Over the past 18 years, astronomers have discovered 1038 planets orbiting distant stars. Disappointingly, though, the vast majority don’t seem like candidates to support life as we know it—they’re either so close to their home star that all water would likely evaporate, or so far away that all of it would freeze, or they’re made up of gas instead of rock and more closely resemble our solar system’s gas giants than Earth.
Or so we thought. Today, a group of scientists from UC Berkeley and the University of Hawaii published a calculation suggesting that we’ve overlooked evidence of a vast number of Earth-sized exoplanets in the habitable zone of their stars, simply because these planets are harder to detect with current methods. They believe that, on average, 22% of Sun-like stars (that is, stars with a size and temperature similar to the Sun) harbor a planet that’s roughly Earth-sized in their habitable zones.
“With about 100 billion stars in our Milky Way galaxy, that’s about 20 billion such planets,” said Andrew Howard, one of the study’s co-authors, in a press conference on the findings. “That’s a few Earth-sized planets for every human being on the planet Earth.”
The team, led by Erik Petigura, came to these conclusions by taking an unconventional approach to planet-finding. Instead of counting how many exoplanets we’ve found, they sought to determine how many planets we’re unable to see.
Exoplanets are detected as a result of rhythmic dimming in a star’s brightness, which indicates that there’s a planet orbiting it and passing between the star and our vantage point. Because of this method, large planets that orbit closely to their stars have been the easiest to find—they block more light, more often—and thus disproportionately dominate the list of known exoplanets.
To estimate the number of exoplanets this technique misses, the Berkeley team wrote a software program that analyzed data from the Kepler mission, an exoplanet-hunting NASA telescope launched into orbit in 2009. Initially, to confirm the program’s accuracy, they fed it the same data from 42,557 Sun-like stars that had already been scrutinized by other astronomers, and it indeed detected 603 candidate planets, all of which had already been found.
When it parsed the data further to find Earth-like planets—using the length of time between dimmings to indicate how far out the planet orbits the star, and the degree of dimming to indicate us how much of the star is blocked by the planet, and thus the exoplanet’s size—it found 10 potential exoplanets that are between one and two times the size of Earth and orbit in what is likely the star’s habitable zone. This, too, aligned with previous findings, showing the program could accurately detect planets.
But what the researchers really wanted to do was determine the overall prevalence of Earth-like exoplanets. To calculate this number, they first had to determine just how many weren’t detected in the survey. “One way of thinking of it is that we’re doing a census of habitable exoplanets, but not everyone’s answering the door,” Petigura explained.
There are a few reasons that a planet might not be detected. If its orbit doesn’t take it into a location that would block the path of light between its star and our telescopes, we’d have no way of seeing it. Alternately, it could successfully block starlight, but the event could be lost amid natural variation in the brightness of the star as we perceive it on Earth.
Both of these possibilities, it turns out, make it disproportionately hard to find Earth-like exoplanets. “Planets are easier to detect if they’re bigger, and closer to their host stars,” Howard said. “Thus it’s no accident that hot Jupiters were the first planets to be discovered.” Simply by virtue of physics, smaller, Earth-sized planets that may orbit a bit farther out are less likely to pass directly in front of their stars, from our perspective.
To finding out how many Earth-like planets we likely miss as a result, the scientists altered the Kepler data by artificially introducing 40,000 more exoplanets similar to Earth—roughly one per star—then feeding the resulting data back into the planet detection software. This time, it only found about one percent of the Earth-like planets introduced, because the vast majority didn’t cause a detectable dimming of their star.
This means that, with current detection methods, 99 out of 100 Earth-like aren’t coming to the door when to answer our interstellar census. Accounting for this level of imperfection, the researchers calculated that far more Sun-like stars are home to a potentially habitable, Earth-sized exoplanet than we previously thought.
It’s important to note that this is a theoretical calculation: The scientists didn’t actually discover these sorts of planets orbiting 22% of the stars. But if the underlying assumptions are accurate, it does give hope to the possibility that we’ll find more potentially habitable planets in the future. In fact, the researchers calculated that if the prevalence of these sorts of planets is uniform across the galaxy, odds are that one can be found tantalizingly nearby—about 12 light years away from Earth.
It’s still unknown whether these planets might have the other ingredients that we believe are likely necessary for life: a protective atmosphere, the presence of water and a rocky surface. But the researchers say another recent finding makes them hopeful that some of them have potential. Earlier this week, scientists found a rocky, Earth-sized exoplanet roughly 700 light-years away. Although that planet is certainly too hot to harbor life, it has density similar to that of Earth—suggesting that at least some of the Earth-sized planets we’ve failed to detect so far have a geologic composition similar to our own planet’s.