1708.02981
Tidal locking of habitable exoplanets
Barnes
Potentially habitable planets can orbit close enough to their host star that the differential gravity across their diameters can fix the rotation rate at a specific frequency, a process called tidal locking. Tidally locked planets on a circular orbits will rotate synchronously but those on eccentric orbits will either librate or rotate super-synchronously. Calculate how habitable planets evolve under 2 commonly-used models and find, for example, that one model predicts that the Earths' 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. 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. 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. The orbits of potentially habitable planets of very late M dwarfs (<0.15 Msun) are very likely to be circularized within 1 Gyr and hence those planets will be synchronous rotators. 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 evolution of the isolated and potentially habitable Kepler planet candidates is computed and about half could be tidally locked. Finally, projected TESS planets are simulated over a wide range of assumptions, and the vast majority of all cases are found to tidally lock within 1 Gyr. 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.
1708.03002
Relativistic astronomy
Zhang, Li
The Breakthrough Initiatives are a program of scientific and technological exploration, probing some big questions of life in the universe. Among them is the "Breakthrough Starshot" program, which aims at proving the concept of developing unmanned space flight (probe) at a good fraction of the speed of light, c. Such a probe is designated to reach nearby stellar systems such as Alpha Centauri within decades, allowing humankind to explore extra-solar systems for the first time. The first prototype "Sprites" of 3.5cm x 3.5 cm chips weighing just 4 grams each, which are the precursors to eventual "starChip" probes, have been recently launched to a low-earth orbit. Point out that due to the relativistic effects, trans-relativistic cameras serve as natural lenses and spectrographs while traveling in space, allowing humankind to study the astrophysical objects in a unique manner and to conduct precise tests on special relativity. Launching trans-relativistic cameras would mark the beginning of "relativistic astronomy".
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