 0.90–1.07 AU, against 0.95–1.00 AU for an Earth-like day; faster rotation narrows the range further. Some fast rotators have sub-freezing global averages yet 34–47% of their surface holds liquid water.
On eccentric orbits, a land planet loses temperate climates beyond about 0.20, while Earth-like and ocean planets keep them to about 0.25.
Why does climate multistability matter for exoplanets?
Habitability is often judged from how much starlight a planet receives. This study shows that two planets receiving identical starlight can end up in completely different climates because of their history or rotation, and that a global average temperature can hide large areas of liquid water.
Different climates would also reflect light differently, so rotation alone could change what telescopes observe.
Because Gen-EBM is fast, it can scan wide ranges of possible planets and flag cases that deserve study with slower, more detailed models. A fair reading is that the existence of several stable climates is the robust message, while the exact distances are model-dependent.
What are the limits of the Gen-EBM climate model?
Read before you quote it
This is a preprint that has not been peer reviewed, and its limits are clear:
- Simple heat transport. Heat moves by 1-D diffusion, with no winds, clouds or weather.
- Water-rich planets only. The radiation curve applies to water-rich planets with Earth-like atmospheres.
- Temperate calibration. Heat capacities were calibrated for temperate climates, so hot-state variability is uncertain.
- Missing long-term feedbacks. The carbonate-silicate cycle, which sets the real outer edge of habitable climates, is not included.
The study does not show which climate any real exoplanet has, since no planet was observed. It also does not show that Earth could flip to another climate.
- It does not show which climate any real exoplanet has; no planet was observed.
- It does not show that Earth could flip into another climate; the carbonate-silicate cycle that stabilises real planets is not modelled.
- Heat transport is simple 1-D diffusion, so atmospheric circulation, clouds and latent heat are not represented.
- The radiation curve applies to water-rich planets with Earth-like atmospheres, not to dry planets or other compositions.
- Heat capacities were calibrated for temperate climates, so post-runaway variability should be read with caution.
- The model is valid only between about 200 and 1056 K, and long-term feedbacks such as the carbonate-silicate cycle are not included.
Takeaways
- saysAt Earth's distance, the same Earth-like planet can settle into three stable climates, a snowball, a mild climate, or a scorching post-runaway state.
- saysFaster spin narrows the range of distances where a mild climate is possible, and land-rich planets lose it on less stretched orbits than ocean planets.
- saysIt's a simple one-dimensional model with no winds or clouds, built for water-rich planets, so the exact numbers are a first map.
- explainsClimate multistability means the same planet, with the same starlight, can settle into more than one stable climate.
- interpretsA fair reading is that a planet's history and rotation may matter as much as its distance from its star.
Ask the paper
In PaperSays you can ask this paper anything, by text or by saying “Hey Aiden”. Three questions listeners asked:
Can a planet have more than one stable climate?
In this model, an Earth-like planet at Earth's distance can be a snowball, temperate or post-runaway, depending on its starting state. Hear the full answer in the app →
How does rotation affect a planet's climate?
Faster rotation weakens heat transport to the poles and narrows the range of distances with temperate climates. Ask in the app →
Why do snowball planets stay frozen?
Ice reflects most sunlight, so in the model snowballs stayed frozen even with nearly twice Earth's sunlight. Ask in the app →
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