Hungarian Researchers Study Unique Trappist-1 Planetary System to Better Understand Our Solar System

For most of history, the only planets known to humanity were those visible in the night sky. However, over the past 30 years, telescopes have been developed with the sensitivity required for researchers to infer the existence of planets beyond our Solar System, known as exoplanets. Almost all exoplanet discoveries, particularly around the 2010s, have been based on photometric measurements of their host stars, tracking changes in the star's light using a technique known as transit photometry, rather than through direct observation of the planets themselves.

"When we only knew our own Solar System, we could assume that planets formed in the places where we find them today. However, after the first exoplanet was discovered in 1995, astronomers had to develop more advanced models of planet formation to explain why these planets appear in the observed configurations," said Gabriele Pichierri, a postdoctoral researcher in planetary science at the California Institute of Technology (Caltech) and lead author of the study.

"Most exoplanets form from the gas and dust discs around newly formed stars and are expected to migrate inward, approaching the inner boundary of the disc. This process results in planetary systems where planets are much closer to their host stars than those in our own Solar System are to the Sun. In the absence of other influences, the planets arrange themselves so that their masses align with each other and with the host star. This is the standard migration process," explained Pichierri. The orbits of the planets create resonances with their orbital periods. If you divide the orbital period of one planet by that of a neighbouring planet, you obtain a ratio of simple integers, such as 3:2. For example, if one planet takes two days to complete an orbit around its host star, the more distant planet will take three days. If the second planet and a more distant third planet are also in a 3:2 resonance, then the orbital period of the third planet will be 4.5 days.

In 2022, Ramon Brasser, a researcher at the Konkoly Thege Miklós Astronomical Institute (KTM CSI) of HUN-REN CSFK, along with three co-authors, studied the dynamics of the Trappist-1 planetary system, located 40 light-years from Earth and hosting seven planets. This publication inspired the researchers to develop a model to explain the "unusual" orbits of the Trappist-1 planets and how they reached their current configuration.

Researchers believe that most planetary systems have undergone significant instabilities during their lifetimes before reaching the configurations we observe today. For example, planets can collide with each other, leading to a complete rearrangement. Such instability has also occurred in our Solar System, but researchers have identified planetary systems that have remained stable, with their original orbits largely intact. In these cases, astronomers can observe the entire dynamical history of the system and attempt to reconstruct it.

In the case of the complex Trappist-1 system, the authors of the recent study suggest that the inner four planets initially evolved independently in the expected 3:2 resonance chain. As the inner boundary of the disc expanded outward, the planets’ orbits shifted from the tighter 3:2 chain to the configuration observed today. The fourth planet, originally at the inner boundary of the disc and migrating outward with it, later reversed direction and moved inward when three additional outer planets joined the system at a later stage of planetary formation.

"The aim of our study was to gain a better understanding of the diversity of exoplanets, their formation and evolution, and to gain deeper insights into the history of our Solar System and why Earth can be inhabited while other planets are not," concludes Ramon Brasser.

• Hungarian discoveries

Hungarian scientists studying exoplanets primarily use data from the Kepler, K2, and TESS space telescopes. Through grants, they also gain access to data from the James Webb Space Telescope and the European Southern Observatory (ESO) telescopes in Chile. Members of the Hungarian-American Hungarian Automated Telescope Network (HATNet) research group have discovered several exoplanets using the HATNet telescope network at Princeton University in the United States. The research is led by Gáspár Bakos, a former researcher at ELTE and KTM CSI, and several current members of Konkoly are actively involved in this network. The exoplanets discovered in this way are designated with the prefix HAT-P.

Figure 1
Distances between the planets in the Trappist-1 system and their orbital frequencies