A growing body of research indicates that there are likely billion of Earth-like planets that we haven't yet discovered.
That's good news for astronomers seeking alien life. Since Earth is our only example of a life-bearing world, scientists try to pinpoint planets like ours when they search for life elsewhere.
That's what NASA's Kepler space telescope set out to do. Kepler scanned the skies from 2009 to 2018, and it found over 4,000 planets outside our solar system. A dozen or so of these planets seem like prime real estate for life.
Kepler's data has produced a growing body of research that indicates there are likely billions more Earth-like planets that we haven't discovered.
Here's why scientists are starting to think planets like Earth might be common.
When astronomers peer across the cosmos for potential outposts of alien life, they look for planets like Earth.
Nine years' worth of observations by the Hubble Space Telescope revealed about 10,000 galaxies in one of the deepest, darkest patches of night sky in the universe.
That means a rocky planet that's roughly the size of Earth. Scientists haven't exactly defined this size range, since they don't yet know how big rocky planets can be.
To be Earth-like, these planets also have to orbit in the "habitable zone" of their stars — the range of distances where the planet could host liquid water on its surface.
The habitable zone, or "Goldilocks zone," around a star is where a planet is neither too hot nor too cold to support liquid water.
A handful of recent discoveries shows that Earths could be common in the universe.
This artist's concept illustrates the idea that rocky worlds like the inner planets in our solar system may be plentiful, and diverse, in the universe.
That means alien life could be common, too.
Most of what we know about exoplanets comes from the planet-hunting Kepler space telescope.
An illustration of NASA's Kepler space telescope.
Kepler, which first launched in 2009, retired last year after it ran out of fuel. NASA passed the planet-hunting torch to the Transiting Exoplanet Survey Satellite (TESS), which launched in April 2018.
Based on Kepler's findings, one NASA scientist estimated that our galaxy alone contains 1 billion Earth-like planets.
From the International Space Station, astronaut Scott Kelly took this photo of Earth and the Milky Way. He posted it to Twitter on Aug. 9, 2015.
Astrophysicist Natalie Batalha sent these rough calculations to the Washington Post in 2015. She noted that it was a conservative estimate.
Since then, further research has indicated that the Milky Way could harbor as many as 10 billion Earths.
This artist's concept of the Milky Way shows the galaxy's two major arms and two minor arms attached to the ends of a thick central bar.
In a study published in August, researchers estimated that an Earth-like planet orbits one in every four sun-like stars.
Those researchers didn't want to rely solely on the planets Kepler found. That telescope's method is better at detecting large planets (like Jupiter) than small planets (like Earth).
Jupiter's Great Red Spot was captured by NASA's Juno spacecraft as it performed a close pass of the gas giant planet on Feb. 12, 2019.
That means that Kepler data probably underestimates the number of Earth-like planets in the cosmos.
That's because Kepler used the "transit method." It watched for tiny dips in a star's brightness, caused by a planet passing in front of it.
In this composite image provided by NASA, the planet Mercury passes directly between the sun and Earth. This May 9, 2016 transit lasted seven-and-a-half-hours.
(NASA's Goddard Space Flight Center/SDO/Genna Duberstein)
Larger planets obstruct more of their stars' light, making them easier to detect. Plus, Kepler's method was biased toward small, dim stars about one third the mass of our sun.
So Ford's team built a simulation of a universe like ours and "observed" its stars as Kepler would have.
A multi-frequency all-sky image of the universe's background radiation.
The simulation gave the scientists a sense of how many exoplanets Kepler would have detected in each hypothetical universe, and which kinds. They then compared that data to what the real Kepler telescope detected in our universe, to estimate the abundance of Earth-sized planets in the habitable zones of sun-like stars.
The result: up to 10 billion rocky, Earth-sized planets in the habitable zones of sun-like stars.
This artist's impression shows an imagined view from nearby one of the three planets orbiting an ultracool dwarf star just 40 light-years from Earth.
"There are significant uncertainties in what range of stars you label 'sun-like,' what range of orbital distances you consider to be 'in the habitable zone,' what range of planet sizes you consider to be 'Earth-like,'" Eric Ford, a professor of astrophysics and co-author of the study, told Business Insider in August 2019. "Given those uncertainties, both 5 and 10 billion are reasonable estimates."
Many of those planets could be Earth-like in other ways, too. Last week, a study found that 87% of Earth-like planets in two-star systems should have a stable axis tilt like Earth's.
An illustration of the binary star system Sirius. Sirius A (left) is the brightest star in the night sky of Earth, and it has a small blue companion called Sirius B.
"Multiple-star systems are common, and about 50% of stars have binary companion stars," Gongjie Li, a co-author on the study, said in a press release. "So, this study can be applied to a large number of solar systems."
That stable tilt is crucial for life on Earth. The tilt of Mars's axis changes wildly over tens of thousands of years, creating drastic shifts in global climate that could prevent life from taking hold.
The surface of Mars.
Some scientists think Mars's changing axial tilt contributed to the disappearance of its atmosphere.
In an autopsy of six dead stars, researchers found that the shredded remains of rocky planets contained oxygen and other elements found in rocks on Earth and Mars.
A star like our sun dies by casting off its outer layers of gas, leaving only the star's hot core behind.
The researchers used telescope data to calculate how much the iron in these rocks had oxidized — the process where iron chemically bonds with oxygen and rusts.
"The fact that we have oceans and all the ingredients necessary for life can be traced back to the planet being oxidized as it is. The rocks control the chemistry," Edward Young, a co-author on the study, said in a press release. "We have just raised the probability that many rocky planets are like the Earth, and there's a very large number of rocky planets in the universe."
Earths might even be common in our own solar system. Venus may have had oceans and a climate like Earth's for billions of years.
An artist's representation of Venus with land and water.
In September 2019, researchers presented the results of five different simulations of the climate history of Venus. In all five scenarios, the planet maintained temperatures between 20 and 50 degrees Celsius for up to 3 billion years.
The researchers think that a mysterious catastrophe about 700 millions years ago transformed Venus into the uninhabitable hothouse it is today.
NASA's Galileo spacecraft took this colorized picture of Venus on Feb. 14, 1990, from a distance of almost 1.7 million miles.
"Something happened on Venus where a huge amount of gas was released into the atmosphere and couldn't be re-absorbed by the rocks," Michael Way, a NASA scientist and study co-author, said in a press release.
It could have been magma bubbling up from below Venus's surface, releasing tons of carbon dioxide into the atmosphere. That would have trapped enough heat to reach the broiling surface temperatures that average 462 degrees Fahrenheit today.
"It is possible that the near-global resurfacing event is responsible for its transformation from an Earth-like climate to the hellish hothouse we see today," Way added.
Even that susceptibility to disaster is, in fact, quite Earth-like.
On the morning of June 22, 2019, astronauts in the ISS captured the plume of ash and gases rising from the erupting Raikoke Volcano on the Kuril Islands in the North Pacific.
A supervolcano eruption or asteroid impact could one day make our planet uninhabitable. That could be the end of life on this Earth, but the research shows there may be plenty more Earth-like planets to spare.
This article originally appeared on Business Insider. Follow @BusinessInsider on Twitter.
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