Activity of the Star TOI-784 and Loss of Atmospheric Mass by Its Planet

Abstract

The properties of the planetary system TOI-784, in which an earth-like planet orbits a low-activity star of solar type, are examined. Archived V-band measurements of the brightness of TOI-784 in the ASAS 4 data base do not yield reliable indications about the variability of the brightness owing to rotation or about the cyclical long-term variability of this object. A possible estimate of the magnitude of the rotation period of the star is Prot/sini = 41.7 ± 11.4 days. The age of the system according to the empirical gyrochronological relations equals 7.8±3.4 billion years. The average value of the atmospheric activity parameter logR′_HK = – 4.99 which corresponds to the level of low-activity stars with analogous (B-V) color index is found and is close to that of the sun. The radius of the planet TOI-784 b is equal to 1.93 R_⊕, its mass is 9.67 M_⊕, and its average density is 7.4 g/cm³ (the planet is rocky). An estimate of the loss of matter from the atmosphere of the planet TOI-784 b was obtained from an approximate formula corresponding to a model of loss by the atmosphere with a limit on energy. Calculations have shown that the rate of loss of matter by the atmosphere of the planet is 4.7 ⋅ 10⁸ g/s. The relatively low rate of loss of matter by the planet’s atmosphere, despite a high effective temperature of the star’s atmosphere and the closeness of the planet to the star, is probably related to a low level of activity of the star.