In a recent paper by Serrano et al. in Nature Astronomy, researchers including Associate Professor Simon Albrecht from the Department of Physics and Astronomy at Aarhus University announced the discovery of a set of four planets orbiting a star outside the solar system. One of the planets is similar to the size of the Earth and has an orbital period of just 13 hours! The paper also presents a first-of-its-kind study on how the planet’s orbit around the star came into being.

In recent years, we have launched space missions to find planets outside the solar system that orbit other stars, known as exoplanets. One such mission is NASA’s Transiting Exoplanet Survey Satellite (TESS), collecting data to search for exoplanets. The TESS satellite was launched in April 2018 and has already discovered over 700 exoplanets. An interesting subgroup of exoplanets, called Ultra-Short-Period (USP) planets have been found that orbit stars in an extremely short time with periods shorter than 1 day! Orbital and fundamental parameters of USP planets have still not been widely studied, and thus makes it is even more exciting to explore them!

A groundbreaking study in the field of USP planets is the study by Serrano et al. (2022). They discovered a system of four planets orbiting a star called TOI-500 from the TESS mission. What makes this discovery even more interesting is that one of the planets TOI-500b is very similar to the Earth and has an interesting story of how its orbit around the star came into being. This planet orbits its star once every 13 hours, making it a perfect example of a USP planet. That’s almost orbiting the star in half a day unlike the Earth taking 365 days. USP planets are known to be born very far from the star with a highly elliptical orbit but come closer eventually via a violent migration process. However, the research by Serrano et al., says that TOI-500b is unique because it seems to have reached its position differently than what is typically expected. In fact, it may not have suffered any violent collisions, and instead of forming in chaotic orbits, as typically expected, it was born directly in a circular and stable orbit. From that nearly circular and stable orbit, the planet continued to migrate slowly from a distant orbit to a closer one around the star maintaining its circular nature. This process took nearly 2 billion years, known as 2 Gyrs (Gyr stands for gigayears)! Serrano et al. discovered these interesting properties via joint analysis of data from the TESS and High Accuracy Radial Velocity Planet Searcher (HARPS) missions. TOI-500 is the first four-planet system known to host a USP Earth-analogue whose current architecture can be explained via a non-violent migration scenario.

P.S. If you’re wondering about the strange-sounding name of the star and the planet, it’s called TOI 500 because it stands for TESS Object of Interest and is the 500th star to be catalogued in the TESS mission! The lowercase letter "b" stands for the planet, in the order in which the planet was found. In the nomenclature of exoplanets, the first planet found is typically named b, with emerging planets called c, d, e, f and so on.

Read the paper.

Other references:

1.https://astronomycommunity.nature.com/posts/toi-500b-a-usp-planet-in-a-4-planet-system-with-a-non-violent-migration-process

2. https://exoplanets.nasa.gov/faq/20/how-do-exoplanets-get-their-names