TRAPPIST-1 planetary orbits not displaced

US, WASHINGTON (ORDO NEWS) — Astronomers using the Subaru telescope determined that planets like the Earth and the TRAPPIST-1 systems do not have significant displacement from the plane of rotation of the central star. This is an important result for understanding the evolution of planetary systems around very small mass stars in general and, in particular, in the history of the TRAPPIST-1 planets, including those close to the inhabited zone.

Stars like the Sun are not static, but rotate around an axis. This rotation is most noticeable when there are such things as sunspots on the surface of a star. In the solar system, the orbits of all the planets are within 7 degrees with the rotation of the sun. In the past, it was assumed that the planetary orbits of the planets would be aligned with the rotation of the star, but there are currently many well-known examples of exoplanet systems where planetary orbits are highly offset from the rotation of the central star. In this connection, the question arises: can planetary systems be formed through planetary migration, or are the observed planetary displacements dynamically interacting?

The TRAPPIST-1 system has attracted attention because it has three small rocky planets located in or near the habitable zone where liquid water can exist. The central star is very cold, with a very low mass, called the M-dwarf, and the 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 of the potentially inhabited planets are actually suitable.

But this is also an interesting system because it does not have any nearby objects that could disrupt the orbits of the planets, which means that their orbits should still be located close to the place where the planets formed. This gives astronomers the opportunity to explore the initial conditions of this system.

If the planet passes, passes between the star and the Earth and blocks a small part of the light from the star, you can determine which edge of the star the planet blocks first. Using the Rossiter-McLaughlin effect, one can measure the displacement between the planetary orbit and the rotation of the star. However, so far these observations have been limited to large planets such as Jupiter or Neptune.

A team of researchers, including those from the Tokyo Institute of Technology and the Astrobiological Center in Japan, monitored TRAPPIST-1 with the Subaru telescope to find a mismatch between planetary orbits and a star. The team took the chance on August 31, 2018, when three of the exoplanets orbiting TRAPPIST-1 passed in front of the star in one night. Two of the three were rocky planets near the habitable zone.

With low-mass stars, usually faint, it was impossible to investigate the stellar tilt (rotation orbit angle) for TRAPPIST-1. But thanks to the ability to collect light from the Subaru telescope and the high spectral resolution of the new IRD infrared spectrograph, the team was able to measure this slope. They found that the slope was low, close to zero. This is the first stellar tilt measurement for a very low mass star such as TRAPPIST-1, as well as the first Rossiter-McLaughlin measurement for planets in the habitable zone.

However, Team Leader Teruyuki Hirano of the Tokyo Institute of Technology warns: “The data suggest that the rotation of the star is aligned with the axes of planetary orbits, but the measurement accuracy was not good enough to completely eliminate a slight orbital shift. Nevertheless, this is the first discovery of the effect with exoplanets in the habitable zone, and more work will better characterize this remarkable exoplanetary system.”

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