Tidally-Locked Planets More Common than Previously Thought, Astronomer Says

Dr. Rory Barnes, an assistant professor in the Department of Astronomy and Astrobiology Program at the University of Washington, arrived at this finding by questioning the long-held assumption that only those stars that are much smaller and dimmer than our Sun could host tidally-locked planets.
Tidal locking results when there is no side-to-side momentum between a body in space and its gravitational partner and they become fixed in their embrace.
Tidally-locked bodies such as the Earth and the Moon are in synchronous rotation, meaning that each takes exactly as long to rotate around its own axis as it does to revolve around its host star or gravitational partner.
The Moon takes 27 days to rotate once on its axis, and 27 days to orbit the Earth once.
Earth’s only permanent natural satellite is thought to have been created by an object the size of Mars, known as Theia, slamming into the proto-Earth at an angle that set the world spinning initially with approximately 12-hour days.
“The possibility of tidal locking is an old idea, but nobody had ever gone through it systematically,” Dr. Barnes said.
“In the past, researchers tended to use that 12-hour estimation of Earth’s rotation period to model exoplanet behavior, asking, for example, how long an Earth-like exoplanet with a similar orbital spin might take to become tidally locked.”
“What I did was say, maybe there are other possibilities — you could have slower or faster initial rotation periods.”
“You could have planets larger than Earth, or planets with eccentric orbits — so by exploring that larger parameter space, you find that in fact the old ideas were very limited, there was just one outcome there,”
He said: “planetary formation models, however, suggest the initial rotation of a planet could be much larger than several hours, perhaps even several weeks.”
“And so when you explore that range, what you find is that there’s a possibility for a lot more exoplanets to be tidally locked.”
“For example, if Earth formed with no moon and with an initial ‘day’ that was 4 days long, one model predicts Earth would be tidally locked to the Sun by now.”
The results of this work 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.
Being tidally locked was once thought to lead to such extremes of climate as to eliminate any possibility of life, but astronomers have since reasoned that the presence of an atmosphere with winds blowing across a planet’s surface could mitigate these effects and allow for moderate climates and life.
“I also considered the planets that will likely be discovered by NASA’s next planet-hunting satellite, the Transiting Exoplanet Survey Satellite (TESS), and found that every potentially habitable planet it will detect will likely be tidally locked,” Dr. Barnes said.
The results will be published in the journal Celestial Mechanics and Dynamical Astronomy, but have been published on arXiv.org ahead of time.


http://www.sci-news.com/astronomy/tidally-locked-planets-05135.html