We now know that Venus is covered with thick clouds and that astronomers misinterpreted their results, but their speculations on the habitability of tidally locked worlds are similar to modern discussions. Some researchers asked “ill tidal friction at the last put a stop to the sure and steady clockwork of rotation, and reduce one hemisphere to a desert, jeopardizing or annihilating all existence ?” (Mumford 1909), while other, more optimistic, scientists suggested “that between the two separate regions of perpetual night and day there must lie a wide zone of subdued rose-flushed twilight, where the climatic conditions may be well suited to the existence of a race of intelligent beings” (Heward 1903). By the late 1800s, astronomers were keenly interested in the possibility that Venus could support life, but (erroneous) observations of synchronous rotation led to considerable discussion of its impact on planetary habitability (Schiaparelli 1891 Lowell 1897 Slipher 19 Webster 1927). The role of planetary rotation on habitability has been considered for well over a century. These results suggest that the process of tidal locking is a major factor in the evolution of most of the potentially habitable exoplanets to be discovered in the near future. Finally, projected TESS planets are simulated over a wide range of assumptions, and the vast majority of potentially habitable cases are found to tidally lock within 1 Gyr. The evolution of the isolated and potentially habitable Kepler planet candidates is computed and about half could be tidally locked. Proxima b is almost assuredly tidally locked, but its orbit may not have circularized yet, so the planet could be rotating super-synchronously today. The orbits of potentially habitable planets of very late M dwarfs ( ) are very likely to be circularized within 1 Gyr, and hence, those planets will be synchronous rotators. For fast-rotating planets, both models predict eccentricity growth and that circularization can only occur once the rotational frequency is similar to the orbital frequency. Lower mass stellar hosts will induce stronger tidal effects on potentially habitable planets, and tidal locking is possible for most planets in the habitable zones of GKM dwarf stars. I calculate how habitable planets evolve under two commonly used models and find, for example, that one model predicts that the Earth’s rotation rate would have synchronized after 4.5 Gyr if its initial rotation period was 3 days, it had no satellites, and it always maintained the modern Earth’s tidal properties. Although these features of tidal theory are well known, a systematic survey of the rotational evolution of potentially habitable exoplanets using classic equilibrium tide theories has not been undertaken. Tidally locked planets on circular orbits will rotate synchronously, but those on eccentric orbits will either librate or rotate super-synchronously. Eventually the tidal torques fix the rotation rate at a specific frequency, a process called tidal locking. Frictional forces inside the planet prevent the bulges from aligning perfectly with the host star and result in torques that alter the planet’s rotational angular momentum. Brian Jackson, Carnegie Department of Terrestrial Magnetism, Washington, D.C.Potentially habitable planets can orbit close enough to their host star that the differential gravity across their diameters can produce an elongated shape. Even so, the Moon’s tidal gravity has slowed our planet’s rotation some, and the length of a day on Earth has probably increased by about two hours over the past 620 million years. The Sun and Moon’s tidal gravity also try to slow Earth’s rotation, but their effects are much smaller than for an extrasolar planet near its host star. ![]() ![]() For extrasolar planets very close to their host stars - about one-tenth Mercury’s distance from the Sun - this gravitational pull eventually tidally locks the rotation, and the length of the planet’s day ends up equal to the length of its year. (The Sun’s tidal distortions make tides in Earth’s oceans.) The mass in that tidal bulge feels the gravitational pull of the host star, which tries to keep the bulge pointed toward the star. If a planet orbits extremely close to a star (as many extrasolar planets do), the star’s gravity stretches the planet into a shape called a prolate spheroid with a tidal bulge that somewhat resembles a football.
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