Astronomers Say Milky Way Has Around 2 Billion “Earth Analog” Planets (That’s the Bad News)

By Patrick Morgan | March 30, 2011 1:35 pm

What’s the News: Based on early Kepler data, astronomers say that the Milky Way galaxy may house at least two billion Earth-like planets—one for every several dozen sun-like stars. As NASA researcher Joseph Catanzarite told, “With that large a number, there’s a good chance life and maybe even intelligent life might exist on some of those planets. And that’s just our galaxy alone — there are 50 billion other galaxies.” But while 2 billion sounds like a lot, it’s actually far below many scientists’ expectation; Catanzarite says his teams’ findings actually show that Earth-like planets are “relatively scarce.”

How the Heck:

  • Using mathematical models to plot the size and orbital distance for all the potential planets spotted during four months’ worth of Kepler data, astronomers extrapolated the data and calculated that 1.4 to 2.7% of the Milky Way’s sun-like stars may have an Earth analog.
  • Two percent of the Milky Way’s roughly one hundred billion sun-like stars means that “you have two billion Earth analog planets in the galaxy,” Catanzarite told National Geographic.

What’s the Context:

Not So Fast:

  • MIT astronomer Sara Seager says that the team “completely underestimates the frequency of Earths.” The calculations are based on only four months of Kepler data—too early to be making an accurate projection.
  • There’s also the fact that Kepler can only detect the size and orbital distance (and occasionally the masses) of planets, which doesn’t tell you whether life as we know it could actually live there; Venus, for example, would roughly like Earth to aliens peering at us from many light-years away, but because of its atmosphere’s runaway greenhouse effect, it’s way too hot to be habitable.

Next Up: The astronomers plan on calculating an even more accurate number once all of Kepler’s data is in.

Reference: Joseph Catanzarite and Michael Shao. “The Occurrence Rate of Earth Analog Planets Orbiting Sunlike Stars.” arXiv:1103.1443v1 Image: Kepler/NASA

  • Rusty Webb

    Ok, stupid question I’m sure. But is the region of space sampled by Kepler thought to be a pretty good analogue for the rest of the galaxy? I mean, aren’t some regions thought to be more friendly towards planet forming?

  • John Lerch

    The final comment seems most relevant–Venus would look earth-like. Indeed so would Mars. We don’t even know how the earth managed to stay hospitable through change’s in the sun’s brightness over 4 billion years.

  • Barry Johnstone.

    Now I’m wondering how the Young Earth creationists would handle this!

  • Iain

    I laughed when I saw “there are 50 billion other galaxies”. They always forget to add ” in the observable universe”. And the difference is? Google Hubble Limit. Or if you want my poor description, space is expanding, the farther away you look the faster it is expanding ( I think that’s just the expansion adding up) so at some point the commulative expansion exceeds the speed of light so we can only see so far, about 14 billion light years worth. Also the light that we see from 14 billion years ago is only just arriving here because of the expansion of space and the objects we are seeing are really 42 billion light years away.
    Einstein said the universe is infinite, just a guess on his part, but I’d put my money on him.

  • Trollcat

    A name like “Catanzarite” is a sure sign someone messed up at Ellis Island… either his ancestor or the paperwork officer :)

  • Trollcat

    Also, is this an April first joke? Just checking…

  • Kakusmate

    Amateur. The grammatic errors in the final paragraph tell the whole story.

    What errors are you talking about? I assume one thing you think is an error is the use of “data is” rather than “data are.” If that is, in fact, what you’re referring to, you’re wrong to say we’re wrong. We’re well aware of the fact that “data” was originally the plural form of the singular “datum,” but it’s now perfectly acceptable in common English to use “data” with singular verb forms, like “information.” Oxford, American Heritage, and Merriam-Webster are all clear on this point.

    Anyway, it’s pedantic to get all huffy over a point like this. But if you’re going to be a pedant, you’ll come off better if you’re actually right.

    —Amos Zeeberg, Discover Web Editor

    PS: “Grammatical” would’ve been better.

  • amphiox

    re @4;

    Well, for all practical purposes , “observable universe” = “universe”. That which cannot be observed does not fall within the realm of science.

