In the Milky Way, There Are As Many Planets As Stars

By Veronique Greenwood | January 12, 2012 12:49 pm


There are at least as many planets in the galaxy as there are stars. And even that is probably a vast underestimate.

That’s the latest bombshell from astronomers looking for planets beyond our solar system. Phil Plait at Bad Astronomy will have a post on this soon, but for now, here’s a little quote salad for you:

“Planets are like bunnies; you don’t just get one, you get a bunch,” said Seth Shostak, a senior astronomer at the SETI Institute who was not involved in this research. “So really, the number of planets in the Milky Way is probably like five or 10 times the number of stars. That’s something like a trillion planets.” (via PopSci)

“We think about one-sixth of stars should have a Jupiter-like planet, half have a Neptune-sized planet, and two-thirds should have an Earth,” said Kailash Sahu, an author of the Nature paper in which this observation was published. (via Wired)

Kepler [a space telescope devoted to the search for planets] has already been finding that small planets are actually quite ubiquitous around stars,” says Scott Gaudi of Ohio State University, who did not contribute to the new research. “That bodes well for our goal of eventually finding an Earth-size planet in the habitable zone. All signs are pointing to the low-mass planets being common, so I think there’s a good chance that we’ll find a system like that in the coming years.” (via Scientific American)

“It just feels like it’s inevitable that Kepler is going to come up with a habitable Earth-sized planet in the next couple of years,” says John Johnson of Caltech, who presented a related study at the American Astronomical Society meeting this week. (via NYTimes)

Image: An artist’s representation of how numerous planets are around the stars of the Milky Way. Courtesy of ESO/M. Kornmesser

  • Jim Johnson

    Not much of a surprise. I’m sure we’ve all pretty much assumed that most stellar systems are about as “busy” with satellites and satellites-of-satellites as our own solar system is.

  • Aidan

    “Habitable” planets however are not just Earth sized and in the goldilocks zone. They need to have an abundance of water, the right kind of atmosphere, a large magnetic field, a round shape, preferably not an elliptical orbit…Etcetera…

    I’m slightly skeptically we’ll find anything withing the next few years, but don’t get me wrong either, I’m 99.99999% positive there are other planets similar to ours…Now if only we had a telescope powerful enough to take images close up. I hope when they do find one though, that they don’t name it anything similar to ‘X-03-beta7s’.

  • Torbjörn Larsson, OM

    @ Aidan:

    “Habitable” is a series of classifications that can range from just having terrestrials with liquid water possible, to actually inhabited planets. The first kind is helpful to find the latter kind, while the latter kind becomes redundant. These are then search strategies.

    Here they look on something else though, “Earth analogs”, which are the ones most likely to yield detection of inhabited planets with the current knowledge. (Of the type case of Earth.) That is a more involved form of search strategy.

  • scribbler

    IF two thirds of the stars have an Earth, why haven’t we found ONE yet???

    That’s the nice thing about “estimates”, they can be as optimistic as the audience will tolerate.

  • MaDeR

    Kailash Sahu: “We think about one-sixth of stars should have a Jupiter-like planet, half have a Neptune-sized planet, and two-thirds should have an Earth,”
    scribbler: “IF two thirds of the stars have an Earth, why haven’t we found ONE yet???”
    We already founded few earth-sized worlds. And yes, context of Sahu quote is that we talking about mass/size only.

  • Jim Johnson

    @scribbler because the smaller the planet, the more difficult to find + the farther from its star, the more difficult to find = small as ours & as far out as ours is obscenely difficult to find. Have patience.

  • dave chamberlin

    We can guess but cannot prove until we go somewhere for a better look that habitable planets are very very very rare. The list of requirements to make an earth like planet which can habor complex life is long and getting longer. Just two of the many requirements. A magnetic field requires a planet that is overwhelmingly metal, an ingredient only produced by supernovas which occur 1/2000th as often as novas. Our entire galaxy creates one every 40 years. We need a huge moon otherwise we would be in tidal lock with the sun and that moon needs to be born of just the right kind of collision so that a centrofuge type reaction occurs so that all the heavy metals stay with earth and the moon is made of the lighter mantle material.Otherwise we get too much mantle material on earth and we dont have the floating of the tidal plates which is crucial to the carbon cycle. The list goes on and on and on. Sorry sci fi fans (I’m one too) but its a universe where complex life is incredibly rare.

  • Sandra

    LMAO at Dave Chamberlin’s absurd pseudo-scientific nonsense.

    Tidal lock with the sun? HAHAHAHAHA!


  • http://n/a RICHARD D. STACY

    The big breakthrough with this panoply of planets will be if we find one or more that are harboring “intelligent” life ! I fervently hope that this will happen and destroy our arrogant idea that we are some kind of special creation.

  • Adam Laceky

    We do, however, need a large moon to

    1) Clear the orbit of asteroids and large meteoroids,
    2) Create tides, which are vital to ocean ecology as we know it, and
    3) Inspire poets and lovers

    In a universe as vast as ours, we can describe a fairly specific scenario and be confident that it has played out (or will) somewhere in the universe.

