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The new study has not yet been peer-reviewed, but it will soon be and is expected to be published in the Astronomical Journal.
“This appears to be a very careful study and addresses really thorny questions of extrapolation from Kepler’s catalog,” says Adam Frank, a physicist and astronomer at the University of Rochester who was not involved in the study. “The goal is to obtain a complete, reliable and accurate estimate of the average number of potentially habitable planets around the stars. They seem to have had a good run in this one. “
Scientists have made several attempts in the past to use Kepler’s data to calculate how many sun-like stars in the galaxy have potentially habitable exoplanets in their orbit. But these studies provided answers ranging from less than 1% to more than 100% (i.e. more planets around these stars). It’s a reflection of how difficult it has been to work with this data, says Steve Bryson of NASA Ames Research Center in California, who led the new work.
Two main problems created this large window: incomplete data and the need to eliminate false detections from the Kepler dataset.
The new study addresses both of these problems. It is the first of its kind to use the full set of Kepler exoplanet data (over 4,000 detections from 150,000 stars), but it also uses star data from Gaia, the European Space Agency’s mission to map every star in the Milky Way. All of this contributed to making the final estimates more accurate, with fewer uncertainties. And that’s after scientists have spent years analyzing Kepler’s catalog to eliminate obscuring elements and make sure only real exoplanets remain. Armed with both Kepler and Gaia data, Bryson and his team were able to determine the rate of sun-like star formation in the galaxy, the number of stars that would likely have rocky planets (with radii from 0.5 to 1.5 times those of Earth) and the probability that those planets would be habitable.
On average, Bryson and his team predict that 37 to 60 percent of the sun-like stars in the Milky Way should host at least one potentially habitable planet. Optimistically, the figure could reach 88%. Conservative calculations put this figure at 7% of sun-like stars in the galaxy (thus 300 million) and based on that number, the team predicts that there are four sun-like stars with habitable planets within 30 light-years of Earth.
“One of the original goals of the Kepler mission was to calculate exactly this number,” says Bryson. “We always meant to.”
Habitability has to do with the chances of a planet having temperatures moderate enough to allow liquid water to be present on the surface (as water is essential for life as we know it). Most studies find this out by measuring the distance of an exoplanet from its host star and if its orbit is neither too close nor too far, the so-called Goldilocks zone.
According to Bryson, orbital distance is a useful metric when you’re looking at a specific star. But when you look at a lot of stars, they will all exhibit different brightnesses that provide different amounts of heat to surrounding objects, meaning their habitable zones will vary. Instead, the team chose to think of habitability in terms of the volume of light hitting the surface of an exoplanet, which the paper calls ‘instellation flux’.
Through stellar luminosity data, “we are measuring the true temperature of the planet, regardless of whether it is in the habitable zone or not, for all planets around all stars in our sample,” says Bryson. You don’t get the same kind of reliable temperature data by working with distances, he says.
Although Bryson claims that the uncertainties in this study are smaller than in previous efforts, they are still quite large. This is mainly due to the fact that the team is working with such a small sample of discovered rocky exoplanets. Kepler has identified over 2,800 exoplanets, only a few of which orbit sun-like stars. It is not an ideal number to use to predict the existence of hundreds of millions of others in the galaxy. “By having so few observations, it limits what you can say about what the truth is,” Bryson says.
Finally, the new study assumes a simple model for these exoplanets that could deviate significantly from real-world conditions (some of these stars may form binary star systems with other stars, for example). Inserting more variables into the model would help draw a more accurate picture, but this requires more precise data that we don’t have yet.
But it’s studies like these that could help us capture that data. Kepler’s focus was to help scientists understand what types of interstellar objects should devote more resources to studying to find extraterrestrial life, especially with space telescopes whose observation time is limited. These are the instruments (such as NASA’s James Webb Space Telescope and ESA’s PLATO Telescope) that could determine whether a potentially habitable exoplanet has an atmosphere or is home to potential biological signatures, and studies such as the latter can help engineers to design telescopes better suited to these tasks.
“Almost every sun-like star in the galaxy has a planet where life could form,” says Frank. “Humanity has been asking this question for more than 2,500 years and now we not only know the answer, but we are honing our knowledge of that answer. This document tells us that there are many planets out there in the right place for the formation of life. “
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