    (Not ruling out the possibility that someday some means might be discovered of observing what previously can’t be observed, of course).

  • Iain

    amphiox – so by your words the moon was a flat disc until the first probe went to the backside and observed that the moon was truly a sphere? Dark matter and energy can’t be observed so they are just mystic mumbo jumbo? String theory is just wishful thinking? I think you need a little more precision in your observations.

  • amphiox


    Firstly, it does not take an observation of the backside of the moon to prove that the moon is not a disc, but a spheroid (it’s not a sphere). And the ancient Greeks did it. With the math and science they had available to them at the time. (They also demonstrated that the earth was round without needing to circumnavigate it).

    Secondly, although we could not observe the far side of the moon in the past, observation of the the far side of the moon was not unobservable in principle. Nothing in the science of Newton (or even Aristotle) ruled out a potential observation of the far side of the moon. Thus, speculation about the far side of the moon fell within the purview of science, along with any speculation with respect to what sort of technology would be required to make that observation, and what kind of engineering would be necessary to build it.

    Thirdly, dark matter and energy ARE observed. By their indirect effects on the movements of other objects, and the expansion of spacetime. ONLY because we have these observations are we able to hypothesize about them. And ONLY because we CAN observe them in this way do we even talk about them at all.

    Fourthly, string theory may very well be wishful thinking. The jury’s still out. Many cosmologists have reservations about string theory and some really are arguing with conviction that it isn’t science. Many are insisting that string theory should not be considered science until the string theorists are able to propose theoretical means by which the theory can be tested.

    What lies beyond the observable universe is considered unobservable because of the speed of light limit. In other words, based on our current understanding of the laws of physics, not only is it unobserved by present capability, it is theoretically completely impossible to observe by any means, ever. Even with infinite time and infinite resources and infinite cleverness and infinite power, we will never, ever, ever, ever be able to observe it, assuming relativity is broadly correct.

    And something that cannot even theoretically be observed cannot be tested, even theoretically. If it cannot be tested, it is not science.

  • win

    si esa informacion es correcta porque no va a ver planetas como la tierra
    en la via lactea y otras galaxias

  • James Thornton

    These cosmologists are really being of disservice to the human race. We have enought problems here on earth. They are only fueling the group of people who say lets make a mess of Earth and then leave it. It is so much easier to fix the problems here on earth than to think of leaving to go to another place to mess up. These places are measured in huge distances away from Earth. How are we going to get there? No body has the answer to the transportation question. We can’t even get to the speed of light. Also according to Physics we can not go faster than the speed of light. Most if not all of these places are many multiples of a Light Year, away from earth. Untill we can get WARP speed, the “Star Treck” speed region of many times the speed of light, going to any of these places is just Science Fiction. We might as well just get the Astronomers doing some thing else like making usefull things from all of the human waste lying around now.

  • phil harvey

    james …really…where are you coming from?. What ‘mess’ are you referring to, and what ‘mess’ do you think the human race will export to space? Get real please.

    The basic fact is real enough though, we as a race HAVE to venture into space for our long term survival. So if you wish to see the human race survive well into the future there can simply be no argument to space exploration and colonisation of other worlds. QED!

    The light barier is not an obstacle to space faring either.

    And please be so kind as not to insult astronomers. They are by far the best and most useful scientists that current and future mankind will depend upon….(them and the rest of the physics and engineering community of course!)

  • phil harvey

    Amphiox. ‘If it cannot be tested it is not science’….I dont think so

  • phil harvey

    John. I dont quite understand how you think it a mystery how the earth has been hospitable over its 4.6byr history! The sun over this time has not exactly altered that much I suspect (it isn’t a vairable star).

    Throughout earth history it has not exactly always been that hospitable though; only over the past 0.5byrs have we seen explotions of life.

  • phil harvey

    Iain, you are correct in stating the need to address the observable universe.

    The light from the outermost observable universe has taken 13.8byrs to reach us. This consitutes the radius of the observable universe. In this time the universe (assuming at least uniform expansion) would be at least 27.6byrs in radius by now in relation to our current timeframe (although likely to be much bigger due to acceleration…and naturally unobservable by us).


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