Billions of Habitable Worlds Likely in the Milky Way


Could there be ‘tens of billions’ of habitable worlds in our own galaxy? That’s the results from a new study that searched for rocky planets in the habitable zones around red dwarf stars. An international team of astronomers using ESO’s HARPS spectrograph now estimates that there are tens of billions of such planets in the Milky Way galaxy, with probably about one hundred in the Sun’s immediate neighborhood, less than 30 light years away.

“Our new observations with HARPS mean that about 40% of all red dwarf stars have a super-Earth orbiting in the habitable zone where liquid water can exist on the surface of the planet,” said Xavier Bonfils, from IPAG, Observatoire des Sciences de l’Univers de Grenoble, France, and the leader of the team. “Because red dwarfs are so common — there are about 160 billion of them in the Milky Way — this leads us to the astonishing result that there are tens of billions of these planets in our galaxy alone.”

This is the first direct estimate of the number of smaller, rocky planets around red dwarf stars. Add this to another recent finding which suggested that every star in our night sky has at least one planet circling it — which didn’t include red dwarf stars – and our galaxy could be teeming with worlds.

This team used the HARPS spectrograph on the 3.6-metre telescope at ESO’s La Silla Observatory in Chile to search for exoplanets orbiting the most common kind of star in the Milky Way — red dwarf stars (also known as M dwarfs). These stars are faint and cool compared to the Sun, but very common and long-lived, and therefore account for 80% of all the stars in the Milky Way.

The Milky Way over the ESO 3.6-metre Telescope, a photo submitted via Your ESO Pictures Flickr Group. Credit: ESO/A. Santerne

The HARPS team surveyed a carefully chosen sample of 102 red dwarf stars in the southern skies over a six-year period. A total of nine super-Earths (planets with masses between one and ten times that of Earth) were found, including two inside the habitable zones of Gliese 581 and Gliese 667 C respectively.

By combining all the data, including observations of stars that did not have planets, and looking at the fraction of existing planets that could be discovered, the team has been able to work out how common different sorts of planets are around red dwarfs. They find that the frequency of occurrence of super-Earths in the habitable zone is 41% with a range from 28% to 95%.

Bonfils and his team also found that rocky planets were far more common than massive gas giants like Jupiter and Saturn. Less than 12% of red dwarfs are expected to have giant planets (with masses between 100 and 1000 times that of the Earth).

However, the rocky worlds orbiting red dwarfs wouldn’t necessarily be a good place to spend your first exo-vacation – or for harboring life.

“The habitable zone around a red dwarf, where the temperature is suitable for liquid water to exist on the surface, is much closer to the star than the Earth is to the Sun,” said Stéphane Udry from the Geneva Observatory and member of the team. “But red dwarfs are known to be subject to stellar eruptions or flares, which may bathe the planet in X-rays or ultraviolet radiation, and which may make life there less likely.”

New Exoplanet Discovered

A new exoplanet was discovered in this HARPS survey of red dwarfs: Gliese 667 Cc. This is the second planet in this triple star system and seems to be situated close to the center of the habitable zone. Although this planet is more than four times heavier than the Earth it is the closest twin to Earth found so far and almost certainly has the right conditions for the existence of liquid water on its surface. This is the second super-Earth planet inside the habitable zone of a red dwarf discovered during this HARPS survey, after Gliese 581d was announced in 2007 and confirmed in 2009.

“Now that we know that there are many super-Earths around nearby red dwarfs we need to identify more of them using both HARPS and future instruments,” said Xavier Delfosse, another member of the team. “Some of these planets are expected to pass in front of their parent star as they orbit — this will open up the exciting possibility of studying the planet’s atmosphere and searching for signs of life.”

Research papers: Bonfils et al. and Delfosse et al.

Source: ESO

38 Replies to “Billions of Habitable Worlds Likely in the Milky Way”

  1. Don’t tell real estate folks about this, talk about driving prices through the floor. Everyone on earth can have a planet of their very own. Would be a long drive to the store to buy wine and cheese, though.

  2. a great example of the importance and application of statistics in astronomy.
    (i am NOT a statistic!)

    super earths could likely have super thick atmospheres and deep oceans with stupendous surface areas.
    the extreme age of the systems would enable the evolution of countermeasures or perhaps even reliance on the flares. at any rate, the study of local red dwarfs is very interesting. not an easy task to find them in the blaze of the background.

    1. @Zetetic Elench

      > [..] not an easy task to find them in the blaze of the background.

