How many Earth-like planets exist? Scientists calculate probabilities

A study published in the Astronomical Journal focused on calculating the probability of the existence of Earth-like planets not only with regard to the size and type of planet but also as regards its distance from the star.

Knowing more precisely the level of this frequency could help to make future observations more profitable, in particular those that will be carried out with future space telescopes, and in general for the search for planets in the habitable zone and therefore also for extraterrestrial life.

The research group, led by Eric B. Ford, professor of astrophysics and astronomy at Pennsylvania State University, used the substantial data set collected over the years by the Kepler space telescope. The main difficulty that the researchers had to overcome, however, lay in the method that Kepler himself used over the years to identify the planets, ie the transit method.

This method makes it easier to find the larger planets closer to the star, but the data is not very useful for finding the smallest and most distant planets. This is precisely why researchers have designed a new study method that simulates universes of stars and planets and then “observes” these simulated universes to determine the number of planets that Kepler would have discovered in each simulated universe.

Danley Hsu, the first author of the study and a student at Penn State, also explains this: “We used the final catalog of the planets identified by Kepler and improved the stellar properties of the European Space Agency’s Gaia spacecraft to build our simulations. Comparing the results with the planets cataloged by Kepler, we have characterized the rate of planets per star and how this depends on the size of the planet and the orbital distance. Our new approach allowed the team to explain several effects that had not been included in previous studies.”

Based on the data, they were able to perform a statistical analysis to estimate the rate of terrestrial-sized exoplanets in the habitable zone around Sun-like stars. The results they obtained seem to show that planets very close to the Earth, from three quarters to one and a half times the size of the Earth, with orbital periods ranging from 237 to 500 days exist around one star every six.

“Knowing how often we should expect to find planets of a certain size and a given orbital period is extremely useful for optimizing exoplanet surveys and designing upcoming space missions to maximize their chances of success,” says Ford.