Mysteries in Science: The mystery behind the birth of planetary systems

Masahiro Ikoma

(Assoc. Professor, Department of Earth and Planetary Science)

We tend to believe that our world is ordinary; however, it might in fact be rather extraordinary. Sometimes we happen to discover another world that is far beyond our imagination. Scientists try to explain theoretically why such a strange world exists, using all their physical and chemical knowledge. Another new world, however, shows up, the presence of which contradicts that theory. It is almost as if nature is toying with us by completely changing our view of the world. This is why science is fascinating.

Until about 20 years ago, the only planetary system that we knew about was our own, composed of a central star called the Sun, which is surrounded by small rocky planets (Mercury, Venus, Earth and Mars), gas giants (Jupiter and Saturn), and ice giants (Uranus and Neptune). According to the Kyoto model of solar system formation, proposed in 1985 by a Kyoto University research team led by Prof. Chushiro Hayashi, the structure of our solar system was a natural result of physics.

In 1995, a planet orbiting another star was discovered beyond the solar system. Such planets are called exoplanets. Since then, the above solar system formation theory has undergone numerous revisions. Within the first decade, many Jupiter-like giant exoplanets were detected. Surprisingly, most of those giant planets, called hot Jupiters, orbit very close to their host stars, unlike Jupiter and Saturn in the solar system (Figure 1). New facts have come to light due to a rapid increase in the number of exoplanets discovered over the past ten years, a result of large-scale exoplanet exploration programs that utilize technology such as space telescopes. Based on the findings, we now know that the vast majority of exoplanets are not hot Jupiters, but actually super-Earths, which are planets slightly larger than the Earth.

These kinds of planets could not exist under the theoretical framework of the Kyoto model, as its fundamental premise is that planets formed in their current locations. This is obviously incorrect, however; instead, planets move from their birthplaces to other areas during their formative period. Since the discovery of hot Jupiters, theorists have found some planetary migration mechanisms. To further complicate matters, alongside hot Jupiters, a myriad of giant planets called cool Jupiters have also been found, which are located far away from their host stars (Figure 1). Moreover, despite these discoveries, we have still not been able to clarify whether super-Earths are rocky planets or ice planets as none exist in our solar system. The origin of such planetary diversity is a complete mystery, casting doubt as to whether a large-scale planetary migration even occurred in the solar system. In order to address how exactly the solar system formed, we must go back to the drawing board.

The next decade is anticipated to be the second golden age of exoplanet science. Planetary exploration so far has revealed the commonness and diversity of planets in the universe. The next stage will be to investigate the detailed characteristics of exoplanets, such as the presence or absence of an atmosphere or water, as well as their components. To achieve this, at least five space telescopes are scheduled to be launched in the coming decade, starting a new era of exploration where a second Earth, and even extraterrestrial life, may be waiting. Will our theory be proven or will it be overturned once again? I hope that in the years that follow, we can work alongside young researchers to challenge the unknown and elucidate these mysteries in nature.

― This article is from the "Mysteries in Science" series in The Rigakubu News ―

Translated by Kristina Awatsu, Office of Communication

― Office of Communication ―