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Ilmari Karonen
Ilmari Karonen
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As several people have indirectly noted, the premise of this question is controversial. There are plenty of studies* suggesting that an Earth-like planet tidally locked to its star can be habitable as long as its atmospheric and/or ocean circulation are sufficient to transport heat from the day side to the night side and keep the temperatures reasonable on both sides.

If this circulation was not present or was insufficient, however, then the temperatures on the side of the planet permanently facing the star would quickly climb above the boiling point of water, while those on the side facing away from the star would drop far below freezing. This would quickly cause all water on the planet's surface to accumulate as ice on the cold side, making both sides inhospitable to water-based lifeforms.

Furthermore, if the temperature on the cold side got low enough (and without any heat circulation it would), the same thing could also happen to nitrogen and oxygen and any other gases necessary for an Earth-like atmosphere. They, too, would freeze on the cold side and fall to the surface, leaving the planet with no atmosphere at all to speak of.

(For comparison, there are craters on the poles of the Moon and even on Mercury that are permanently shielded from sunlight, just like the whole night side of a tidally locked planet would be. In the absence of an atmosphere to circulate heat, these craters are some of the coldest places in the whole solar system.)

To some extent these are both potentially runaway processes: the less liquid water there is on the surface, and the thinner the atmosphere gets, the weaker the heat transport via air and water circulation will become. Thus, while a planet with a sufficiently thick atmosphere and deep oceans may be able to stay habitable, if the night side ever gets cold enough the whole system may cross a critical point from one stable state to another and the whole hydro- and atmosphere may freeze out on (geologically) very short timescales.

You could compare this with the runaway greenhouse effect that is believed to have occurred on Venus at some point in its early history, transforming it from a vaguely (proto-)Earth-like state to the super-hot hellscape it is today. The end state of the "runaway freeze-out", while quite different, is no less hospitableinhospitable to life.

*) See for example:

As several people have indirectly noted, the premise of this question is controversial. There are plenty of studies* suggesting that an Earth-like planet tidally locked to its star can be habitable as long as its atmospheric and/or ocean circulation are sufficient to transport heat from the day side to the night side and keep the temperatures reasonable on both sides.

If this circulation was not present or was insufficient, however, then the temperatures on the side of the planet permanently facing the star would quickly climb above the boiling point of water, while those on the side facing away from the star would drop far below freezing. This would quickly cause all water on the planet's surface to accumulate as ice on the cold side, making both sides inhospitable to water-based lifeforms.

Furthermore, if the temperature on the cold side got low enough (and without any heat circulation it would), the same thing could also happen to nitrogen and oxygen and any other gases necessary for an Earth-like atmosphere. They, too, would freeze on the cold side and fall to the surface, leaving the planet with no atmosphere at all to speak of.

(For comparison, there are craters on the poles of the Moon and even on Mercury that are permanently shielded from sunlight, just like the whole night side of a tidally locked planet would be. In the absence of an atmosphere to circulate heat, these craters are some of the coldest places in the whole solar system.)

To some extent these are both potentially runaway processes: the less liquid water there is on the surface, and the thinner the atmosphere gets, the weaker the heat transport via air and water circulation will become. Thus, while a planet with a sufficiently thick atmosphere and deep oceans may be able to stay habitable, if the night side ever gets cold enough the whole system may cross a critical point from one stable state to another and the whole hydro- and atmosphere may freeze out on (geologically) very short timescales.

You could compare this with the runaway greenhouse effect that is believed to have occurred on Venus at some point in its early history, transforming it from a vaguely (proto-)Earth-like state to the super-hot hellscape it is today. The end state of the "runaway freeze-out", while quite different, is no less hospitable to life.

*) See for example:

As several people have indirectly noted, the premise of this question is controversial. There are plenty of studies* suggesting that an Earth-like planet tidally locked to its star can be habitable as long as its atmospheric and/or ocean circulation are sufficient to transport heat from the day side to the night side and keep the temperatures reasonable on both sides.

If this circulation was not present or was insufficient, however, then the temperatures on the side of the planet permanently facing the star would quickly climb above the boiling point of water, while those on the side facing away from the star would drop far below freezing. This would quickly cause all water on the planet's surface to accumulate as ice on the cold side, making both sides inhospitable to water-based lifeforms.

Furthermore, if the temperature on the cold side got low enough (and without any heat circulation it would), the same thing could also happen to nitrogen and oxygen and any other gases necessary for an Earth-like atmosphere. They, too, would freeze on the cold side and fall to the surface, leaving the planet with no atmosphere at all to speak of.

(For comparison, there are craters on the poles of the Moon and even on Mercury that are permanently shielded from sunlight, just like the whole night side of a tidally locked planet would be. In the absence of an atmosphere to circulate heat, these craters are some of the coldest places in the whole solar system.)

To some extent these are both potentially runaway processes: the less liquid water there is on the surface, and the thinner the atmosphere gets, the weaker the heat transport via air and water circulation will become. Thus, while a planet with a sufficiently thick atmosphere and deep oceans may be able to stay habitable, if the night side ever gets cold enough the whole system may cross a critical point from one stable state to another and the whole hydro- and atmosphere may freeze out on (geologically) very short timescales.

You could compare this with the runaway greenhouse effect that is believed to have occurred on Venus at some point in its early history, transforming it from a vaguely (proto-)Earth-like state to the super-hot hellscape it is today. The end state of the "runaway freeze-out", while quite different, is no less inhospitable to life.

*) See for example:

Source Link
Ilmari Karonen
Ilmari Karonen
  • 2.6k
  • 14
  • 18

As several people have indirectly noted, the premise of this question is controversial. There are plenty of studies* suggesting that an Earth-like planet tidally locked to its star can be habitable as long as its atmospheric and/or ocean circulation are sufficient to transport heat from the day side to the night side and keep the temperatures reasonable on both sides.

If this circulation was not present or was insufficient, however, then the temperatures on the side of the planet permanently facing the star would quickly climb above the boiling point of water, while those on the side facing away from the star would drop far below freezing. This would quickly cause all water on the planet's surface to accumulate as ice on the cold side, making both sides inhospitable to water-based lifeforms.

Furthermore, if the temperature on the cold side got low enough (and without any heat circulation it would), the same thing could also happen to nitrogen and oxygen and any other gases necessary for an Earth-like atmosphere. They, too, would freeze on the cold side and fall to the surface, leaving the planet with no atmosphere at all to speak of.

(For comparison, there are craters on the poles of the Moon and even on Mercury that are permanently shielded from sunlight, just like the whole night side of a tidally locked planet would be. In the absence of an atmosphere to circulate heat, these craters are some of the coldest places in the whole solar system.)

To some extent these are both potentially runaway processes: the less liquid water there is on the surface, and the thinner the atmosphere gets, the weaker the heat transport via air and water circulation will become. Thus, while a planet with a sufficiently thick atmosphere and deep oceans may be able to stay habitable, if the night side ever gets cold enough the whole system may cross a critical point from one stable state to another and the whole hydro- and atmosphere may freeze out on (geologically) very short timescales.

You could compare this with the runaway greenhouse effect that is believed to have occurred on Venus at some point in its early history, transforming it from a vaguely (proto-)Earth-like state to the super-hot hellscape it is today. The end state of the "runaway freeze-out", while quite different, is no less hospitable to life.

*) See for example: