Researchers from Tokyo Institute of Technology, the Astrobiology Center of the National Institutes of Natural Sciences, and the University of Hawaii performed spectroscopic observation of two recently-discovered young planetary systems by using the new infrared spectrograph IRD mounted on the Subaru Telescope, and they determined that the orbital axis of the planet and the axis of rotation of the star were virtually aligned in both of these young planetary systems. This is the first time in the world that orbital inclination has been obtained about a young planet with an age of around 20 million years; this is extremely important data for understanding the evolution of planetary systems.
The search for planets orbiting stars other than the Sun (exoplanets) has focused on main sequence stars that are similar to the Sun. One of the reasons for this is that these types of stars have low levels of surface activities, such as flares and starspots, which makes it easier to find planets. Thanks to improvements in observational techniques in recent years, however, exoplanets orbiting near young stars immediately after formation have begun to be discovered.
Since it is thought that primordial information related to the formation of planets is still relatively unchanged in the case of young planets, they are valuable observation targets for investigating the origins of planetary systems. In particular, theories suggest that the orbital inclination of a planet (the angle between the orbital axis of the planet and the axis of rotation of the star, Figure 2) changes over time due to gravitational interactions between planets and tidal interactions with the star. While the orbital inclinations of over 100 planetary systems have already been investigated, all but one of these observations were of main sequence planetary systems with an age of 1 billion years or greater (as of the release date of this article). It is necessary to examine the orbital inclination of young planetary systems in order to determine the kinds of orbits that planets have when they are first formed.
A team of researchers, consisting of members from the Tokyo Institute of Technology, the Astrobiology Center (ABC), and the University of Hawaii focused on two young stars with recently-discovered planets, "AU Microscopii" (AU Mic) and "K2-25." AU Mic and K2-25 belong to young stellar groups, the Beta Pictoris Moving Group (age around 23 million years) and the Hyades Star Cluster (age around 600 million years), respectively (see Note 1). A transiting planet (see Note 2) about the size of Neptune has been found around each star. Although the two target stars are dim and hard to observe in the visible light range because of their low surface temperatures, they are bright and easily observable in the infrared region. Furthermore, it is also expected that the surface activities in young stars have less effect in the infrared region. The research team therefore carried out observations using the new infrared spectrograph IRD (Infrared Doppler) of the Subaru Telescope.
Using the Doppler shadow technique (see Note 3), in which the motion of the shadow of a transiting planet in the stellar spectrum is examined while taking the Doppler effect into account, the team determined that the orbital axis of the planet and the axis of rotation of the star are well aligned in both cases. In particular, planet around AU Mic (AU Mic b), whose age is estimated to be around 20 million years, has become the youngest planet whose orbital alignment is known.
The fact that the orbital planes of these young planets are not inclined has important implications for the interpretation of previous observation results. Although the orbital plane of the planets in the Solar System is not significantly inclined, it is known that about one-third of systems where the orbital inclination of planets has been measured have large inclinations. The mechanism behind this phenomenon and its timing have been under research for a long time. In the present study, the fact that the orbital planes of these young planets were not inclined suggests that the orbits of planets are not inclined immediately after formation, and that instead, in some systems, the orbital plane becomes inclined after some time has passed after formation. However, observation of young planetary systems has only just begun, and it is expected that the origin of inclined planets will be further clarified by performing similar observations of more young planetary systems in the future.
These research results were published as "Limits on the Spin–Orbit Angle and Atmospheric Escape for the 22 Myr Old Planet AU Mic b" by T. Hirano et al. in The Astrophysical Journal Letters (August 7, 2020), and "Zodiacal Exoplanets in Time. XI. The Orbit and Radiation Environment of the Young M Dwarf-Hosted Planet K2-25b" by E. Gaidos et al. in The Monthly Notices of the Royal Astronomical Society Letters (August 14, 2020).
Note 1: While it is difficult to determine the age of a star, especially for a young star, the ages of the two planetary systems are assumed to be virtually the same as that of the stellar groups to which they belong.
Note 2: Exoplanetary systems where a portion of the surface of the star is obscured periodically by the transit of the planet in front of the star are known as "transiting planetary systems." Transits can be observed if the orbital plane of a planet is nearly aligned with the line of sight of the observer.
Note 3: "Doppler shadow" is a way to analyze the shadow of a planet in the absorption lines of the host star’s spectrum. The stellar absorption lines are broadened due to the Doppler effect of the stellar rotation. When a transiting planet blocks a small portion of the stellar surface, the shadow of the planet appears in the broadened stellar absorption lines. The orbital axis of the planet can be determined by analyzing the time variation of the planet's shadow. Because the "Rossiter-McLaughlin effect" represents a phenomenon where the shapes of the absorption lines are modified or stellar radial velocity appears to be changed by the planet's shadow, the Doppler shadow can be described as a way to capture the Rossiter-McLaughlin effect.