TOI-500, a four planet system with a peculiar migration process

The inner planet is an Earth analogue with an orbital period of only 13 hours. Against all odds, its ultra-short period orbit can be explained via a "non-violent" migration pathway

A recent study published in the prestigious journal Nature Astronomy announces the discovery of a unique planetary system featuring 4 planets in orbit around the star TOI-500. This is the first system known to host an Earth analogue with a period shorter than 1 day and 3 additional low-mass planets whose architecture can be explained via a non-violent and smooth migration scenario. This process has never been demonstrated to work for such a peculiar planetary system comprising 4 planets. The study entitled "A low-eccentricity migration pathway for a 13-h-period Earth analogue in a four-planet system" is led by Luisa Maria Serrano and Davide Gandolfi of the Physics Department of the University of Turin and involves an international team of researchers, including astronomers from several European countries, the USA, Japan, and Chile, as well as Elisa Goffo and Enrico Bellomo also from the University of Turin.

The inner planet, dubbed TOI-500b, is a so-called ultra-short period (USP) planet, as its orbital period is only 13 hours. It is regarded as an Earth analogue, that is, an Earth-like rocky planet with radius, mass, and density comparable to those of our planet. "In contrast to Earth, though, its proximity to the star makes it so hot (about 1350 °C) that its surface is most likely an immense expanse of lava", says Enrico Bellomo.

TOI-500b was initially identified as a planet candidate by NASA’s Transiting Exoplanet Survey Satellite (TESS), a space telescope designed to look for planets in orbit around nearby bright stars using the transit method. This method measures the minute decrease of the brightness of a star as the planet crosses the stellar disk as seen from the telescope. TOI-500b was subsequently confirmed as a planet thanks to a one-year-long observing campaign carried out by the University of Turin with the HARPS spectrograph at the European Southern Observatory (ESO), as part of a large observing program led by Davide Gandolfi. The analysis of the TESS and HARPS data has provided precise measurements of the mass, radius, and orbital parameters of the inner ultra-short period planet TOI-500b. "The HARPS measurements have also allowed us to detect 3 additional low-mass planets orbiting TOI-500 every 6.6, 26.2, and 61.3 days. TOI-500 is a remarkable planetary system to understand the dynamical fate of planets", says Davide Gandolfi.

The novelty presented by the newly published paper lies in the migration process that led the planetary system to its current configuration. "It is commonly accepted that ultra-short period planets did not form in their current-day orbits, as the innermost regions of their natal protoplanetary disk have inadequate density and temperature to form planets. They must have originated further out and then migrated inwards close to their host star", says Elisa Goffo. Although there is no consensus on the migration process, it is often believed to occur via a violent process, involving planet-planet scattering, which would shrink and excite the orbits of the planets.

In their study the authors show that the planets orbiting TOI-500 may have always been on nearly circular orbits, and then migrated inwards following a so-called secular and quasi-static migration process that lasted about 2 billion years. "This is a quiet migration pattern, in which planets move slowly on orbits closer and closer to their star, without bumping into each other and without exiting their orbits", explains Luisa Maria Serrano.

This paper demonstrates the importance of coupling the discovery of systems hosting USP planets with numerical simulations to test possible migratory processes that may have led the planets to their current orbital configuration. "Acquiring data across a long time baseline makes it possible to unveil the inner architecture of systems similar to TOI-500 and understand how planets have settled into their orbits", conclude Luisa Maria Serrano and Davide Gandolfi.

The study: https://www.nature.com/articles/s41550-022-01641-y