      Thankfully, many of our neighbouring red dwarfs will be further out from the center of the galaxy than we are, so there should be plenty to study in the darker regions of space all around us.

      When I left school and headed to University in the late 1980’s – not a single exo-planet had yet been discovered. It’s truly amazing what has been accomplished since those not so distant days. Sadly, our ability to explore the universe around us is not developing nearly as well as our ability to detect and observe more of it’s hidden details.

      Ultimately, politics is still the biggest barrier to human progress – and wars one of the biggest sinks for funding that could do a huge amount of good in the world.

      1. We would have to harvest every last resource in our solar system just to get there. I’m not sure what political climate would help with that. Perhaps if the “who needs Jupiter anyway” party won.

  3. “contact” was right – there are billions of worlds similar to our own just in our galaxy. If so dim and cold stars like M dwarfs take 80% of all stars in the galaxy, it would be wise to search them – we can say which is a flare star and which is not. All we need is patience and time plus better tech also, but this is now in a boom state.
    I never thought that imagination will someday become reality! Some 100-200 years and maybe we will find a way to travel to them 😉

    1. Traveling would be nice, but it needs much, much more compared to observing. Imagination is cheep, if you don’t have to bother with the details.

  4. so that mean there are 64 Billion habitable planets in the Milky Way.
    In february 2011 we started with 2 billion planets in the habitable zone. now is it 64 Billion.
    next year it will probably be 100 Billion.

    what i do not get they find 9 super earth planets, 2 that are in the habitable zone. gliese 581 d is just in the outer edge of the habitable zone
    than they come to the conclusion of 40%

    that also mean that 40% of all the stars of kepler must have a habitable planet.
    Why is the number of habitable planets than still so low what we have found with kepler so far ?

    1. Because we really can’t detect earth sized planets yet. Based on our own system, smaller is more common.

    2. yes that is an artifact of our detection technology. we see the closest and largest planets because they have the most significant effect on their stars. the outer planets have lower and slower effect ergo we need time and sensitivity to detect them radially. throw in the effects or orbital resonances and it only becomes more so.
      the better the sample the better the statistical projection.

  5. Using a clue from Zetetic, I wonder if the inhabitants of one of these worlds would look at G2 stars and say “but how could anyone even exist there without the xrays we glory in on regular intervals?” And “wouldn’t a bright star like that burn their retinas?” I mean, it might be nice to check out your own skeleton on a regular basis. Zzzzzzzap!

    1. That`s what I thought – why we assuming that life in other worlds is earth-like? Even here on Earth we have life forms which are completly different (based on arsen not phosphorus).

      At this stage of our civilization development we should focus on closer words i.e. Solar System and how to use it`s resources because we can`t reach or even communicate with these other worlds with current technology.

      1. Just to clarify, no, no life uses arsenic here on earth. It was just an experiment to see if they could. (AFAIK) But yeah, life could take some pretty weird turns ie: maybe it all exists underwater somewhere else but they get some form of food from something that delights in x-ray baths, floating on the surface.

  6. This article is a little misleading…
    Just because a world is within the Goldylocks zone around a star DOESN’T mean that it contains the necessary atmosphere, surface, ocean and temperature to sustain Earth-based lifeforms.
    Hence making it “non-habitable” I’m not sure why people are constantly assuming that every new planet discovered within a Goldylocks zone could support us.

    That said, the likelihood of there being Earth-like planets out there is massive in its own right – probably ranging in the billions, but physicists and astronomers figured that out years ago. Hopefully, they’ll just scout out the surrounding neighbour hood and then spend their money on something a little more constructive 🙂

  7. now to wait till m-class stars will be subdivided according activity. Maybe a bit more or less metalicity per mass and the (in)stability improves to allow for more favorable life circumstances.

  8. The large number of planets in the habitable zone increases the probability we might find a planet with biological signatures. If such a planet is within 25 light years we might be able to send a probe there.

    Nobody is going to any of these planets. I doubt that human beings are really going to engage in interstellar travel. So these planets do not constitute “real estate,” and virtually all of them are probably hostile to human life. We might be lucky if 1% of these 64 billion planets have biological activity. Even if one is within 25 light years I doubt humans could live there, and such alien life could be toxic as well. Alien microbes might “see” us as food in the same way mold spores “see” a loaf of bread.


      1. Of course. There would be that number in our galaxy. Maybe some fraction is within viewing distance, or where we can get a signature of biological activity in the atmosphere.


