A new study indicates some 60 percent of the galaxy's most ubiquitous stars likely host planets smaller than Neptune and about 6 percent host Earth-size worlds orbiting in the so-called "Goldilocks" zone where liquid water -- and life as we know it -- are possible, astronomers announced Wednesday.
The stars in question are red dwarfs with a quarter the mass of the sun and just 2 percent the luminosity. But they are commonplace, making up about 75 percent of the stars closest to the sun.
Courtney Dressing, a graduate astronomer who led the study at the Harvard-Smithsonian Center for Astrophysics, told reporters a statistical analysis of 4,000 red dwarfs studied by NASA's planet-finding Kepler space probe shows the nearest Earth-like planet must be orbiting a red dwarf within a stone's throw of the sun.
"We're able to use this estimate, that 6 percent of red dwarfs host Earth-like planets, in order to figure out how far away from the Earth you would need to look in order to find an Earth-like planet orbiting a red dwarf," she said. "We find that the answer to the question ... is actually quite close. The nearest Earth-like planet is expected to be about 13 light years away."
To put that in perspective, if the 100,000-light-year-wide Milky Way galaxy was the size of the United States, and if Earth was located on one side of Central Park in New York City, "we find the nearest Earth-like planet is just across the park," Dressing said.
John Johnson, an assistant professor of astronomy at the California Institute of Technology, called Dressing's findings "extraordinarily exciting."
"What we need to do is take the next step in designing the next generation of instruments that will allow us to gather these small planets from the sky, study their properties in detail and provide us with an understanding of how Earth-size planets form," he said.
"It will give us that grander galactic context and it puts us hot on the trail of finding life elsewhere in the galaxy. ... Keep in mind that all of these planets that we now know are out there, these are physical locations, places throughout the galaxy where life could have emerged."
Maybe. Ubiquitous or not, red dwarfs only emit a fraction of the sun's energy.
"So if you picture the sun as a thousand-watt light bulb, these stars would be Christmas tree lights, they would emit one or two watts of energy," said David Charbonneau, a Harvard astronomer. "So planets at the right distance to be the same temperature as the Earth have to be tucked in close to these stars."
Planets in such close orbits likely would become "tidally locked" over time, leaving just one hemisphere facing the warmth of its star while the opposite side faces deep space.
In addition, red dwarfs are active stars and large sunspots could affect the amount of light reaching a close-orbiting planet as the star rotates, producing large-scale effects on a planetary surface.
Red dwarfs also emit torrents of ionizing ultraviolet radiation, possibly enough to blast away a nearby planet's atmosphere if not sterilize it.
But a thick atmosphere and a deep ocean could mitigate those effects, protecting the surface and allowing heat from the day side to migrate around the planet.
In any case, Charbonneau said carefully reasoned arguments as to why red dwarfs could not host habitable worlds may reflect a sun-centric viewpoint that does not mirror reality.
"I think that with hindsight we see that as a myopia from having grown up around a sun-like star," he said. "If there is one message from the past 15 years of exo-planet discoveries it is that there is no guarantee our solar system is common place, there is an enormous diversity of architectures.
"Basically, anytime we have the sensitivity to find planets around a different kind of star, we find planets."
Red dwarfs are remarkably stable and if a planet survived the energetic birth of its diminutive host, it likely would survive for tens of billions of years.
"What is so exciting about the announcement today is that it raises the real possibility of ... finding planets that are like the Earth ... but are actually much, much older, and actually seeing what that the consequence would be for a 10- or 11-billion-year-old Earth-like planet," Charbonneau said.