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.
Relevant Links
Source: Subaru Telescope
