Whenever I think of a tidally locked planet, like say Mercury, the only image that comes to mind is one that is barren of all life. You see for tidally locked systems the face of the smaller body is always pointing towards the larger one, like our Moon is towards Earth. For planets and suns this means that the surface of the tidally locked planet would typically turn into an inferno with the other side becoming a frigid wasteland, both devoid of any kind of life. However new research shows that these planets might not be the lifeless rocks we once thought them to be and, in fact, they could be far more Earthlike than we previously thought.
Scientists have long theorized that planets of this nature could potentially harbour a habitable band around their terminator, a tenuous strip that exists between the freezing depths of the cold side and the furnace of the hot side. Such a planet wouldn’t have the day/night cycles that we’re accustomed to however and it would be likely that any life that evolved there would have adapted to the permanent daylight. There’d also be some pretty extreme winds to contend with as well due to the massive differences in temperature although how severe they were would be heavily dependent on the thickness of the atmosphere. Still it’s possible that that little band could harbor all sorts of life, despite the conditions that bookended its environment.
However there’s another theory that states that these kinds of planets might not be the one sided hotbeds that we initially thought them to be. Instead of being fully tidally locked with their parent star planets like this might actually still rotate thanks to the heavy winds that would whip across their surface. These winds would push against the planets surface, giving it enough rotation to overcome the tidal locking caused by the parent star’s gravity. There’s actually an example of this within our own solar system: Venus which by all rights should be tidally locked to our Sun. However it’s not although it’s extremely long days and retrograde rotation (it spins the opposite way to every other planet) hints at the fact that its rotation is caused by forces that a different to that from every other planet.
Counterintuitively it seems that Venus’ extremely thick atmosphere might be working against it in this regard as the modelling done shows that planets with thinner atmospheres would actually experience a higher rotational rate. This means that an Earthlike planet that should be tidally locked would likely not be and the resulting motion would be enough to make the majority of the planet habitable. In turn this would mean that many of the supposedly tidally locked planets we’ve discovered could actually turn out to be habitable candidates.
Whilst these are just beautiful models for now they can hopefully drive the requirements for future craft and observatories here on Earth that will be able to look for the signatures of these kinds of planets. Considering that our detection methods are currently skewed towards detecting planets that are close to their parent stars this will mean a much greater hit rate for habitable candidates, providing a wealth of data to validate against. Whether we’ll be able to get some direct observations of such planets within the next century or more is a question we won’t likely have an answer to soon, but hopefully one day we will.