The first planets discovered outside our solar system were strange and unknown worlds. Giants like Jupiter, or hot like Venus. Astronomers have since discovered, for example, planets where winds of 10.000 miles per hour raise metallic clouds that rain molten iron. A heterogeneity that almost causes dizziness.
What we are really looking for are more Earth-like planets. Are those with the "right" conditions for life as we know it rare in the galaxy? As the planetary census progresses, the evidence points to the exact opposite.
This week, in a paper written by 44 astronomers led by Steve Bryson of NASA and published onAstronomical Journal, scientists have come up with a new estimate of how common Earth-like planets are in the Milky Way.
The operation, called eta-Earth, is an attempt to estimate the number of Earth-sized rocky planets in the habitable zone of sun-like stars. Also known as the “Goldilocks” or rather “Goldielocks” zone, the habitable zone is the area where temperatures are neither too hot nor too cold for the formation of liquid water on planetary surfaces.
It is also where we think life is most likely to arise.
The criteria adopted by the research team
The team drew their estimate from the complete history of exoplanet discoveries made by the Kepler Space Telescope from 2009 to 2018. Kepler examined over 150.000 stars, found more than 4.000 candidate exoplanets, and confirmed over 2.800. Scientists have also spent years maniacally scrutinizing Kepler's data, correcting any minor inconsistencies to increase the certainty that only real, confirmed exoplanets remain.
Data from the European Space Agency's Gaia project, which mapped the position and brightness of one billion stars, was also used to improve habitable zone estimates.
Combining data from Gaia and Kepler, the team defined the habitable zone with the "flow of instellations," which is an elegant way of telling how much sunlight actually hits a planet's surface, rather than orbital distance. This makes a more precise estimate, not simply "statistical" but based on the actual livability of the planet.
Another clarification to make: the scientists' estimate was very cautious. 37 to 60% of sun-like stars would have Earth-like planets in habitable areas. The most optimistic scenario would be a whopping 88%.
And in short? How many Earth-like planets are there "around here"?
Taking the document's more conservative lower bound, 7% of the galaxy's 4 billion sun-like stars could have Earth-like planets in the habitable zone.
Translated, it means at least 300 million Earth-like planets in the Milky Way.
If we had Star Trek's warp engine and a map, we would have one earth-like, potentially habitable planet for every 26 inhabitants. The key word here is "potentially".
It is clear that this estimate will undergo other changes in the years to come: although this study reduces the uncertainties of the past years, there is still a fairly wide range. And, more importantly, they are still planets too far away to be observed in detail (although the eventual launch of the James Webb Space Telescope could change the tables).
Other discoveries could also broaden our reach.
Finding life in the clouds of Venus, the icy methane lakes of Titan or the underground ocean of Europe it could expand what we call habitable. Perhaps liquid water on the surface is not always needed. Perhaps we will add the moons of the outer solar system to the planets.
Whatever happens, the search for life in the galaxy is about to get interesting. This study suggests that the most basic conditions for life may have occurred hundreds of millions of times in our galaxy alone. It is a conservative estimate, yet it is already a huge number. And it is likely that it will grow.