60 Billion Habitable Planets in the Milky Way Alone? Astronomers say Yes!

A new study suggests that the number of habitable exoplanets within the Milky Way alone may reach 60 billion.

Previous research performed by a team at Harvard University suggested that there is one Earth-sized planet in the habitable zone of each red dwarf star. But researchers at the University of Chicago and Northwestern University have now extended the habitable zone and doubled this estimate.

The research team, lead by Dr. Jun Yang considered one more variable in their calculations: cloud cover. Most exoplanets are tidally locked to their host stars – one hemisphere continually faces the star, while one continuously faces away. These tidally locked planets have a permanent dayside and a permanent nightside.

One would expect the temperature gradient between the two to be very high, as the dayside is continuously receiving stellar flux, while the nightside is always in darkness. Computer simulations that take into account cloud cover show that this is not the case.

The dayside is covered by clouds, which lead to a “stabilizing cloud feedback” on climate.  It has a higher cloud albedo (more light is reflected off the clouds) and a lower greenhouse effect. The presence of clouds actually causes the dayside to be much cooler than expected.

“Tidally locked planets have low enough surface temperatures to be habitable,” explains Jang in his recently published paper. Cloud cover is so effective it even extends the habitable zone to twice the stellar flux. Planets twice as close to their host star are still cool enough to be habitable.

But these new statistics do not apply to just a few stars. Red dwarfs “represent about ¾ of the stars in the galaxy, so it applies to a huge number of planets,” Dr. Abbot, co-author on the paper, told Universe Today. It doubles the number of planets previously thought habitable throughout the entire galaxy.

Not only is the habitable zone around red dwarfs much larger, red dwarfs also live for much longer periods of time. In fact, the Universe is not old enough for any of these long-living stars to have died yet. This gives life the amount of time necessary to form. After all, it took human beings 4.5 billions years to appear on Earth.

Another study we reported on earlier also revised and extrapolated the habitable zone around red dwarf stars.

Future observations will verify this model by measuring the cloud temperatures. On the dayside, we will only be able to see the high cool clouds. A planet resembling this model will therefore look very cold on the dayside. In fact, “a planet that does show the cloud feedback will look hotter on the nightside than the dayside,” explains Abbot.

This effect will be testable with the James Webb Space Telescope.  All in all, the Milky Way is likely to be teeming with life.

The results will be published in Astrophysical Journal Letters (preprint available here).

31 Replies to “60 Billion Habitable Planets in the Milky Way Alone? Astronomers say Yes!”

  1. and if these planets are tidally locked, they would not need a moon to stabilize the axis of rotation (earth/moon system) and maintain a relatively constant climate (as is the case with the earth moon system). Life should flourish and evolve EVERYWHERE!!

  2. There’s life out there. In vast abundance. Now all we have to do is find it.

    Exciting times.

  3. IF the planet is constantly covered by clouds, wouldn’t it be much harder for photosynthesis? I know life exists in almost every corner of the earth, including the hottest, coldest, and deepest parts of the ocean under the highest amounts of pressure, but habitable to me means habitable to humans. This means plant life must exist, but how can it without proper photosynthesis?

  4. It must be remembered that Venus is in the habitable zone and its rotation is so slow as to maybe approximate some of these tidally locked planets around M-class stars. Venus is anything but habitable,

    1. As I understand it the main hypothesis is that Venus was once habitable, up to perhaps 2 billion years of age.

      The reason why it lost its habitability is likely because it lacked the strong geomagnetic field that we have, which meant it lost its water from photolysis and solar wind interaction. (Arguably so, since the current hothouse makes for a changed mantle-core behavior.)

      Venus is a nice prototype for tidally locked planets, since it shows clouded and/or dense atmospheres prevent a zonal temperature gradient and high surface winds. We wouldn’t like it, but large animals could stand the wind speeds: “The winds near the surface of Venus are much slower than that on Earth. They actually move at only a few kilometres per hour (generally less than 2 m/s and with an average of 0.3 to 1.0 m/s), but due to the high density of the atmosphere at the surface, this is still enough to transport dust and small stones across the surface, much like a slow-moving current of water.[1][21]” [ http://en.wikipedia.org/wiki/Atmosphere_of_Venus#Circulation ]

      1. One thing that I think is very apparent is that the diversity and complexity with planetary formation and structure is enormous. When I was in high school there was a book in the school library that I found fascinating on astrophysics, stars, planets, neutron stars and so forth. They presented this idea about planetary formation and it had a protoplanetary disk that was thick in the middle and tapered off near the star and the outer region. This meant that planets close and far would be smaller, sort of what is seen in our solar system. The stellar wind would then blow light gases away from the interior region and the nearby small planets would be rocky. Consequently the theory predicted stellar systems of planets rather similar to our solar system. Of course what we have found of extrasolar systems is this is knocked out of the water.

