Can you believe the news? It’s estimated that at least one-sixth of the stars in the Milky Galaxy harbor Earth-sized planets. Since our galaxy contains at least 100 billion stars, that means a minimum of 17 billion planets. Next time you look up into the night sky, consider how many worlds your gaze encompasses.
Francois Fressin, of the Harvard-Smithsonian Center for Astrophysics (CfA), presented the study Tuesday in a press conference at a meeting of the American Astronomical Society in Long Beach, Calif. His work is based on a new analysis of data from NASA’s Kepler spacecraft which has been monitoring over 145,000 stars in the direction of the constellations Cygnus and Lyra for the past 22 months.
Kepler spots planets by looking for periodic fadings when an extrasolar planet crosses in front of (transits) its host star. Called the transit method, a planet’s size is calculated by measuring precisely how much the star’s light is dimmed. Timing the intervals between repeat transits yields an orbit. As of this week, Kepler has tallied up more than 2,700 potential planets with more than 100 already confirmed.
Through a clever simulation, Fressin and colleagues determined that 90% of Kepler’s candidates are the real ticket, with the remainder false positives. They then extrapolated that figure to the heavens at large and arrived at these amazing statistics:
* 50% of stars have a planet of Earth-sized or larger in a close orbit. That proportion goes up to 70% when larger planets in wider orbits are included.
* Extrapolating the Kepler results with other ongoing surveys and techniques that to date have uncovered 854 exoplanets indicate that all sun-like stars have planets.
The team then grouped the planets into these categories: 17 percent of stars have a planet 0.8 -1.25 times the size of Earth in an orbit of 85 days or less. About one-fourth of stars have a super-Earth (1.25 – 2 times the size of Earth) in an orbit of 150 days or less. (Larger planets can be detected at greater distances more easily.) The same fraction of stars has a mini-Neptune (2 – 4 times Earth) in orbits up to 250 days long.
Folks, we’ve got company. 17 billion? Even if 99 percent are either too close or too far from their host suns to support any form of life, that still leaves 17 million open to the possibility. Astronomers predict the best places to find not just Earth-sized planets but versions friendly to life are in the Goldilocks zone, where liquid water can exist without vaporizing before you get to the end of this sentence.
The sweet zone for life has expanded in recent years as scientists have uncovered bacteria completely at home in acid and boiling water. Deinococcus radiodurans, listed in the Guiness Book of World Records as the planet’s toughest bacterium, can survive cold, acid, dehydration, a vacuum and powerful doses of radiation. Yeah! That’s what I love about life.
“Earths and super-Earths aren’t picky. We’re finding them in all kinds of neighborhoods,” says co-author Guillermo Torres of the CfA. And they include not only sun-sized stars but also red dwarfs that are smaller and cooler than the sun. Less common are big Jupiter-sized worlds even though that’s what most ground surveys of exoplanets have found. Kepler’s broad sample shows that only 5 percent of stars are orbited by gas giant planets with periods of 400 days or less.
We all enjoy looking up at night and letting our sense of wonder take us away. The next time you’re out, consider a planetary journey. Pick any star and the chances are 3 out of 4 it’s orbited by an alien world.