An important discovery: TRAPPIST-1 habitable planet orbit is not misplaced!

[Mobile software: Bo Ke Yuan] Astronomers using the Subaru Telescope have determined that the Earth-like planets in the TRAPPIST-1 system are not significantly misaligned with the star's rotation, which is an important discovery for understanding the evolution of planetary systems around extremely low-mass stars, especially the history of the TRAPPIST-1 planets, including planets near the habitable zone. Stars like the Sun are not stationary, but rotate around an axis, and this rotation is most obvious when there are features such as sunspots on the surface of the star. In the solar system, the orbits of all planets are aligned to within 6 degrees of the Sun's rotation.

In the past, it was assumed that planetary orbits would be aligned with the rotation of the star, but there are now many known exoplanetary systems where the planetary orbits are severely misaligned with the rotation of the central star. This raises the question: Can planetary systems form out of alignment, or can the observed misaligned systems start out aligned and then get pushed out of alignment due to some kind of perturbation? The TRAPPIST-1 system is of interest because it has three small, rocky planets in or near the habitable zone, where liquid water could exist.

The central star is a very low-mass, cool star known as an M dwarf, and these planets are located very close to the central star. Therefore, this planetary system is very different from our Solar System. Determining the history of this system is important because it can help determine if any potentially habitable planets are actually habitable. But it's also an interesting system because it doesn't have any nearby objects that could disrupt the orbits of the planets, which means the orbits should still be near where the planets first formed, giving astronomers a chance to study the system's pristine conditions.

Because the star rotates, the side that is rotating into the line of sight is moving toward the observer with relative velocity, while the side that is rotating out of sight is moving away from the observer with relative velocity. If a planet passes between a star and Earth, blocking a small portion of the light from the star, it is possible to tell which side of the star the planet blocked first. This phenomenon is called the Rossiter-McLaughlin Effect. Using this method, the misalignment between the orbit of a planet and the rotation of the star can be measured. However, until now, these observations have been limited to large planets like Jupiter or Neptune.

A team of researchers, including members from Tokyo Institute of Technology and the Japan Astrobiology Center, observed TRAPPIST-1 with the Subaru Telescope to look for misalignments between the planetary orbits and the star, when three exoplanets orbiting TRAPPIST-1 transited in front of the star overnight. Two of the three planets are rocky planets near the habitable zone. Since low-mass stars are usually dim, it has been impossible to detect the stellar inclination (spin-orbit angle) of TRAPPIST-1. But thanks to the light-gathering power of the Subaru Telescope and the high spectral resolution of the new infrared spectrometer IRD, the team was able to measure the inclination.

The study found that the inclination is very low, close to zero, which is the first time that the stellar inclination of an extremely low-mass star like TRAPPIST-1 has been measured, and the first Rossiter-McLaughlin measurement of a planet in the habitable zone. However, the leader of the team, Teruyuki Hirano of Tokyo Institute of Technology, said: "The data show that the star's rotation is consistent with the planet's orbital axis, but the accuracy of the measurement is not enough to completely rule out the possibility of a small spin-orbit misalignment. Nevertheless, this is the first time that the influence of an Earth-like planet has been detected, and more research will better characterize this exoplanet system."