On October 29, 2020, research teams such as NASA and the SETI Institute announced that the observation data of the space telescope “Kepler” showed that about half of the stars similar to the sun in the galaxy (Milky Way). He announced the results of his research that there may be rocky planets with liquid water on their surface.

If this estimate is correct, there could be as many as 300 million habitable exoplanets in the galaxy, some of which are relatively close, within 30 light-years of the Sun. There is a possibility.

A paper summarizing the research results was published in the journal “The Astronomical Journal”.

  • Kepler 452b

    An imaginary view of the exoplanet “Kepler 452b” revolving around Kepler-452, a G-type main sequence star located about 1400 light-years away from the Earth in the direction of Cygnus. It is considered to be an exoplanet close to the Earth in the habitable zone of a star similar to the Sun. (C) NASA Ames / JPL-Caltech / T. Pyle

Is there a rocky planet with liquid water in about half of the sun-like stars in the galaxy?

The Kepler is a space telescope launched by NASA in 2009 that aims to discover planets outside the solar system (exoplanets).

Kepler uses the fact that an exoplanet orbiting a star crosses in front of the star as seen from a telescope, and the brightness of the star becomes slightly darker to detect the exoplanet. Observations were made by a method called “law”. This method can also estimate the size, mass, density, etc. of a planet, and by analyzing the light of a star that changes depending on the atmosphere of the planet, the composition of that atmosphere can also be known.

Kepler went out of operation in 2018 after repeating extended missions, but the vast amount of data collected is still being analyzed. More than 2,800 exoplanets have been identified so far, and hundreds of them are in the “habitable zone” where water can stably exist on the surface of planets in a liquid state in the planetary system. It is known to exist in. Thousands of candidates are awaiting confirmation, and it is suggested that there are billions more exoplanets.

And this time, a research team consisting of researchers from NASA, SETI Institute, British Columbia University, etc., has set the range of ± 1500 ° C from the observation data of Kepler, centering on stars of age and temperature similar to the Sun. We investigated exoplanets with a size of 0.5 to 1.5 times the radius of the earth that orbits a certain star, and narrowed down the planets with a lot of rocks (rock planets, terrestrial planets).

As a result, conservative estimates have led to the estimation that about half of the sun-like stars in the galaxy may have rocky planets capable of retaining liquid water on the surface. In this case, there are as many as 300 million habitable exoplanets in the galaxy, four of which could be within 30 light-years of the Sun, the closest being about 20 light-years away. In addition, calculations based on optimistic estimates will increase to about 75%.

Previous studies have attempted to estimate the number of habitable exoplanets in the galaxy, but this study introduces new elements to improve its accuracy.

Previous studies have estimated whether the exoplanet is habitable or not, depending heavily on the distance from the star. However, in this study, in addition to the distance, the temperature of the star, the type of light emitted from the star, and the type of light absorbed by the exoplanet are also considered, and as a result, whether it is in the habitable zone or not is determined. It is said that it was possible to find it more accurately than before.

This is based on Kepler’s data, the flux of stars collected by the European Space Agency’s (ESA) astrometric space telescope “Gaia”, that is, the total amount of energy released in a specific area at a certain time, and the stars. It is said that it was made possible by combining data on the amount of energy that falls on the planet.

Steve Bryson of NASA Ames Research Center, who led the research team, said, “Kepler suggested that there are already billions of exoplanets, but the study shows that a significant number of those planets are rocks. It turns out that the planet may be in a habitable zone where water can exist in a liquid state. “

“Of course, we can’t make a final conclusion (whether or not there is liquid water), and whether or not there is water on the surface of the planet is one of many factors that indicate the existence of life. But it’s very exciting to be able to calculate and show the results with such high reliability and accuracy of the number of exoplanets with habitable potential. “

  • Kepler 452b

    Imaginary view of the space telescope Kepler (C) NASA / Ames Research Center / W. Stenzel / D. Rutter

One step closer to solving the Drake equation?

“This study is needed to provide a reliable estimate of the number of extraterrestrial planets in the habitable zone in the galaxy,” said Jeff Coughlin of the SETI Institute, one of the research teams. This is the first time that all the pieces have been taken into consideration. This is an important term in solving the “Drake equation”. “

The Drake equation is an equation used to estimate the number of civilizations in our galaxy that can communicate with humankind, and was proposed by American astronomer Frank Drake in 1961. Due to the many uncertainties, it is considered a thought experiment in astrobiology rather than a mathematically rigorous equation, but efforts to improve its accuracy continue and SETI (Extraterrestrial Knowledge) It is also a guide for many studies (search for extraterrestrial life).

It’s still a long way to find all the 300 million computationally estimated exoplanets in this study, find out if they’re really habitable, have life, and come up with answers to the Drake equation. It looks like it will take.

However, NASA is currently operating the “TESS (Transiting Exoplanet Survey Satellite),” which is the successor to Kepler. TESS observes a much wider area than Kepler and aims to discover more exoplanets.

In addition, the space telescope “CHEOPS” launched by ESA and the Swiss Space Office in December last year aims to observe extrasolar planets detected and discovered by Kepler and others in more detail.

In addition, NASA and ESA are planning and considering a number of plans aimed at discovering and exploring extraterrestrial planets, and even the world’s largest radio telescope “FAST” built by China in 2016 is extraterrestrial intellectual. Lifeform exploration is underway.

There is no doubt that we are slowly approaching the goal of knowing whether we are alone or not in this galaxy.

  • Kepler 452b

    An imaginary view of TESS launched as a successor to Kepler. Aiming to observe a much wider area than Kepler and discover more exoplanets (C) NASA’s Goddard Space Flight Center—


・[2010.14812] The Occurrence of Rocky Habitable Zone Planets Around Solar-Like Stars from Kepler Data
・ About Half of Sun-Like Stars Could Host Rocky, Habitable-Zone Planets | NASA
・ How Many Habitable Planets are Out There? | SETI Institute
・ Drake Equation | SETI Institute