Stephen Kane has been researching planets around other stars for more than 20 years and has discovered and characterized hundreds of exoplanets, including Kepler-186f, which is the smallest planet yet to have been found in the Habitable Zone of a star. His work covers a broad range of exoplanet detection methods, and he is an expert on the topic of planetary habitability and the habitable zone of planetary systems.
Second Earth Candidates
Over the past 20 years, the number of known planets outside of our solar system has risen dramatically. Our technological capabilities have caught up to our ideas, allowing us to discover not only more planets, but smaller planets that are rocky like the Earth. One of the greatest contributors to this vast increase in planet detections are the results from the Kepler mission. Kepler is the first NASA mission dedicated entirely to the discovery of planets orbiting other stars.
This figure shows the habitable zone for stars of different temperatures, as well as the location of terrestrial size planetary candidates and confirmed Kepler planets described in new research from SF State astronomer Stephen Kane. Some of the Solar System terrestrial planets are also shown for comparison. Credit: Chester Harman
The results from Kepler have shown us that rocky planets are amongst the most common type of planet in the universe. The “Holy Grail” of planet hunting however is the discovery of Earth-size planets which are just the right distance from their star such that habitable conditions might exist at the surface. We call this region around stars the Habitable Zone, and it provides us with a way to focus our efforts on the planets that have the best chances for harboring life.
The newly published results contain catalogs of different classes of planets that lie within the Habitable Zones of their host stars. Our most conservative list contains 20 planets that are the most similar to the Earth in terms of size and the amount of radiation received from their stars. Undoubtedly, there is a large diversity amongst these planets in other respects, such as the composition of their atmospheres and the amount of water at the surface. For example, the atmosphere and surface conditions of the Earth have changed drastically since its formation four and a half billion years ago, so the results of observing a direct Earth analog would depend on when in the planetary history we were looking. The lists in the new study thus provide the starting point for not just finding a new Earth, but learning what the diversity of “Earth-like” planets really is.