![]() Indeed, the dark side could have lacked starlight-the main energy source for life on Earth-for billions of years, casting doubt on whether life could have ever emerged there. The dark side could be so cold that water and would-be atmospheric components (e.g., carbon dioxide, nitrogen, or methane) are frozen, certainly an inhospitable environment for life as we know it. On a tidally locked planet, one side is always facing a star while the other is cloaked in perpetual darkness. Tidal locking influences planetary bodies in dramatic ways that at a glance, do not paint a promising picture for the existence of life on these planets. Such processes likely occur between exoplanets and stars as well, turning planets that used to rotate faster into tidally locked satellites. The only reason its rotational and orbital periods are slightly out of sync is that Mercury has a highly eccentric orbit (as opposed to the Moon, whose orbit around Earth is practically circular). And Mercury, which rotates three times around its axis for every two revolutions around the Sun, is almost tidally locked to the Sun. Other moons within our solar system are also tidally locked to their home planets, such as Jupiter’s moon Io and Saturn’s moon Enceladus. This illustration shows the gravitationally induced process by which a planet (or a moon) can become tidally locked to its host star (or planet). Get the most fascinating science news stories of the week in your inbox every Friday. When the Moon formed more than 4 billion years ago, it rotated more quickly than it does today, but over time, tidal torques slowed its rotation until it became tidally locked to Earth. Torques act on spinning bodies to slow or speed their rotation. This misalignment produces a torque on the Moon. Because rock bends and flows slowly, as the solid Earth rotates, its rocky tidal bulges pull ahead of the lunar gravitational force that causes them (Figure 1). The same sort of deformation manifests itself in Earth’s oceans, where the Moon’s tidal forces produce watery bulges that travel around Earth as it rotates, leading to alternating high and low tides.Įarth also has rocky tides that respond to the Moon’s pull, but solid rock is harder to deform than water, so the corresponding bulges aren’t as noticeable. This force deforms the Moon, reshaping it from a perfect sphere into something a little more akin to an American football: slightly squashed at the poles, with a bulge at its equator facing Earth and another on its far side. As the Moon orbits Earth, Earth’s gravity tugs at it. This tidally locked state is a consequence of gravity. Because the Moon’s rotation and orbit around Earth both last a little less than a month, we always see the same craters and plateaus on the Moon’s surface while the opposite side is hidden from our sight. This physical quirk affects many planets and moons, including Earth’s Moon. What Is Tidal Locking?Ī tidally locked object rotates around its axis exactly once during its orbit around a host planet or star. Would such a tidally locked exoplanet be able to support life? We don’t know, but the question raises another challenge for astronomers trying to characterize exoplanets and assess their habitability. If so, one side of the planet could be burning hot while the other side is brutally cold. But as we look for habitable planets, there are other factors to consider, including whether a planet is tidally locked-that is, whether it always shows the same face to its host star. Rocky planets with stable orbits and long-lived host stars are potential targets in the search for extraterrestrial life. Outer space also harbors planets that are relatively similar to Earth, and excitement rises whenever a new “Earth-like” planet is discovered. Some of these exoplanets are, at least by our standards on Earth, bizarre: Astronomers have found diamond planets worlds whose surfaces are pelted by sapphires, rubies, and glass and planets hotter than some stars. In the past 20 years, astronomical observations have revealed thousands of planets outside our solar system.
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