Ever since astronomers confirmed the presence of planets beyond our solar system, the so-called exoplanets, humanity has wondered how many of them could harbor life. We are now one step closer to finding an answer. According to new research using data from NASA’s now-retired planet-hunting mission, the Kepler Space Telescope, about half of stars similar in temperature to our sun could have a rocky planet capable of supporting liquid water in its surface.
Our galaxy contains at least 300 million of these potentially habitable worlds , according to the most conservative interpretation of the study results, published Nov. 3 in The Astronomical Journal . Some of these exoplanets could even be our interstellar neighbors, with at least four potentially within 30 light-years of our Sun and the closest is probably at most about 20 light-years from us.
These are the minimum numbers for such planets based on the most ‘pessimistic’ estimate that 7% of Sun-like stars host such worlds. However, based on the expected average rate of 50%, there could be many more.
This research helps us understand the potential of these planets to have the elements to support life. This is an essential part of astrobiology, the study of the origins and future of life in our universe.
The study is signed by NASA scientists who worked on the Kepler mission together with collaborators from around the world. NASA retired the space telescope in 2018 after it ran out of fuel. Nine years of telescope observations revealed that there are billions of planets in our galaxy, more planets than stars.
According to Steve Bryson, lead author and researcher at NASA’s Ames Research Center in California’s Silicon Valley, “Kepler already gave us the data that there were billions of planets, but now we know that a good part of those planets could be rocky and habitable. Although this result is far from a final value, and the water on the surface of a planet is only one of many factors that support life, it is extremely exciting that we have calculated that these worlds are so common with such confidence and precision. ” .
For the purposes of calculating this occurrence rate, the team observed exoplanets between a radius of 0.5 and 1.5 times that of Earth, focusing on planets that are likely to be rocky. It also focused on stars similar to our Sun in age and temperature.
The result is a wide range of different stars, each with its own particular properties that influence whether the rocky planets in its orbit are capable of supporting liquid water. These complexities partly explain why it is so difficult to calculate how many potentially habitable planets there are, especially when even our most powerful telescopes can barely detect these tiny planets. That is why the researchers needed a new approach.
Previous estimates of the frequency of such planets, also known as the occurrence rate, ignored the relationship between the star’s temperature and the types of light emitted by the star and absorbed by the planet.
The new analysis accounts for these relationships and provides a more complete understanding of whether a given planet might be capable of supporting liquid water and potentially life. That approach is made possible by combining Kepler’s final planetary signal dataset with data on the energy output of each star from an extensive trove of data from the European Space Agency’s Gaia mission.
According to Ravi Kopparapu, a co-author of the paper and a NASA scientist: “We always knew how to define habitability simply in terms of the physical distance from a planet to a star, so that it is not too hot or too cold, which left us making many assumptions. Gaia’s star data allowed us to look at these planets and their stars in a whole new way. “
Gaia provided information on the amount of energy that falls on a planet from its host star based on the flow of a star, or the total amount of energy that is emitted in a given area during a given time. This allowed the researchers to approach their analysis in a way that recognized the diversity of stars and solar systems in our galaxy.
Although the exact effect is still being investigated, a planet’s atmosphere calculates how much light is needed to allow liquid water to also enter a planet’s surface. Using a conservative estimate of the effect of the atmosphere, the researchers estimated an occurrence rate of around 50% – that is, about half of Sun-like stars have rocky planets capable of harboring liquid water on their surfaces. An alternative optimistic definition of the habitable zone estimates around 75%.