      1. To be honest I suspect Homo sapiens is going to check out of reality, in other words go extinct, long before the sun starts to broil this planet. The average duration of a complex mammalian species is a few million years, and our species has 150,000 years of bio-history with a hominid lineage going back 2.5 million years to Australopithecus. There is no way I can know, but I think we will be just another evolutionary dead end a million years from now.

        I find it interesting that people are so concerned about this sort of astro-eschatology involving the duration of the sun and so forth, and an imperative to colonize other stars. Right now there are lots of indicators we are doing things which could put the kibosh on us in the next few centuries. In fact we are heading into a resource depletion problem right now that could result in world wars in the next few decades. Further, modern society looks to the future not much further than a decade or so. Our economic reality is based on quarterly and yearly returns and profits. Nobody actually running this world, by which I mean the real owners, give a fig about the sun going out a billion years from now. BTW, by the real owners I mean the corporate heads, bankers, CEOs, and the whole financial elite. The politicians are just their vassals. The people who run things could care less, and in fact could care less about these immediate issues which will impact us in a few decades to a few centuries.


      2. Yes you are absolutely right.

        Why to worry about the death of the sun 5 billion years in the future when right now the capitalist elite is making billions at the expense of the future of us and our descendants, by over-exploitation of earth resources? (I am thinking about fossil fuels, of course).

        They care only about short-term profits, and care nothing about habitability of our planet. Maybe the greatest challenge to habitability of a planet is the emergence of those self-destructive behaviour in any intelligent civilization that emerges anywhere.

      3. Unfortunately corporate power today is just a current manifestation of socioeconomic elitism and corruption. History is paved with many other examples of this, and the Soviet experiment proved to be just another example of such corruption. Our species seems almost incapable of escaping this, and even if we do in some limited manner it only holds for a rather brief period of time.


      4. Ok I agree with you, maybe we can be extinct then what? We humankind go to just complain,that this is most likely possibility and wait for the extinction, or we go to find a solution ever that unlikely (in your point of view) like many scientists and engineers work right now for it?

        well I already pink my choice, fight at end and try to find the solution, support the scientist and engineers to find the solutions for humankind problems to not be extinct, one of the first steps on that direction is the solar system colonization

        OK I understand your point of view the economy isn’t seems good,politics every time get worse and worse,the US most advanced human space program,isn’t have the deserve support,but all this can have a solution,when people get tired of it,and wake up to do something, instead of complain like religious that the doomsday is coming,this human specie can still have a chance to triumph as survival specie,this world will much better

      5. My point is that it is silly to worry about our existence billions of years from now. This is particularly given our ever growing predicament here. It makes little sense to me.

        To be honest through my adult life this country has gone in completely the wrong direction. We have squandered our energy and resources, traded away real concerns to chase after phantom enemies, and now we have GOP politicians who seem to want high birth rates and think we can drill our way out of our energy problems. At some point this will all go beyond a point of no returnn.

        Putting small bases or colonies on the moon or even Mars will do nothing, for without “ground control” these installations can’t work. It will remain that way for considerable time into the future, assuming there is much future left.

        I think the prospect for building our way into immortality is problematic. This includes our entire species as well. Our species is ultimately mortal, where our options available involve choices in the foreseeable future, and not likely into vast times into the future.


    1. “We might be lucky if 1% of these 64 billion planets have biological activity”

      Are you stating that any life is uncommon, or just complex life?

      Microbes can live in a great variety of extreme environments. Just look at all the “extremophiles” here on Earth. This life probably is common even outside the habitable zone, hiding in hydrothermal vents and deep rocks.

      Complex (multicellular)life, on the other hand, is much more fragile. It needs a narrower range of temperatures, and an oxygen atmosphere to grow. A planet must have a reasonably stable climate for this. Various geological conditions are needed, among the most important is active plate tectonics, because it permit the “CO2-silicate thermostat” mechanism to work . Diana Valencia modeled the mantles of super earths, and found that they have more active plate tectonics than earth. This is good news for super-earth habitability.

      With respect to the toxicity of alien life, I think that most alien microbes are harmless. The “toxicity” of microbes is the capacity to invade the body causing diseases. Most viruses and bacteria here on earth are harmless to humans. The few ones that are harmful, have adapted for millions of years to us, acquiring the capacity to infect our body. Take viruses as an example: they have proteins in their surfaces that must match the proteins in our cells to infect them. If there is no match, the virus is not pathogenic to humans. Viruses have a narrow range of hosts for this reason (for example, hepatitis B virus only infect humans, chimpanzees and gorillas; most influenza A viruses infect only birds, just a few strains have adapted to humans, causing pandemics ).
      Alien life will be so different from terrestrial life that is highly unlikely that any alien microbe has the capacity to infect earthly life, just as a plant pathogens have no capacity to infect humans(or any other animal).

