Evidence of tidal locking in super-Earth LHS 3844b
In a study published March 28 in the Astrophysical Journal, astronomers found that the planet has 1:1 tidal timing, meaning that it always faces the star in the same direction that the moon faces Earth. This provides the most convincing evidence to date that this means that Many rocky exoplanets were thought to be highly closed due to their orbital elements, but observational evidence was lacking. That’s it! When a planet orbits very close to its star, the side in front of the star experiences a much stronger gravitational pull than the side opposite. Over time, this imbalance, called tidal forces, slows the planet’s rotation and eventually brings it into perfect sync with the planet’s orbit. This means that the time it takes for a planet to rotate once around its axis is the same as the time it takes for a planet to rotate once around its star. In such conditions, the planet always faces the star in the same hemisphere, creating permanent day and night sides. However, not all short-period exoplanets need to rotate synchronously. The evolution of a planet’s rotation is complicated by many factors, including the planet’s internal structure, the presence of an atmosphere and ocean, and the potential influence of companion planets. For example, Mercury was long thought to rotate synchronously, based on early tidal theories and observations of its surface features. However, radar observations in the late 20th century showed that Mercury was in a 3:2 spin-orbit resonance (Colombo & Shapiro 1966). While it is easy to measure an exoplanet’s orbit, determining its rotation around it is much more difficult, especially if the planet has an atmosphere that obscures its rotating surface. To prove the tidal locking hypothesis, Xintong Lyu (Peking University) and his colleagues focused particularly on exoplanets close to the star. In 2019, the team used the Spitzer Space Telescope (2003-2020) to measure the intensity of light coming from this super-Earth, called LHS 3844b. Since LHS 3844b
likely has no atmosphere, Lyu and his co-authors realized that these measurements could provide information about the temperature of the planet’s Earth-facing surface.
LHS 3844b is a super-Earth with a radius of 1.32 Râ and an orbital period of 11.1 hours. It is one of the smallest short-period exoplanets for which a thermal phase curve has been measured. Spitzer’s observations of the planet at 4.5 μm (infrared range) rule out many possible atmospheric scenarios and limit the total atmospheric thickness to less than 10 bar (Kreidberg et al. 2019; Whittaker et al. . 2022). These observations are consistent with atmospheric escape models, which predict that the planet lost its atmosphere due to strong stellar winds and high X-UV flux. The most plausible interpretation is that LHS 3844b is a bare rocky planet, which rules out possible degeneration during asynchronous rotation and atmospheric heat redistribution, making LHS 3844b a good candidate for testing the tidal-locking hypothesis. It becomes ideal. Previous analysis of LHS 3844b found that its phase curve not only excludes a thick atmosphere, but is also consistent with a circular orbit, synchronous rotation, and a relatively low-albedo basaltic surface (Kreidberg et al. al. 2019; Whittaker et al. 2022). . . Researchers are examining Spitzer’s phase curve for LHS 3844b using a thermal model of a planet without an atmosphere, analyzing the effects of asynchronous rotation, eccentricity, tidal dissipation, and surface composition. Planets that are not (yet) tidally synchronized are known to be warmer due to the conflict between their rotation and the strong tidal forces they exert. However, the researchers found that LHS 3844b’s surface is relatively cold. This is to be expected for a planet that does not experience tidal heating and is therefore synchronous. Because no strong tidal warming has been observed, Lyu et al. rule out rapid asynchronous rotations (including Mercury’s 3:2 spin-orbit resonance) and limit the planet’s eccentricity to values below 0.001. Additionally, the phase curve of LHS 3844b hints at two other possible effects. That is, the planet is still subject to very weak tidal heating due to its weak but non-zero eccentricity (although in this case this would require the presence of an undetected orbital companion); or The dayside surface of LHS 3844b has darkened due to changes in radiation. Of these two of his scenarios, Lyu and his colleagues believe that change is the most likely, but the other cannot be ruled out either. For them, these results support the hypothesis that short-period rocky exoplanets are well tidally confined and that spatial changes can significantly alter them given the atmosphere-less exoplanet surfaces.
This is the most convincing evidence of the phenomenon available with currently available information and tools. Future observations will allow us to test and refine this interpretation in various ways. Measurements of radial velocity can be used to constrain the planet’s eccentricity and rule out the presence of companion planets. Similarly, Webb observations can identify surface weathering by its effect on the planet’s secondary eclipse spectrum, whereas tidal heating may be due to its effect on the planet’s secondary eclipse spectrum, which can be reduced thanks to a more accurate thermal phase curve than Spitzer’s. Such measurements should be possible using Webb not only for LHS 3844b, but also for other candidate exoplanets of the same type (the Rocky Mountains, which have no atmosphere near their star), and in particular for GJ 1252b. , TRAPPIST-1b, and TRAPPIST-
1c.
source: https://doi.org/10.3847/1538-4357/ad2077