Note: If you need a refresher on the habitability argument, click here.

Here’s an article entitled “Tidal heating shrinks the ‘goldilocks zone’: Overlooked factor suggests fewer habitable planets than thought”. It appeared in Nature, the most prestigious peer-reviewed science journal.

The gist of it is that tidal forces can alter orbits so that planets don’t spend all of their orbit in the habitable zone. If planets go outside the habitable zone, it damages their supply of liquid water, and any life chemistry going on in there is disrupted.

Excerpt:

A previously little-considered heating effect could shrink estimates of the habitable zone of the Milky Way’s most numerous class of stars — ‘M’ or red dwarfs — by up to one half, says Rory Barnes, an astrobiologist at the University of Washington in Seattle. That factor — gravitational heating via tides — suggests a menagerie of previously undreamt-of planets, on which tidal heating is a major source of internal heat. Barnes presented the work yesterday at a meeting of the American Astronomical Society’s Division on Dynamical Astronomy in Timberline Lodge, Oregon.

The habitable zone is the orbital region close enough to a star for a planet to have liquid water, but not so close that all of the water evaporates. For our Sun, the zone extends roughly from the inner edge of the orbit of Mars to the outer edge of that of Venus. For smaller, cooler stars, such as M-class dwarfs, the zone can be considerably closer to the star than Mercury is to the Sun. And because close-in planets are easier to spot than more distant ones, such stars have been a major target for planet hunters seeking Earth-like worlds.

There’s just one problem with finding habitable planets around such stars, says Barnes. Because tidal forces vary dramatically with the distance between a planet and its star, closer orbits also result in massively larger tidal forces.

Since planets do not have perfectly circular orbits, these tidal forces cause the planet to flex and unflex each time it moves closer to or further from its star; kneading its interior to produce massive quantities of frictional heat. Substantial heat can be produced, he added, with even slight deviations from a perfectly circular orbit. And, Barnes notes, other factors — such as the rate of the planet’s rotation and its axial tilt — can also influence heat production.

A similar tidal process makes Jupiter’s moon Io the most volcanic body in the Solar System. “I’m just scaling that Io–Jupiter system up by a factor of 1,000 in mass,” Barnes said at the meeting. “It’s the same process, on steroids.”

So, stars that are smaller and cooler will have a habitable zone that is closer to the star, exposing them to more tidal forces. More tidal forces makes their orbits less likely to stay circular – within the habitable zone around the star. These variations cause an increase in heat production on the planet. Too much heat means that the planet is unable to support liquid water on the surface, making it inhospitable for life. Therefore, solar systems with less massive stars can be ruled out as possible sites for life, because of these tidal forces.