A Neptune-like exoplanets in a close in orbit.. Published by AJ: “An Accurate Mass Determination for Kepler-1655b, a Moderately Irradiated World with a Significant Volatile Envelope” of R. D. Haywood in collaboration with OAPA
The Solar System is particularly ordered, with the small rocky planets lying close to the Sun and the giant gaseous planets (with a radius larger than 3.9 R⊕) orbiting at larger distances. The study of exoplanets, however, has thought to us that the architecture of exoplanetary systems can be very different than that of our Solar System.
Considering planets between Earth (R=1R⊕) and Neptune (R=3.9R⊕), it is not clear which parameters determine the transition between rocky and gaseous planets. Some studies suggest that planets smaller than 1.6 R⊕ are typically rocky planets, while other studies suggest that the fundamental parameter is the stellar flux irradiating the planet: planets close to the host star are more irradiated and loose their atmosphere quickly.
Unfortunately the determination of the parameters of small planets orbiting in such close orbits is hampered by the stellar magnetic activity. Photospheric spots and faculae, in fact, contaminate the precise spectroscopic measures necessary to determine planets properties by the analysis of radial velocities (the Doppler effects induced by the oscillations of the star around the center of mass of the planet+star system). Nowadays, last generation instruments such as Harps-N (Telescopio Nazionale Galileo) provide the excellent resolution needed to tackle this problem.
In the study: “An Accurate Mass Determination for Kepler-1655b, a Moderately Irradiated World with a Significant Volatile Envelope” of R. D. Haywood (Harvard-Smithsonian Center for Astrophysics), recently published by Astronomical Journal with the collaboration of the astronomers G. Micela and L. Affer of the Astronomical Observatory of Palermo, are analyzed 95 high resolution spectra from Harps-N of the star Kepler-1655 and its planet Kepler-1655b. The authors have characterized the central star (a Sun-like G0V star) and its magnetic activity, being thus able to determine mass and radius of its planet (between 2 and 8 M⊕ and 2.2 R⊕, respectively). Besides, from the estimate of the rotation period (about 12 days) they have calculated the orbit radius and thus the stellar flux irradiating the planet (about 155 more intense than that irradiating the Earth today). Kepler-1655b is thus a Neptune-like planet moderately irradiated by its star, being an important target to determine whether Neptune-like planets in close orbits can loose their atmosphere because of the irradiation from their central star.
The figure (link) shows the distribution radius vs. mass for planets with mass in the 0.1–32 M⊕ range and radius between 0.3–8 R⊕. Credit: Haywood et al. 2018, AJ