There are many unanswered questions about our place in the Universe. Why are we here? What is the likelihood of our existence? Could there be others like us out there in the galaxy?
One of the numbers that could help us answer these questions is this: How many rocky planets like Earth are orbiting stars like the Sun at a temperate distance amenable to life as we know it? Now, we have an answer, based on data from the retired Kepler space telescope.
It's around 300 million.
That's not necessarily an exact number, but it does give us a rough baseline to work from as we search for potentially life-supporting worlds in our galaxy.
"Kepler already told us there were billions of planets, but now we know a good chunk of those planets might be rocky and habitable," said astronomer Steve Bryson of NASA's Ames Research Center.
"Though this result is far from a final value, and water on a planet's surface is only one of many factors to support life, it's extremely exciting that we calculated these worlds are this common with such high confidence and precision."
When trying to narrow down which exoplanets might harbour life, we look for what we know. And the only planet we know for sure that supports life is our own – Earth. There could be any number of detailed factors that play into our presence here, such as the presence of a moon, or a massive gas giant like Jupiter; but as a starting point, astronomers tend to use just the following three.
Is the exoplanet rocky, like Earth, Mars, and Venus? Does it orbit a star like the Sun, not too hot and not too active with flares that could lash planets with radiation? And does it orbit that star in the not-too-cold-not-too-hot Goldilocks zone, neither so far that any liquid water on the surface would freeze, nor so close that any surface water would evaporate?
One of Kepler's main goals was to help us determine how many exoplanets that fit these three parameters might be out there in the Milky Way galaxy. Bryson and his team used all four years of the original Kepler mission data, from May 2009 to May 2013, to make the best estimate yet of this number.
In that first mission (K2 was the second, extended mission, not included in the team's calculations), Kepler identified 4,034 candidate exoplanets, of which more than 2,300 were later validated. But the space telescope had a harder time spotting smaller, rocky planets than anticipated.
The stars the telescope studied ended up being far more variable in brightness than the Sun, which meant that smaller exoplanetary transits that dim the star's light – the signature Kepler used to identify exoplanet candidates – likely ended up indistinguishable from the stellar variability in many cases, thus missing real planets and also generating false positives. Software called Robovetter corrected for these problems for objects with orbits of less than 500 days, but, the team noted, many habitable exoplanets could have much longer orbits.
So, they derived a method of determining a star's Goldilocks zone based on planet radius and photon flux – the number of photons per second per area unit from the star (derived from data from the Gaia survey) hitting the surface of the hypothetical exoplanet.
"We always knew defining habitability simply in terms of a planet's physical distance from a star, so that it's not too hot or cold, left us making a lot of assumptions," said planetary scientist Ravi Kopparapu of NASA's Goddard Space Flight Center.
"Gaia's data on stars allowed us to look at these planets and their stars in an entirely new way."
The researchers confined their search to exoplanets between 0.5 and 1.5 times the mass of Earth and stars between 4,800 and 6,300 Kelvin (4,530 to 6,025 degrees Celsius; 8,180 and 10,880 degrees Fahrenheit) in effective temperature (the Sun has an effective temperature of 5,780 Kelvin).
The team found that around half these stars, based on their calculations, should have rocky, Goldilocks-zone exoplanets. That's roughly 300 million stars in the Milky Way, based on our current counts.
Given the restrictions on the stars, that's not the whole picture. Previous estimates of the number of potentially habitable worlds based on Kepler data have come back with much higher numbers. But astrobiologists believe that the closer a system's characteristics are to Earth's and the Sun's, the better the chances of finding a place where life could thrive.
So really drilling down in granular detail is important for ongoing and future searches for potentially habitable worlds.
"Knowing how common different kinds of planets are is extremely valuable for the design of upcoming exoplanet-finding missions," said astronomer Michelle Kunimoto of the Massachusetts Institute of Technology.
"Surveys aimed at small, potentially habitable planets around Sun-like stars will depend on results like these to maximise their chance of success."
The research will be published in The Astronomical Journal, and is available on arXiv.