In the last few decades the study of exoplanets has increased by leaps and bounds. 4,296 confirmed discoveries in 3,188 systems and an additional 5,634 candidates awaiting confirmation. Because of this, scientists could get a better idea of the number of potentially habitable planets that could be out there. A popular destination is stars like our own, known as yellow G-type dwarfs.
Recently, an international team of scientists (led by researchers from NASA Ames Research Center) combined data from the now defunct Kepler Space Telescope and the Gaia Observatory of the European Space Agency (ESA). This revealed that half of the sun-like stars in our universe could have rocky, potentially habitable planets, the closest of which could be in our cosmic backyard!
The study, describing its results, "The Occurrence of Planets in Rocky Habitable Zones around Solar-like Stars from Kepler Data," was recently published online and will be published shortly in the Astronomical Journal. The study was carried out by NASA scientists, in which colleagues from universities, institutes, observatories and laboratories around the world participated.
Illustration of Kepler-186f, a recently discovered possibly Earth-like exoplanet that could host life. Photo credit: NASA Ames / SETI Institute / JPL-Caltech / T. Pyle
The Kepler mission, which was in operation from 2009 to 2018 (when the fuel ran out), is responsible for most of the exoplanet discoveries. Based on the number of planets found, scientists now estimate that there could be more planets than stars in the Milky Way (the latest estimates suggest that there are between 100 and 400 billion stars). Steve Bryson, researcher at NASA's Ames Research Center and lead author of the study, stated:
“Kepler already told us that there are billions of planets, but now we know that a good part of these planets could be rocky and habitable. Although this result is far from definitive and water on the surface of a planet is just 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 are. "
To calculate how often there are potentially habitable Earth-like planets in our galaxy, the team focused on stars in Kepler's final dataset that were similar in age (about 4.6 billion years) and our Sun's temperature – plus or minus up to 815 ° C (15,000 ° F). They also studied planets with Earth radii between 0.5 and 1.5, which are most likely rocky.
From this, the team concluded that there could be up to 300 million potentially habitable planets in the Milky Way alone. In addition, their results suggest that four of these exoplanets could be within 30 light years of the solar system – the closest of which could be only 20 light years away from us. Overall, their analysis included a wide variety of stars, each with their own properties that affect habitability.
Artist's impression of the habitable zone around different types of stars. Photo credit: NASA
This is typical of exoplanet and astrobiological research, where several overlapping features are taken into account to determine potential habitability. Combined with the limitations of current telescopes, the resulting complexity of this research is why it is so difficult to calculate the number of potentially habitable planets that could be out there.
For example, previous estimates of the abundance of potentially habitable planets (also known as the rate of occurrence) failed to take into account the relationship between the brightness of a star and the type of light it emits – what is known as "blackbody radiation". Brighter, more massive stars emit radiation in the blue, UV, X-ray, and gamma-ray parts of the spectrum, making them less safe for habitable planets.
Weak, low-mass stars emit most of their radiation in the red, infrared, or microwave and radio wave regions of the spectrum. Stars like our sun emit most of their radiation in the yellow-green part of the spectrum. To take this relationship into account in their study, the research team combined data from the Kepler mission for planetary transits with astrometric data from ESA's Gaia mission.
Ravi Kopparapu, scientist at NASA's Goddard Space Flight Center and co-author of the study, said:
"We have always known that habitability is defined simply by the physical distance a planet is from a star so that it is not too hot or too cold. So we made many assumptions. Gaia's data on stars allowed us to identify these planets and to look at their stars in a whole new way. "
Artist's impression of an exoplanet system containing a planet with surface water. Photo credit: NASA / Goddard Space Flight Center
The Gaia mission has for the past seven years studied the positions, distances, and movements of over 1 billion objects outside of our solar system – including stars, exoplanets, quasars, comets, etc. The addition of this data provided information based on the amount of energy the planet absorbs on the flow of a star (the total amount of energy emitted in a given area over a given time).
The research team also considered the role of exoplanet atmospheres, which affects how much light is needed to allow liquid water to remain on a planet's surface. While the exact nature of this effect is still under investigation, the team used both conservative and optimistic estimates of the extent of the habitable zones and found that 50% to 75% of Sun-like stars have rocky planets that can absorb liquid on their surfaces.
As more exoplanets are discovered and our ability to study them improves, astronomers will be able to tighten the chances of finding planets that could support life. This has led to a burgeoning field of astrobiology specifically concerned with the search for extraterrestrial and extrasolar life. With a little luck, we will soon have proof that we are not alone in the universe.
Further reading: NASA