| Authors : | Mayank Narang (TIFR), Manoj P (TIFR), H.C. Bhatt (IIA retd), Motohide Tamura (The University of Tokyo), Thomas Henning (MPIA), E. Furlan (Caltech), T. Sivarani (IIA), Ravinder K. Banyal (IIA), Blesson Mathew (Christ Univ) |
| Abstract : | When did the first planets form in the galaxy? The first few generations of stars were too metal-poor to form any planets. As the metallicity of the ISM and the stars progressively increases, small, rocky Earth-like planets are formed first. However, for the formation of Jupiter-like planets, a 10-15 Earth-mass core needs to form first. Therefore, Jupiter-like gas giants are likely to have formed much later when the Galactic metallicity have increased significantly. How do we investigate this scenario? It's challenging to measure the ages of main-sequence stars, and conventional methods such as isochrone fitting cannot be employed to measure its age accurately. However, using the dispersion in the peculiar velocities of stars, it is possible to measure the average ages of the stars. We used the velocity dispersion of exoplanet host stars as a proxy for their ages to study the evolution of planetary systems on the galactic scale. We find that as the planet mass increases, the average age of the host stars decreases, indicating that the Jupiter-like planets are preferentially found around younger stars. We also find that as the host star metallicity increases, the age of the host stars also decreases, indicating that the metallicity enrichment of the ISM is strongly correlated with the formation of Jupiters in the Galaxy. We further probe this by investigating the occurrence rate of planets in the Kepler field as a function of their host star metallicity. We find that Jupiter-like planets are most common around metal-rich systems, confirming similar results obtained using different samples and methods. Based on these findings, we argue that there is a critical threshold in metallicity that is required to form a Jupiter-like planet, and that threshold of [Fe/H] > -0.2 was only reached in the last 5-7 Gyrs in the ISM. |
