One of the main questions that have excited human beings throughout history is “Are we alone?”. Are we the only planet with life in this whole universe? Are there planets around other stars than the sun? For a long time, these questions have intrigued us. The main difficulty of detecting planets outside the solar system was that these planets are really faint. Hence, a significant amount of methods and technology development was needed to find planets outside the solar system. The first planet outside the solar system was found in 1995. Since then, the exoplanet hunters have discovered more than 4000 planets outside our solar system. These planets have various interesting features. For example, most of these planets are so close to the host stars that they can orbit around the stars in a few days. On these planets, one year is like only a few days. There are planets orbiting around two stars simultaneously, it is literally land on which the sun never sets. And then, there are planets that are so close to their host stars that they started evaporating. With 4000 extra-solar planets and counting, some of the key questions still remain unanswered. How do planets form? How does life form on these planets? Does habitability depend on how the planets form? To answer the first question, scientists still need to understand the statistics of different types of planets. Even though there are 4000+ planets discovered to date, only about 70 planets are discovered that
To understand the cause of habitability, scientists are approaching the problem from two different directions. In one direction, they are trying to learn more details about the different type of planets and the planetary dynamics. The era of exoplanet study has advanced from detecting the exoplanets to precisely measuring the mass of the planets and their host stars as well as the separation between them. This would give a better understanding of different type of planetary systems around the different type of stars: which stars are more likely to host which kind of planets? On the other hand, as technology improved over the years, it is now possible not only to detect the extra-solar planets but also to characterize the atmosphere of these exoplanets. One branch of the study of exoplanets is devoted to detecting and understanding the presence of different chemical elements in the atmosphere of these exoplanets, especially the signs of water. In the last 25 years, the field of exoplanet study has advanced from detecting the first exoplanet to searching for the signature of life outside the solar system. We live in an exciting time to see how the next 25 years can improve our understanding of how life started.
Aparna Bhattacharya is working in the Gravitational Microlensing Centre at NASA Goddard Space Flight centre since 2016.