      1. We are really working with unknown unknowns, so it is hard to estimate a prior probability in any Bayesian manner. I think life does exist outside this planet. If we look hard and long enough we may find a planet similar to Earth around some star. The signatures would include an oxygen atmosphere, trace amounts of methane and so forth. Even a CO_2 atmosphere might carry signatures of biology.

        My sense or hunch is that prokaryotic biology might exist with some abundance in the universe. There might be a fair number of planets with such life, including Mars or some interiors of these Jovian-Saturnian moons. I suspect that complex life, such as on Earth, is far more rare. I also suspect that life is tough on any planet around a dwarf star. Many of these stars violent flare up, planets around them may not have long term stable orbits, and the part about tidal “squeezing out water” might also make these planets into Venusian type planets. I suspect there are a lot more Venus and Mars analogues out there than analogues to Earth.

        It is the case that microbes have co-evolved with hosts. The famous “lock & key” mechanism is standard in how a parasite infects a cell. The problems with alien microbial life is that it might be so divergent from Earth life that it would interact with Earth life just as mold does with a loaf of bread. The converse might hold as well, where Earth microbes on Mars or inside Encaledus might regard microbes there as “snacks.” In other words alien microbes might just plain eat us. Our immune systems might also have a horrible time trying to decode what is taking us down. Mind you I am not saying this would happen, but I can’t discount the prospect.


      2. “We might be lucky if 1% of these 64 billion planets have biological activity”

        No. More like 0.00000001%

  9. I find it difficult to get excited about this aspect of astronomy. Unless we can actually travel to other stars, all this talk is just hypothetical ideas. Even if there are Earth-like habitable worlds out there, we will never see them up close. I fail to see the excitement in research where we will never, ever, know for certain if the findings are correct. Scientists are putting a lot of faith in what their data (might) be telling them.

    1. It was once hypothesized that we would never know what stars were made of, because they’re too far away to be sampled. But thanks to spectroscopy we do. I hold out hope that one day our telescopes will be good enough to resolve features on exoplanets as well as we can resolve features on Mars.

  10. I’m glad science is starting to formulate the likelihood of an inevitable future encounter with life from elsewhere. I have always been pragmatically philosophically about this. I honestly don’t think it will be any bigger deal than discovering new life in Earth’s forests and oceans, which happens every day and no one seems to care. I think we’ll shrug and just go on our way, realizing all along that yes, look it all of those stars – it just had to be so, and that would be the end of the discussion. It would be nice if there is actually communicative intelligence in the alien life and we can all brainstorm about how the Universe came into being and what sort of God neither of us can see would create such a vast cosmos. But let’s hope they aren’t as religious as we are – I hate senseless wars as much as anyone does.

  11. I think one of the main issues is that we are mortal and will die before anything really happens. Most people don’t care about whether humans in 100 years will make it to another planet because they wont be alive.

    But with bio-engineering and genetic this and that, if we could design our bodies to live longer, healthier…etc etc….then time wouldn’t be an issue. I wouldn’t mind going on a 1000 year space voyage. 🙂

    Make our bodies more adaptable, survivable etc and we might be strong and healthy enough to make long time consuming trips.

    Just a thought.

  12. By nature, man has the thirst for knowledge, and if so, it searches for life on other planets and exoplanets, he will find it.
    The search for signals (SETI for example) at a reasonable cost, must be promoted.
    For over 40 years, I maintained that life lived more planets there were people on our own.
    I also really want to know more about our universe!
    I invite you to visit my web site :
    It describes a scenario in cosmology.
    You will discover at least new geometric shapes.

  13. By nature, man has the thirst for knowledge, and if so, it searches for life on other planets and exoplanets, he will find it.
    The search for signals (SETI for example) at a reasonable cost, must be promoted.
    For over 40 years, I maintained that life lived more planets there were people on our own.
    I also really want to know more about our universe!
    I invite you to visit my web site :
    It describes a scenario in cosmology.
    You will discover at least new geometric shapes.

  14. By nature, man has the thirst for knowledge, and if so, it searches for life on other planets and exoplanets, he will find it.
    The search for signals (SETI for example) at a reasonable cost, must be promoted.
    For over 40 years, I maintained that life lived more planets there were people on our own.
    I also really want to know more about our universe!
    I invite you to visit my web site :
    It describes a scenario in cosmology.
    You will discover at least new geometric shapes.

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