The orbital flatness of planetary systems

Artist's concept of the seven planets that orbit the star in the TRAPPIST-1 solar system. Astronomers used the system's extreme flatness to constrain the evolution and properties of the protoplanetary disc. Credit: NASA/JPL-Caltech/R. Hurt, IPAC
All the planets in the solar system orbit the Sun more or less in a plane. The Earth's orbit defines the plane at zero degrees. Mercury's orbit has the greatest angle, with a 7 degree inclination. Pluto's orbit is 17. 2 degrees). As the protoplanetary disk of dust and gas dissipates, the orbital characteristics of planets change. Young planets also migrate within the disk due to the mutual gravitational effects and effects of material in it. Astronomers know that the orbital appearances of planetary systems reflect their evolutionary history.

TRAPPIST-1 is a planetary system that contains seven planets of Earth size orbiting a small star. It lies approximately forty light-years from Sun. The star has a mass of.09 solar masses. The TRAPPIST telescopes first detected the system. Follow-up observations using the Spitzer IRAC camera and K2 mission have also confirmed the planet masses and other properties. The system's orbital tilt is just 0.072 degrees, which is remarkable. This extreme flatness could be a significant constraint in the formation and evolution the system. The system is compact, with the nearest planet orbiting at.06 astronomical unit from the star. Mercury orbits five times further away in our solar system. The planets' mutual gravitational attraction will have a significant influence on details such as the orbital inclinations.

CfA Astronomers Matthew Heising and Dimitar Sassolov, Lars Hernquist and Ana Luisa Ti Humphrey used 3D computer simulations of the gaseous disc and planets to explore a variety of formation models, including some that were not suggested in previous studies. The scientists were particularly interested in determining what the minimum disk mass for TRAPPIST-1 could have been, as they know that the gaseous protostellar disc influences the migration properties. They adapted the computer code AREPO which was used in the past primarily to perform cosmological simulations.

According to astronomers, the seven planets formed in a sequential fashion. Each one was created at a distance from the star, where water can freeze. Then it moved inwards, slowly accreting as it goes. It stops when the orbits of other planets are influenced. A modest disk mass of about.04 solar masses is needed. The models address the distribution of material within each disk and the astronomers can exclude disks with more than fifteen times that value. This new work shows how simulations can be used to infer amazing details about how planetary systems formed and evolved.

Further exploration of the First ALMA animation featuring circling twin young stars

Matthew Z. Heising and colleagues, How Flat can a Planetary Systems Get? I. The Case of TRAPPIST-1. The Astrophysical Journal (2021). Matthew Z. Heising and co, How flat can a planetary system get? I. TRAPPIST-1: The Case, (2021). DOI: 10.3847/1538-4357/abf8a8