17 Billion Earth-sized Planets – You Gotta Be Kidding!

The center of the Milky Way rises high over the domes of the European Southern Observatory in Chile. A new study suggest there could be at least 17 billion Earth-sized planets orbiting stars in our galaxy. The laser beam creates an artificial star to monitor atmospheric turbulence. Data from the “star” is used to adjust the telescope’s mirror to cancel out the blurring effects of the atmosphere. Credit: Yuri Beletsky / ESO

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.

Illustration of the variety of Earth-sized planets detected by Kepler. Credit: C. Pulliam and D. Aguilar (CfA)

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.

Graph showing the fractions of different kinds of planets around Milky Way stars. At least one-sixth of our galaxy’s stars have Earth-sized planets in tight orbits. Credit: F. Fressin, CfA

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.

A selection of planets discovered by Kepler, many of which are similar in size to Earth. Credit: NASA/Kepler Mission/Wendy Stenzel

“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.

9 Responses

  1. Giorgio Rizzarelli

    That’s great! The obvious questions is: is it possible to give a sort of estimate, using the new available data, for the number of Earth-size planets in habitable zone in the galaxy? And how does it change if we consider also exomoons?

      1. Giorgio Rizzarelli

        Indeed no such planet (habitable and earthsize) was found until now. Partly because, as you underlined in the article, Kepler can’t reveal such planet. You anticipated me writing in the article “even if 99%..”. I was wondering if it’s possible to do anyway a probabilistic estimate based on the available data. For example when they discovered Alpha Cen Bb earthsize planet, it was said that, even if it’s close to the star, there’s a good chance to find other rocky planets in the system, also likely in habitable zone, because rocky planets tend to be in company. Of course this is very approximate for now, but getting always a better estimate of numbers in Drake formula is a great tentation. Incredible progress has been done about exoplantes in very recent years.
        And forget what I said about exomoons… they would probably have no atmosphere for they have small gravity, if they’re like Solar System moons (although bigger moons of bigger planets cannot be excluded, and some moons can have a consistent atmosphere as shows Titan).

  2. Lynn

    Hi Bob
    I read at Nasa about a large asteroid belt around the star Vega may have been found and it say’s the discovery of an asteroid belt-like band of debris around Vega makes the stat similar to another star Fomalhaut, then in another article it say’s about vast debris disk encircling the star Fomalhaut, Fomalhaut B will intetsect the belt around 2032 on the outbound leg of it’s orbit and during the crossing icy and rocky debris in the belt could crash into the planet’s atmosphere and create the type of cosmic fireworks seen when comet shoemaker crashed into Jupiter, so is Fomalhaut and Fomalhaut B the same and what planet does it mean, I know it’s not ours, hope you can explain it a bit better please as u put it down in a lot easier terms that I understand better and you will know what I’m talking about as I don’t have a clue :-/ thanks

    1. astrobob

      Fomalhaut is the parent star, Fomalhaut b is the planet. It appears that Fomalhaut b’s elongated or stretched-out orbit might take it through the star’s version of the asteroid belt (debris disk). If so, it’s possible the planet could get smacked.

  3. Lynn

    See Bob I knew you would describe it in the way I got to understand, either that I’m getting a bit dopey in my older year’s lol. Thanks Bob 🙂

  4. Wayne Hawk

    Aloha Astro Bob!

    I love the following paragraph in your article:

    “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.”

    The reason? I think it’s great scientists are only beginning to think “outside the box”, so to speak and have “expanded the ‘sweet’ spot” in solar systems for life. Was it one of your articles that I read where bacteria are not only living but thriving on the “space junk” we’ve left behind on the moon? I’d have loved to have been a “fly on the wall” and seen the reaction of the scientist/scientific team that discovered that! It’s those little discoveries like that and the fact that Jupiters’ moon, Io, may very well have some form or life under its crust of ice that basically have forced the minds of these scientists to “expand” a bit and realize that living organisms can be pretty tough when they evolve in what would be too hostile an environment for most of Earth’s life.

    Almost all my life I’ve wondered why the scientific community seems to insist that conditions always have to resemble Earths in order for life to exist? Why are their minds so closed to be able to believe there may be some forms or life that do NOT require the same things (oxygen, liquid water, etc.) that the life forms on Earth do? Why couldn’t there be some form of life that breathes methane and is able to process whatever chemical it needs to survive from THAT gas in the same way we can process our nitrogen rich atmosphere and draw out the oxygen us human beings need? Or some other gas or liquid (perhaps even a solid!)?

    I’ll never forget a newspaper cartoon of “The Far Side” where it showed the American-African child-scientist who was always looking through his backyard telescope. In this particular one, it showed him looking through his telescope in one panel (or several) and in the last panel it shows him looking up and either screaming or speaking VERY loudly to the heavens, “I DON’T KNOW…THINGS JUST SEEM A BIT TOO ORDERLY OUT THERE TO HAVE HAPPENED RANDOMLY!!” Heh-heh…get it?

    It boggles the mind to think that our planet has managed to stay in the ONE spot for all these billions of years for countless forms of life to not only manage to simply “be” but to evolve into multi-celled organisms like the dinosaurs and us humans…doesn’t it? To think that had this planet not been hit by the “Mars-sized” object that created our Moon, our orbit may not have managed to “settle” into the “sweet spot” orbit that it finally did for all the life to have developed! I realize that some of us humans do know how extremely fortunate we are to have our (mostly) blue planet and just how rare it is to be just the right size to be in just the right orbit from the type of “star” that’s in our particular solar system but far more humans probably haven’t even had that thought…nor are able to comprehend such thinking!

    I have no intention of changing the subject matter, Bob, however, if the majority of the human race would/could have the thought of just how we managed to “come to be” (I know it’s hard not to fall back on our religious beliefs for this, but try, just for this example) and realize the fact that this planet is the ONLY planet right now, that we can comfortably live and we’d better start taking better care of it. Of course if we don’t, and we destroy our environment to the point where we all go extinct, I believe that the Earth has amazing powers of “healing” itself so some other “intelligent” life form could evolve given enough time.

    It’s articles that are written like this that really gets the “gears” moving in my head. Thank you for putting this out today. The sad thing about us looking for Earth-like planets is if we ever DO find one that we could live on, we don’t have the technological know-how to ever “get” to it within a persons’ lifetime. Hopefully we can figure out THAT little snafu if/when we need to leave our filthy planet (or perhaps we need to leave due to a NEO collision instead).

    Boy, aren’t I a “downer”! Sorry, just my train of thought!

    1. astrobob

      Thanks Wayne for your comments. I think many scientists are a conservative lot. They need as much evidence as possible before they’re willing to go out on the limb and risk their careers. That carefulness can be a very good thing as it gives science legitimacy, but it can also close some minds to possibilities untested and unrealized … until others come along to make those discoveries.

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