Earth-like exoplanet in unique planetary system

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Illustration exoplanet TOI-500 b (NASA/JPL Exoplanet Catalog)
Illustration: exoplanet TOI-500 b (NASA/JPL Exoplanet Catalog)

Astronomers have discovered a unique planetary system around the star TOI 500, 155 light years from Earth. The innermost of the four planets is similar to Earth in several ways, but has an orbiting period of just 13 hours and a temperature of over 1300 degrees Celsius. It is believed to have formed further out, and then migrated close to the star in a slow and "quiet" process, lasting billions of years. Until now, it has never been shown that such a scenario could expain the existence and architecture of such a peculiar planetary system​.

Judith Korth, one of four Chalmers astronomers involved in the study, recently published in Nature Astronomy, explains why this planetary system is of particular interest:

“Its architecture is unique. TOI-500 hosts four low-mass planets where the innermost planet has an orbital period of around 13 hours (TOI-500b). Such ultra-short-period planets (USPs) usually show a particular architecture of high-inclined orbits with respect to the outer planets in the system and are thought to be the outcome of so called high-eccentricity migration, where very elliptical orbits gradually become more and more circular from the star’s tidal forces”, says Judith.

“The planets in the TOI-500 system, however, show orbits on a similar plane, and thus, TOI-500 is the first system that could have formed via a different formation scenario, namely the low-eccentricity migration described in the article”.

Slow and steady migration towards the star​

The scientific community unanimously agrees that a planet like TOI-500b could not have formed in its current position, but that it must have originated in a more external area of ​​the protoplanetary disk, and then migrated much closer to its star. However, there is still a lot of debate on the migration process, but it is common opinion that it usually takes place in a violent way, a process that can involves collisions between planets which set the planets on non-circular and inclined orbits, migrating towards smaller orbits that become increasingly circular.

In the recent article, however, the authors present simulations with which they demonstrate that the planets around TOI-500 may have formed on almost circular orbits further out in the system, and then performed a slow and steady migration during 2 billions years, in which the planets, without colliding with each other, move along orbits that remain almost circular but gradually smaller and smaller.

The research, published in the prestigious journal Nature Astronomy was led by Luisa Maria Serrano and Davide Gandolfi of the Physics Department of the University and featured Chalmers astronomers Judith Korth, Carina Persson, Iskra Georgieva and Malcolm Fridlund.

TOI similar to Earth - and also very different

The planet closest to the star, named TOI-500b, is a so called Ultra-Short Period (USP) planet , as its orbital period is just 13 hours . It is also considered an Earth analogue, that is, a rocky planet similar to the Earth in radius, mass and density. However, its proximity to the star makes it so hot (around 1350 degrees Celsius) that its surface is most likely an immense expanse of lava.

“TOI-500b has a size and mass similar to Earth but in reality, it is very different from Earth due to its short orbital period. It is called an Earth-analog, meaning that it has a similar bulk density as our Earth. This does not mean that the planet is also as habitable as our Earth. It is quite the opposite, due to its vicinity to the star the planet is very hot and its surface consists most likely of a lava ocean”, says Judith Korth and continues.

“However, another similarity to our own Earth could exist for TOI-500b. It could have a secondary atmosphere. I think this will trigger further atmospheric studies in the future which may also give us information about our own atmosphere”.

TOI-500b was initially identified by NASA 's TESS (Transiting Exoplanet Survey Satellite) space telescope which searches for exoplanets using the so called transit method. This method identifies planets that periodically obscure their home star, causing a decrease in the light received on Earth. The planet was subsequently confirmed thanks to an intense observation campaign conducted by European Southern Observatory (ESO). The data cover an entire year and their analysis, combined with that of the TESS data, made it possible to measure the mass, radius, and orbital parameters of the inner planet.

An extraordinary planetary system ​​

“The same measurements also made it possible to discover 3 additional planets, with orbital periods of 6.6, 26.2 and 61.3 days. TOI-500 is an extraordinary planetary system for understanding the dynamic evolution of planets”, says project leader Davide Gandolfi, University of Turin.

Judith Korth, of the Department of Space, Earth and Environment at Chalmers, was involved in the dynamical studies:

“I studied if the system shows transit timing variations that could help us to constrain the planetary and orbital parameters. Unfortunately, this was not the case since the dynamics of the system are dominated by the secular dynamics rather than the resonant dynamics. Furthermore, I studied the long-term stability of the system and tested if we could refine the upper mass limits of the outer planets since we have only the Msini (minimum mass) from the radial velocities. Since the system could have formed via low-eccentricity migration, I also studied the dynamics within a smaller range of mutual inclinations but for a longer time span.”

The article demonstrates the importance of combining the discovery of systems hosting close USP-type planets with numerical simulations to test the possible migratory processes that may have brought them to the current configuration.

“Acquiring data over long periods of time allows us to study the internal architecture of systems similar to TOI-500 and to understand how the planets settled on their orbits”, concludes Davide Gandolfi, University of Turin.

Read the article A low-eccentricity migration pathway for a 13-h-period Earth analoguein a four-planet system in Nature Astronomy.

The text is written by Christian Löwhagen, Chalmers, based on the press release from the University of Turin: Dalla missione della NASA alle osservazioni UniTo: TOI-500, un sistema planetario di quattro pianeti con un processo di migrazione peculiare - Il pianeta più vicino alla stella è molto liknelse alla Terra...