    A Jovian planet with several moons. Two of those moons develop intelligent life. At some point they become aware of each other’s existence. They see city lights, for instance. They start imagining what the people on the other moon are like. They have science fiction and poetry and legends about the other moon. This would probably inspire them to build telescopes and take a closer look. They’d probably invent radio, and start talking to each other. At least one would eventually develop a space program. And one day, they would blast off and land on the other moon. Take me to your leader.

    After that, all bets are off.

    In a universe as vast as ours…

  • scribbler

    I have all the patients in the world. I’m not ruling one out. I’m just saying that so far, nothing has been close. The news reels are all “Earth-like planet found!” and then when they take a closer look, it’s all “Oops! Maybe next time!”

    It’s cool.

    Just for me, I prefer a more measured approach…

    “The greatest mystery was not that we have been flung at random among the profusion of the earth and the galaxy of the stars, but that in this prison we can fashion images of ourselves sufficiently powerful to deny our nothingness.”― Maurice Friedman

    Wouldn’t a greater mystery be thinking it happened twice???

  • Brian

    Life came to exist because a bunch of conditions were met. Life is not preordained.. it is all about cause and effect. Since we know that there are many different types of stars out there, and many different types of planets around them, it’s only a matter of time before we find another one that has seen all the different conditions that Earth has, and because of that, will likely have some form of terrestrial-esque life inhabiting it. However, finding one of these planets does not necessarily mean that there will be complex life. For humans alone to become what we are today, many more conditions had to be met for that to happen.

    Intelligent life existing in the cosmos is random at best. It’s extremely hard to find it, since the conditions that are required for it to exist are rare, and vast in number.. and usually, you need every condition to be met to see the outcome we’re looking for.

    But this is just life as WE know it.. meaning carbon-based life… which lives off oxygen and hydrogen. There could be a form of intelligent life out there who would find Earth conditions toxic. NASA scientists have already announced the discovery of life that can thrive in a pure methane environment… so why is it so hard to fathom many different forms of life in the universe that we can’t even conceive of? Just as our planet consists of lifeforms that breathe air and drink water, there could be life out there that lives off arsenic. The universe is a big place.. we haven’t even found the edge of it yet… and there are billions and billions of galaxy clusters out there which consist of hundreds of billions of galaxies, which all contain billions of stars, and trillions of planets. We are not the only form of life.

  • Pat

    Just think, in a few paragraphs we went from a reasonable discussion of what we actually know, to solving Fermi’s paradox. Well, at least some of us did.

  • scribbler

    I think the evidence supports the presumption that life is unique to the Earth, especially if one limits life to “Earth-like” planets.

    Beaches are huge places with many billions of grains of sand. Just because there is a huge number of them and we haven’t looked at each and every one does not give a “scientist” free hand to make up whatever he or she WISHES to be true about them. It isn’t a game of “you can’t prove me wrong, so I can say whatever I like” anymore. It’s a game of “The evidence, ALL OF IT, so far says you are wrong…” 😉

  • scribbler

    Just look in our own solar system. Mars and Venus are in the habitable zone and are about the same size as Earth and circle the same star, getting the same radiation. Both are extremely hostile to life as we know it.

    There are many, many millions of variables that exclude life on a given planet. To paste ones hopes solely on three or four is irresponsible, in my book…

  • mapS

    Hope isn’t an element that needs to be a part of this in any case. Keep it scientific; if you project your desires on it you’ll see what you like until there is assurance one way or the other. Personally I am quite skeptical, but moreso I am skeptical about the POV of a certain breed of “militant liberal atheist” who seem to look to the skies to give them the magic ingredient to defeat the evil religions.

    Whether life is pre-ordained or not is irrelevant.

    scribbler, you have to start somewhere though. It’s not about hope, it’s about looking in the right places.

  • scribbler

    All for looking…

    Just being realistic about what has been found so far…


    I think the greatest value with finding another Earth is that it would give us someplace else to colonize. That would be something to work toward. As for aliens, IF we ever find any, personally, I just hope they are tasty…


  • Tanner

    Discovering so many new stars and planets in MW may or may not mean we find life elsewhere in my lifetime. But it does mean that the total mass of our galaxy and the other galaxies are far greater than estimated just a couple of years ago.

    So what do all these new discoveries have to say about the total mass of the universe and the resulting impact on dark matter/dark energy? I’ve been looking around for this but haven’t found anything.

    Has anyone seen new studies on the so called “missing mass” in our universe?

  • Richard Dinning

    I wonder if all those planets and dark stars might be the missing matter needed to hold galaxies together, removing the need for dark matter as something special.

  • scribbler

    The Sun contains 99.85% of all the matter in the Solar System. The planets contain only 0.135% of the mass of the solar system.

  • Tanner

    Even if that proportion is constant throughout the MW, adding all these new stars/solar systems reduces need for DM/DE in calculations. Maybe it isn’t there at all, or if it is there, perhaps it is a much smaller proportion of total than currently estimated.
    Also some new debate about mass of photons. Even if photon mass is tiny the quantity throughout the galaxy is enormous. How many photons has our sun released in 4 billion years? More stars mean more black holes. Photons with mass mean more mass in black holes. In total, more mass accounted for, less to find.

    Just wondering if new estimates need to be done, and if these new estimates would challenge our understanding of the behavior, type, quantity, etc., of matter in the universe.


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