An article published in the journal “Astronomy & Astrophysics” reports a study of the exoplanet TOI-1807 b, a rocky planet discovered in 2020 thanks to NASA’s TESS space telescope. A team of researchers led by the Italian National Institute of Astrophysics and the University of Padua used the HARPS-N spectrograph installed on the Galileo National Telescope in the Canary Islands to conduct follow-up examinations of TOI-1807 b. The conclusion is that it’s a slightly larger exoplanet than Earth but the main peculiarity is that it’s the youngest of the type with an ultra-short period orbit, as it’s estimated to be around 300 million years old and its year lasts about 13 hours.
Over the past few years, among the thousands of exoplanets discovered there are several of the ultra-short period type, which have a year that lasts less than one Earth day. However, so far, these exoplanets were all over one billion years old while the star TOI-1807, also known as BD+39 2643, is far younger. For this reason, the discovery of an exoplanet that orbits it in less than one Earth day, which took place in 2020 thanks to the TESS space telescope, was particularly interesting from the beginning.
A star like TOI-1807, a bit smaller than the Sun, is very active when young, making it much more difficult to examine with the HARPS-N spectrograph. For this reason, the researchers developed a specific strategy to study young star systems. In particular, they used very advanced techniques to separate the signals generated by stellar activity from those generated by the exoplanet, which are much weaker.
The results of the analysis of 170 spectral emissions detected from the star TOI-1807 over two years made it possible to apply the radial velocity method. It made it possible to accurately estimate the mass of the exoplanet TOI-1807 b, a fact that is added to the measurement of its size, obtained thanks to its transits in front of the star from the Earth’s point of view.
The conclusions are that the exoplanet TOI-1807 b has a radius that is about 1.5 times the Earth’s and a mass that is about 2.5 times the Earth’s. These pieces of data made it possible to calculate the planet’s density and to estimate its composition, which should include a core composed of 25% iron.
Instead, no atmosphere was detected, so more than a super-Earth, TOI-1807 b looks like a super-Mercury. Its proximity to its star is almost certainly the cause of the lack of atmosphere, as the process called photoevaporation can strip the hydrogen and helium atmosphere typical of young planets from a planet so close. Many such planets are likely to form as mini-Neptune, with a very thick and extended atmosphere, only to lose it and turn into super-Earths.
The mechanisms leading to the existence of ultra-short period planets are not yet clear. The migration of planets is a subject under development thanks to the discovery of many systems with multiple planets that can influence each other but at least for now, we only know one that orbits the star TOI-1807. The technique developed to examine TOI-1807 b will help to study more exoplanets close to active stars to better understand the processes of planetary formation and evolution. The conditions on TOI-1807 are totally unsuitable for the development of Earth-like life forms but advances in research and study of exoplanets will help to better understand where life forms can emerge as well.
