Conceptual illustration of a young planetary system orbiting near a star other than the Sun.

Key points of presentation:

  • The transits of two young planets (AU Microscopii b and K2-25b) in front of their stars was analyzed by infrared analysis using the Subaru Telescope with Infrared Doppler (IRD) instrument, and it was found that the orbital axis of these planets are aligned with the axis of rotation of the star.
  • This was the first time in the world that orbital inclination was obtained about a young planet with an age of around 20 million years.
  • This is in contrast to the discovery of many planetary systems around stars with ages of 1 billion years or greater where the orbits are not always aligned.
  • This observational result suggests that planets do not have inclined orbits immediately after they first form, but rather, that the orbital plane becomes inclined after some time has elapsed since formation in some 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 surface activity, which makes it easier to find planets. However, exoplanets orbiting near young stars immediately after formation have recently begun to be discovered. Researchers from Tokyo Institute of Technology, the Astrobiology Center of the National Institute 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 (Infrared  Doppler) equipped on the Subaru Telescope, and it was determined that the orbital axis of the planet and axis of rotation of the star were virtually aligned in both of these young planetary systems. One of the two observed planets (AU Microscopii b) was found to be the youngest planet that was confirmed to have an uninclined orbital plane. This is the first time that this kind of orbital information about a young planet has been obtained, and is extremely important data for understanding the evolution of planetary systems.

It is known that generally, the properties of planets, such as their orbit and atmosphere, gradually change with time. Since it is thought that primordial information related to the formation of planets, such as where they form and what kind of atmosphere they obtain, is still relatively unchanged in the case of young planets, they are a valuable observation target for investigating the origins of planetary systems. In particular, the orbital inclination of the planet (the angle between the orbital axis of the planet and the axis of rotation of the star) is known to change over time due to gravitational interactions between planets and tidal interactions with the star. As a result, 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 formed. The orbital inclinations of over 100 planetary systems have already been investigated, and it was found that the orbital axis of the planet and axis of rotation of the star were highly aligned in some cases, and greatly misaligned in some cases. However, virtually all of these observations were of main sequence planetary systems with an age of 1 billion years or greater.

Our research team focused on two young stars, "AU Microscopii" and "K2-25", whose transiting planets have recently discovered [Note]. These are young stars that belong to the Beta Pictoris Moving Group (age around 23 million years) and the Hyades Star Cluster (age around 600 million years), respectively, where transiting planets of around the size of Neptune have been found orbiting each star. Young stars are cooler than main sequence stars, and since the two target stars in this research in particular are low-temperature stars, although they are dim and hard to observe in the visible light range, they are bright and easily observable in the infrared region. Furthermore, it is also expected that the effects of the activity factor in young stars are reduced in the infrared region. Our research team therefore carried out observations using the new infrared spectrograph IRD of the Subaru Telescope, and discovered that the orbital axis of both planets was well aligned with the axis of rotation of the star. This was determined using the Doppler shadow technique, in which the shadow of the planet is examined while taking into account how the Doppler effect changes the spectrum during the transit.

Although the orbital plane of a planet is not inclined immediately after a planetary system is formed (left), after a period of time, there are both cases where the orbital plane is inclined (lower right) and cases where it is not inclined (top right).

The fact that the orbital planes of planets are not inclined in these two young systems has important implications for the interpretation of previous observation results. Although the orbital plane of the planets in the solar system is virtually not inclined, it is known that many (around one-third) of systems where the orbital inclination of planets has been measured have a large inclination (i.e., the orbital axis of planet is very different from the axis of rotation of the star). 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 planets in young planetary systems 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 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 in The Astrophysical Journal Letters (August 7, 2020), and in The Monthly Notices of the Royal Astronomical Society (August 14, 2020), as "Limits on the Spin–Orbit Angle and Atmospheric Escape for the 22 Myr Old Planet AU Mic b" by T. Hirano et al., 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., respectively.

 

[Note] 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".

 

Related release: Subaru telescope, NAOJ

 

Paper info.

Journal: The Astrophysical Journal
Title: "Limits on the Spin–Orbit Angle and Atmospheric Escape for the 22 Myr Old Planet AU Mic b"
著者: T. Hirano et al., including M. Kuzuhara, M. Omiya, M. Tamura
DOI: 10.3847/2041-8213/aba6eb
archive: https://arxiv.org/abs/2006.13243

 

Journal: The Monthly Notices of the Royal Astronomical Society
Title: "Zodiacal Exoplanets in Time. XI. The Orbit and Radiation Environment of the Young M Dwarf-Hosted Planet K2-25b"
著者: E. Gaidos et al., including M. Kuzuhara, M. Omiya, M. Tamura
DOI: 10.1093/mnrasl/slaa136
archive: https://arxiv.org/abs/2007.12701