        The current issue with planets in the habitable zone is probably similar. The diversity of surface and interior configurations for such planets is probably larger than we can even imagine. I have no doubt that out of 60 billion planets in the habitable zone that some percentage of them may be similar to Earth. However, observing a planet in the habitable zone may weakly qualify a planet to be Earth-like. To hold complex life it probably must be in a fairly stable orbital configuration. I doubt there can be a lot of gravitational billiard ball interactions that knock orbits chaotically around, maybe in and out of the habitable zone. With these M-class stellar systems, such as Gliese 667 the planets are in close to each other, and they are super-Earths that makes them gravitationally interact a lot. So I suspect it may be a tiny percentage of these planets that hold complex life such as Earth. Of course there may be far more which have elementary life forms. Mars might be such a planet. The Jovian and Saturnian moons with subsurface oceans might have some basic life forms that can live in very low energy flow conditions.

        I am not exactly a “lucky Earth” advocate, and certainly no panegyric of extreme forms of this argument such as Gonzalez who claims condition on Earth are so rare as to require intelligent design or creationism. However, I don’t think the universe is brimming over with planets like Earth. There may be a fair number of planets a bit like Mars and even more like Venus. There may be planets with physical and chemical conditions on their surface outside of anything we have pondered.


      2. “”However, I don’t think the universe is brimming over with planets like Earth.””

        Probably not, and I also think even if there are many perfect earth like planets, the majority of them are devoid of any life whatsoever. My opinion is, though this article tells us there may be many HZ bodies to look for life on out there, the notion is akin to discovering there are over 40,000 mine shafts in Nevada so therefore, excitedly, there is “probably” gold ore in a high number of them so the mine fields of Nevada are teaming with gold.. Yes, we can find gold in mine shafts but mine shafts don’t cause the gold thus majority of goldless mine shafts. I think there are probably billions of HZ bodies out there, but whether are not life occurs on them is due to unknown criteria in addition to the myriad of conditions you point out.

      3. A planet in the HZ is certainly a necessary condition, but it is most certainly not sufficient. We have far too little data to determine what the statistics are for planets in the HZ to harbor life, and probably even less frequently complex life.


      4. There are a variety of reasons(many of which you detailed in your post) that Venus is no longer habitable, but size relative to Earth isn’t one of them. They are virtually the same size, with Venus’s diameter 12,104 kilometers, and Earth’s 12,756 kilometers.

      5. I suspect that the slow rotation of Venus played a role. Venus may have had a moderate environment early on. However, the long duration of solar radiation on any given side probably caused lots of water vapor which over time caused the atmosphere to have lots of this IR trapping gas. This may have pushed the environment towards an increasingly hot condition.


      6. Good post. One thing to consider with regards to larger Earth sized planets is that although they will retain more atmosphere and more clouds presumably, they will also retain a thicker coat of CO2. Larger size therefore is more of an advantage for planets on the colder end of the habitable zone. Venus is a cautionary tale in this regard since it’s smaller size should have been an advantage. It’s lack of rotation does not seem to have been a significant factor according to this study, as as you explained. It’s achilles heel would be it’s lack of a magnetic field as you referred to. Another important factor affecting how soon Venus turned into a hothouse is how much water it started with, which at this point we have no way of knowing. It is also of great interest as to why Venus and Earth’s interiors turned out so differently despite their similar size, endowing only one with a protective magnetic field.

  5. Hmm, but isn’t there a big problem with precipitation on tidally-locked worlds? Evaporation occurs on the light side, moisture moves to the dark side and precipitates out. There it freezes and basically never returns to the light side. Over time, the light side loses all it’s water…and no water means no life (as we know it). Am I wrong here?

  6. Photosynthesis is now known to be far less of a factor needed to support life than previously considered, taking into account tidal force friction and geothermal processes and such as substitutes.

    1. True, but in all fairness to Mr. Sias’ point: ‘alt-photosynthesis’ sources of energy for animals or plants are very very limited compared to, say, ocean algae or terrestrial plants. At least that’s the case on Earth…

  7. Taking into account the theoretical age of Red Dwarves to be into the trillions (far longer than the BBC series lasted), statistically that would suggest that a truly TEEMING Universe would exist much farther into the future than we should ever last… Not that there isn’t butt tonnes now, of course, but advanced civvies would reasonably be MUCH more abundant with more stewing….

  8. Since it nearly doubles the latest estimate, it would be more like 200 billion habitables.

    But my main reaction is not how large a factor we will end up multiplying the habitables with. Instead it is the maximum number of planets the habitable zone (HZ) can contain.

    Our own system likely started out with 3 habitables. The maximally packed red dwarf Gliese 667C had 3 with the old models of HZ. And I have already noted that we can expect to see at most 4-5 habitables around larger stars like our Sun, if the HZ is packed.

    This confirms that we will see some HZs that have more than a few habitables. At places in our universe there will be modestly separated binaries that happen to have their HZs packed with super-Earths. What fascinating systems they would be, if all planets are not already inhabited one could terraform, having 6 or more planets to travel between.

    We should perhaps call them Firefly systems.

  9. If the universe is designed, or a virtual reality, the distancing of galaxies is ingenious b/c it prevents low-intelligence species from reaching each other and eventually arming up against one another or blowing each other up.

  10. The atmosphere would probably circulate warm air to the night side and cool air to the day side.

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