Observational evidence finds that the giant “black” Earth “LHS ​​3844 b” rotates synchronously

Many “exoplanet” orbit close to their stars, so their orbital periods and orbits match, like Earth's moon, always facing the same side toward the star.Synchronous rotation(tidal lock). However, the synchronous rotation of the exoplanets remains only an estimate in most cases. In particular, “super-Earths,” which are rocky planets larger than Earth, have not been verified by observation.

A research team led by Xintong Liu of Peking University has discovered that one of the super-Earths is “LHS 3844BBy applying observational data from the Spitzer Space Telescope infrared to a planetary model, we verified whether evidence of synchronous rotation could be found. As a result, we were able to rule out possibilities other than synchronous rotationLHS 3844 b became the first super-Earth to be observed with synchronous rotation.

■Direct evidence of synchronous rotation is difficult to obtain.

The front side of Earth's Moon is always facing Earth, and we cannot see the back side. This phenomenon occurs because the Moon's rotation period is lengthened by the tidal force exerted by the Earth, and is fixed at a value that corresponds to its orbital period. This kind of phenomenon isSynchronous rotation“It is called. Examples of synchronous rotation are not limited to Earth's moon, but there are many other examples, such as Jupiter's Galilean moon, and Pluto's moon Charon.

Exoplanets, which orbit celestial bodies other than the Sun, are known to often orbit close to their stars over periods ranging from several hours to several days. These planets are also believed to rotate synchronously due to tidal forces from the stars. However, it is not easy to measure the rotation periods of exoplanets that are close together, even if they are several light-years away, so most examples of synchronous rotation have not been confirmed by observation.

In particular, there were no known examples of the synchronized rotation of “super-Earths,” which are rocky planets larger than Earth. If a super-Earth orbits close to a star, this increases the chance of synchronous rotation, but this alone is not enough. For example, Mercury was long thought to rotate synchronously, but in fact it rotates three times in two cycles, meaning that its period and rotation period are in a 2:3 resonance relationship. This phenomenon occurs because the stabilization of the rotation period due to tidal forces can have values ​​other than 1:1 synchronous rotation. Therefore, super-Earths orbiting close to stars do not necessarily orbit synchronously.

Unlike hot Jupiters, which are planets similar to Jupiter and composed mainly of gas, super-Earths, which are thought to orbit synchronously, have almost lost their atmosphere and are exposed to radiation from stars and cosmic rays from outer space. This rain falls directly onto the Earth's surface, making It causes significant weathering of rocks. If we can understand how the rocks of super-Earths weather, this will also provide clues to the degree of weathering of rock-based celestial bodies in the solar system. Therefore, whether or not a super-Earth rotates synchronously is a key indicator for determining the degree of weathering of rocks in that planetary system, but the research faces difficulties because no actual example of synchronous rotation has been found.

■Observational evidence has been discovered that “LHS 3844 b” is rotating synchronously!

A research team led by Mr. Liu conducted a search for one of the super-Earths, LHS 3844 b. LHS 3844 b is one of the most widely observed and studied exoplanets, and was named “Kua'kua” in the “2022 Exoplanet Naming Campaign” conducted by the International Astronomical Union (IAU) in 2022. Name.

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Several properties of LHS 3844 b were estimated in a 2019 study that analyzed observational data from the infrared Spitzer Space Telescope operated by the National Aeronautics and Space Administration (NASA). For example, the daytime temperature is 770 °C (1040 K), while the night temperature is almost absolute zero (0 K), indicating an extreme environment where the temperature difference between day and night is up to 1000 °C. Because there is a large temperature difference between day and night, and the night side is particularly cold, it is suggested that LHS 3844 b has no atmosphere to transfer heat, and that it rotates synchronously, with day and night permanently fixed. will be done. However, as of 2019, this remains only an estimate.

Therefore, Liu et al. applied the LHS 3844 b data to a planetary model assuming no atmosphere, and compared cases with and without synchronous rotation to find the model that best matched the actual observed data they explored. Results,Observational data can be best explained by rotation synchronous with permanently constant day and night.It turns out that this study makes LHS 3844 b the first super-Earth with observational evidence of synchronous rotation.(※1).

■Is LHS 3844 b black and hot due to space weathering?

On the other hand, the results of this study also raised new questions. In a 2019 study, it was thought that LHS 3844 b was quite dark in color and may have been covered in basalt, which is dark lava. However, the absorption of the star's heat by the black basalt does not explain the high surface temperature of LHS 3844 b. The simplest explanation is that tidal forces heat the interior.

However, in this study, the eccentricity of the orbit of LHS 3844 b was studied(※2)It was also shown that the value is less than 0.001, which is very close to the ideal circle. Because its orbit is very close to a perfect circle, there is very little heat generated by tidal forces. The simplest explanation to avoid this discrepancy is that there are other planets besides LHS 3844 b, and their tidal forces are generated by disrupting their orbits. A similar situation occurs on Io, one of Jupiter's Galilean moons. Although Io's orbit is close to a perfect circle and rotates synchronously, the orbit is perturbed by the gravity of the other Galilean moons, causing heating due to tidal forces.

However, there are also more likely scenarios. LHS 3844 b has no atmosphere, so it is exposed to solar wind and cosmic rays produced by charged particles.Space weathering“It is advancing strongly. As a result, rocks become darker, like Mercury and the Moon in our solar system, making it easier to absorb heat. Blaming space weathering for the warming of LHS 3844 b is a more plausible scenario than assuming an undiscovered planet might exist.” It does not actually exist. Liu and others also believe that space weathering is a strong candidate, and that tidal heating is less likely.

It is not yet known what type of material is produced by the space weathering that occurred in LHS 3844 b. The main cause of the black color is graphite in the case of Mercury, and metallic iron in the case of the Moon, but current observational data suggest that either material could be the cause, so it cannot be determined. Additional observations about LHS 3844 b are needed to narrow down the candidates. As a result, if we can obtain not only data on the matter on the surface, but also more detailed data on its orbit, we will be able to rule out the theory of undiscovered planets, which currently remains a candidate to explain the surface temperature.

*1…There is also a non-zero probability that LHS 3844 b is not rotating synchronously, but is instead rotating very slowly, a “pseudo-synchronous rotation”. However, even if LHS 3844 b was rotating at a rate disproportionate to its orbital period, its speed would be one revolution every 211 years (the orbital period of LHS 3844 b is about 0.46 days, so it would rotate about 170,000 times) and since it is thought to be slower than One per cycle, it is quickly stabilized by tidal forces. There are also studies that estimate that planets composed primarily of rock do not rotate in a pseudo-synchronous manner, so the probability of such rotation is considered very low.

*2…Deviation is the value that indicates how far the orbit is from the perfect circle. 0 is a perfect circle, 0 or more but less than 1 is an ellipse, and less than 1 the orbit is closed. If it is equal to 1, it becomes a parabola, and if it is greater than 1, it becomes a hyperbola, and the orbit is no longer closed.

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Text / Riri Ayaka Editing / sorae editorial department