The region around a star where an Earth-like planet can maintain liquid water on its surface is known as the habitable zone or the ‘Goldilocks’ zone. Previous studies of Earth-like planets in the habitable zones of stars generally assume ocean covered planets that resemble the present Earth. If such an ocean planet is too far from its star, it leads to an ice-albedo feedback which ends in the complete freezing of the planet. If the same planet is too close to its star, a runaway greenhouse effect occurs which ends in the complete evaporation of the planet’s oceans. Now imagine another kind of habitable planet whose surface is predominantly land, with only small areas of surface water. A planet like this is can be called a land planet.
Although a land planet is probably covered by vast deserts, it can support localized regions with abundant water and such regions can exist for example, near the poles of the planet. In our own solar system, the closest analogy to a land planet is Saturn’s moon Titan. Titan has lakes of methane on both its poles and between the poles of Titan is a vast desert that spans the tropics and temperate zones. The surface of Titan is far too cold for liquid to exist, resulting in liquid methane playing its role on Titan as water does on Earth. A rather engaging paper by Abe et al. 2011 that is entitled “Habitable Zone Limits for Dry Planets” studies the possibilities that land planets can have wider habitable zones than ocean planets. This means that a land planet can be nearer or further from its parent star than an ocean planet and still be capable of supporting habitable Earth-like surface conditions.
In this article, the comparison between an ocean planet and a land planet assumes that each planet orbits a Sun-like star that is identical to ours. The complete freezing of an Earth-like ocean planet occurs when the Sun is dimmed to 90 percent of its present luminosity while the complete freezing of a land planet only occurs when the Sun is dimmed to 77 percent of its present luminosity. In other words, a land planet has a greater resistance to complete freezing than an ocean planet. This is due to the fact that a land planet will tend to be less reflective than an ocean planet. One reason for this is that a land planet has fewer clouds than an ocean planet because it is less humid. The other reason is that less snow accumulates on a land planet than on an ocean planet because the atmosphere is drier and the daytime temperatures are higher for a land planet. Fewer clouds and less snow cover make a land planet less reflective than an ocean planet to incoming insolation from its parent star. A less reflective planet means a higher surface temperature. For this reason, a land planet can be further than an ocean planet from its parent star before complete freezing occurs. Therefore, the outer boundary of the habitable zone of a land planet is larger than for an ocean planet.
Moving now to the inner boundary of the habitable zone, liquid water can remain stable on an ocean planet until the Sun is brightened to 135 percent or more of its present luminosity. For a land planet, liquid water can remain stable on its surface until the Sun is brightened to 170 percent or more of its present luminosity. This means that the inner boundary of the habitable zone of a land planet is closer in to its parent star than for an ocean planet since a land planet can be nearer to its parent star than an ocean planet before a runaway greenhouse effect occurs. For an ocean planet, a runaway greenhouse effect occurs when there is enough water vapour in the atmosphere such that the atmosphere becomes optically thick to outgoing thermal radiation. This causes the ocean planet to absorb more energy from its parent star than it can radiate away, eventually causing the surface of the planet to become sterilizingly hot.
Compared to an ocean planet, the case for a land planet is rather different. The low latitude region of a land planet is expected to have an extremely low humidity and effectively no surface water. This allows the low latitude region of the land planet to absorb more energy from its parent star than it can radiate away, without leading to a runaway greenhouse effect. Furthermore, for an extremely dry land planet, all its surface water can evaporate without a significant contribution of water vapour into the atmosphere to trigger a runaway greenhouse effect. Since a runaway greenhouse effect may not occur for a land planet, the equivalent runaway greenhouse effect threshold can be defined as the maximum insolation the land planet can receive, beyond which all surface water and surface ice completely evaporate, including even those at the poles.
A land planet with no permanent surface water can still sustain a hydrated layer of surface soil by the deposition and subsequent melting of frost. At night, it may be cold enough for frost to form, especially within the pore spaces of the surface soil. During the day, the frost can melt into liquid water and moisturize the surface soil. This mechanism is particularly effective for a land planet with a thin atmosphere since a thin atmosphere is much less effective at damping daily temperature fluctuations than a thick atmosphere. Nights on a land planet with a thin atmosphere can get exceptionally cold, thereby creating an environment that is very conducive for the formation of frost. Additionally, a thinner atmosphere will reduce the rate of energy transport from the equator to the poles of a land planet. This stabilizes any polar ice caps against evaporation and reduces the input of water vapour into the planet’s atmosphere which further prevents the onset of a runaway greenhouse effect.
To conclude, the habitable zone for a land planet around its parent star is considerably larger than it is for an ocean planet around the same star. One key consideration that can be of importance is that the presence of clouds creates a major uncertainty as to the true limits of the habitable zone of a planet around its parent star. Clouds can warm or cool a planet, whereby high clouds have a warming effect and low clouds have a cooling effect. Reducing the coverage of high clouds and increasing the coverage of low clouds pushes the inner limit of a planet’s habitable zone closer to its parent star. Alternatively, increasing the coverage of high clouds and reducing the coverage of low clouds pushes the outer limit of a planet’s habitable zone further from its parent star.
The Sun’s luminosity increases at a rate of about 9 percent per billion years. As the Sun brightens, it might be possible that an ocean planet like the Earth can lose most of its water and become a land planet without passing through a sterilizing runaway greenhouse effect. However, this depends on how much water an ocean planet like the Earth can lose before it reaches the threshold for a sterilizing runaway greenhouse effect. Still, even if an ocean planet can successfully evolve into a land planet, the surface temperature of the planet during the transition phase can reach up to between 300 to 400 degrees Kelvin. Such conditions are marginally habitable as only thermophilic microbial life on Earth can exploit such conditions. Nevertheless, the possibility of the Earth becoming a land planet in the far future adds an extra billion years or so to the continuous habitability of the Earth even as the Sun evolves to a higher luminosity.
