November 25, 2020

Half the Milky Way’s sun-like stars could be home to Earth-like planets

Nearly 4,300 exoplanets have been discovered by astronomers, and it’s quite obvious now our galaxy...

Nearly 4,300 exoplanets have been discovered by astronomers, and it’s quite obvious now our galaxy is filled with them. But the point of looking for these new worlds is more than just an exercise in stamp collecting—it’s to find one that could be home to life, be it future humans who have found a way to travel those distances or extraterrestrial life that’s made a home for itself already. The best opportunity to find something like that is to find a planet that resembles Earth.

And what better way to look for Earth 2.0 than to search around stars similar to the sun? A new analysis of exoplanet data collected by NASA’s Kepler space telescope, which operated from 2009 to 2018, has come up with some new predictions for how many stars in the Milky Way galaxy that are comparable to the sun in temperature and age are likely to be orbited by a rocky, potentially habitable planet like Earth. When applied to current estimates of 4.1 billion sun-like stars in the galaxy, their model suggests there are at minimum 300 million with at least one habitable planet. 

The model’s average, however, posits that one in two sun-like stars could have a habitable planet, causing that figure to swell to over 2 billion. Even less conservative predictions suggest it could be over 3.6 billion.

The new study has not yet been peer-reviewed, but it will be soon, and it is due to be published in the Astronomical Journal.

“This appears to be a very careful study and deals with really thorny issues about extrapolating from the Kepler catalogue,” says Adam Frank, a physicist and astronomer at the University of Rochester, who was not involved with the study. “The goal is to get a complete, reliable, and accurate estimate for the average number of potentially habitable planets around stars. They seem to have made a good run at that.”

Scientists have made several attempts in the past to use Kepler data to work out how many sun-like stars in the galaxy have potentially habitable exoplanets in their orbit. But these studies have provided answers that ranged from less than 1% to more than 100% (i.e., multiple planets around these stars). It’s a reflection of how hard it’s been to work with this data, says Steve Bryson of NASA Ames Research Center in California, who led the new work.

Two major issues have created this large window: incomplete data, and the need to cull false detections from the Kepler data set.

The new study addresses both of these problems. It’s the first of its kind to use the full Kepler exoplanet data set (more than 4,000 detections from 150,000 stars), but it’s also using stellar data from Gaia, the European Space Agency’s mission to map every star in the Milky Way. All that helped make the final estimates more accurate, with smaller uncertainties. And this is after scientists have spent years analyzing the Kepler catalogue to strip away obscuring elements and ensure that only real exoplanets are left. Armed with both Kepler and Gaia data, Bryson and his team were able to determine the rate of formation for sun-like stars in the galaxy, the number of stars likely to have rocky planets (with radiuses 0.5 to 1.5 times Earth’s), and the likelihood those planets would be habitable.

On average, Bryson and his team predict, 37 to 60% of sun-like stars in the Milky Way should be home to at least one potentially habitable planet. Optimistically, the figure could be as high as 88%. The conservative calculations pull this figure down to 7% of sun-like stars in the galaxy (hence 300 million)—and on the basis of that number, the team predicts 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 compute exactly this number,” says Bryson. “We have always intended to do this.” 

Habitability has to do with the chances a planet has temperatures moderate enough for liquid water to exist on the surface (since water is essential for life as we know it). Most studies figure this out by gauging the distance of an exoplanet from its host star and whether its orbit is not too close and not too far—the so-called Goldilocks zone.

According to Bryson, orbital distance is a useful metric when you’re examining one specific star. But when you’re looking at many stars, they’ll all exhibit different brightnesses that deliver different amounts of heat to surrounding objects, which means their habitable zones will vary. The team instead chose to think about habitability in terms of the volume of light hitting the surface of an exoplanet, which the paper calls the “instellation flux.” 

Through stellar brightness data, “we are measuring the true temperature of the planet—whether or not it is truly in the habitable zone—for all the planets around all the stars in our sample,” says Bryson. You don’t get the same sort of reliable temperature figures working with distances, he says. 

Though Bryson claims this study’s uncertainties are smaller than those in previous efforts, they are still quite large. This is mainly because the team is working with such a small sample of discovered rocky exoplanets. Kepler has identified over 2,800 exoplanets, only some of which orbit sun-like stars. It’s 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,” says Bryson.

Lastly, the new study assumes a simple model for these exoplanets that could depart dramatically from conditions in the real world (some of these stars may form  binary star systems with other stars, for example). Plugging more variables into the model would help paint a more accurate picture, but that requires more precise data that we don’t really have yet. 

But it’s studies like these that could help us acquire that data. The whole point of Kepler was to help scientists figure out what kinds of interstellar objects they ought to devote more resources to studying to find extraterrestrial life, especially with space-based telescopes whose observation time is limited. These are the instruments (such as NASA’s James Webb Space Telescope and the ESA’s PLATO telescope) that could determine whether a potentially habitable exoplanet has an atmosphere or is home to any potential biosignatures, and studies like this latest one can help engineers design telescopes more 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, we are refining our knowledge of that answer. This paper tells us there are a lot of planets out there in the right place for life to form.”

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