To be considered Earth-like, a planet must be rocky, roughly Earth-sized and orbiting Sun-like (G-type) stars. It also has to orbit in the habitable zones of its star—the range of distances from a star in which a rocky planet could host liquid water, and potentially life, on its surface.
“My calculations place an upper limit of 0.18 Earth-like planets per G-type star,” says UBC researcher Michelle Kunimoto, co-author of the new study in The Astronomical Journal.
There are about 400 billion stars in the Milky Way.
There are 28 billion (seven percent) G-type stars.
Six billion stars may have Earth-like planets in our Galaxy.
Exoplanets like Earth are more likely to be missed by a planet search than other types, as they are so small and orbit so far from their stars. That means that a planet catalog represents only a small subset of the planets that are actually in orbit around the stars searched. Kunimoto used a technique known as ‘forward modeling’ to overcome these challenges.
This study does not consider earth-like planets around other stars. Earth-like planets could be around red dwarf stars. The major advantage that red dwarfs have is they produce light energy for a very long time. It took 4.5 billion years before humans appeared on Earth, and life as we know it will see suitable conditions for 5 billion more years or so. Red dwarfs can exist for trillions of years, because their nuclear reactions are far slower than those of larger stars. Life both would have far longer to evolve and to survive. The total amount of habitable zone around all red dwarfs combined is likely equal to the total amount around Sun-like stars given their ubiquity. Red dwarf stars are the smallest, coolest, and most common type of star. Estimates of their abundance range from 70% of stars in spiral galaxies to more than 90% of all stars in elliptical galaxies. It is estimated that 73% of solar systems in the Milky way are Red Dwarf systems.
Abstract
We present exoplanet occurrence rates estimated with approximate Bayesian computation for planets with radii between 0.5 and 16 R ⊕ and orbital periods between 0.78 and 400 days orbiting FGK dwarf stars. We base our results on an independent planet catalog compiled from our search of all ~200,000 stars observed over the Kepler mission, with precise planetary radii supplemented by Gaia DR2-incorporated stellar radii. We take into account detection and vetting efficiency, planet radius uncertainty, and reliability against transit-like noise signals in the data. By analyzing our FGK occurrence rates as well as those computed after separating F-, G-, and K-type stars, we explore dependencies on stellar effective temperature, planet radius, and orbital period. We reveal new characteristics of the photoevaporation-driven “radius gap” between ~1.5 and 2 R ⊕, indicating that the bimodal distribution previously revealed for P < 100 days exists only over a much narrower range of orbital periods, above which sub-Neptunes dominate and below which super-Earths dominate. Finally, we provide several estimates of the "eta-Earth" value—the frequency of potentially habitable, rocky planets orbiting Sun-like stars. For planets with sizes 0.75–1.5 R ⊕ orbiting in a conservatively defined habitable zone (0.99–1.70 au) around G-type stars, we place an upper limit (84.1th percentile) of less than 0.18 planets per star.
SOURCES - UBC, Astronomical Journal, Wikipedia
Written By Brian Wang, Nextbigfuture.com
Brian Wang is a Futurist Thought Leader and a popular Science blogger with 1 million readers per month. His blog Nextbigfuture.com is ranked #1 Science News Blog. It covers many disruptive technology and trends including Space, Robotics, Artificial Intelligence, Medicine, Anti-aging Biotechnology, and Nanotechnology.
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Brian, something is messed up with the password reset on this site. I get the email but then click the button in the email and just get the following text in a :
{“success”:false,”type”:”EmailResetPasswordEnd_EmailIsAlreadyVerifiedByOtherUserError”,”code”:401,”data”:{},”status”:”Email ‘[email protected]’ is already verified by other user! (endpoint: GET /reset-password-end)”}
Impossibly optimistic. A lot of things had to happen for this galaxy, our star, our solar system, and our planet to come into existence, especially as early as they have after the Big Bang; yet the first intelligence to spread beyond its birth system (something we haven’t done yet) probably precludes any other species from arising naturally in that galaxy.
Both smaller (so long red dwarfs, Goldilocks zones are too small, planets move, tidal effects are too strong, the flares too nasty and close, and planetary cores cool, so time is not on life’s side) and bigger stars won’t cut it and our star is pretty rare even in it’s weight clas: (https://www.youtube.com/watch?v=TAQKJ41eDTs&list=PLinphdV9a3zH8PI50iB8ul2bcOAI-IfD0)
Also the percentages of elements on the planet are essential (and some, such as phosphorous and iodine, for example, might be very rare in large parts of the galaxy due to the rare–and different–types of supernovas that produce them), and life needs to arise soon or not at all due to radioactive decay issues. Also, the collision that formed the Moon was essential for many reasons (but a super unlikely event in itself). And it all had to stay relatively stable for all that time and it took this long for us to get here (and even that wouldn’t have happened without a long series of near-extinction events to accelerate evolution without quite wiping out everything) and yet life doesn’t have much more than another billion years without intelligent intervention.
As I said, methanogens and extrophiles are still based on liquid water, the same carbon chemistry, DNA (or maybe RNA), etc. They’re pretty much still “just like us”.
Assuming you mean Newtonian. This is not correct. On pure mathematical bases, assuming you can transform your entire culture toward the singular objective of developing the technology and all associated technologies, low-to-mid-sublight is entirely feasible on some sort of Newtonian momentum exchange. This is a social engineering problem more than anything, mobilizing resources, physical and intellectual. And there are stars within 40 light years of interest.
We have been studying methanogens and extrophiles for a very long time, so this whole “we only know how to look for life that looks just like us” stuff is a very outdated objection, and is not at all accurate anymore. This isn’t 1950.
The chemistry that can be seen in an atmosphere is far from proof (and that is the only useful thing we are likely to detect even with very large telescopes). We have no idea what chemistries are possible without life. We conjecture that oxygen is the telltale sign…but we don’t know.
The best evidence would be a several thousand megaton fusion or antimatter explosion away from a star or other large celestial body. But if we detected something like that, it probably would be a bad idea to point them in our direction.
I see no reason there would be detectable signals from other solar systems. The power required to make a signal we could detect at this distance would be absurd. They would have no need to ever broadcast like that. The only way it could reach, would be some kind of tight beam which would not be likely to be pointed in our direction. But chances are they would use some other form of communication…possibly based on quantum entanglement, or something we haven’t thought of to communicate with one another that we can’t even eavesdrop on.
But the point of the calculation was to say how many there might be, not how many we might be able to find. And if that is what we are speculating about, like I said, we have no basses for a calculation, as it is not reasonable to decide liquid water is required. Initial immersion in a liquid or a gas is probably required for life…but even that is not guaranteed. And there are a lot of possible liquids or gasses.
You state that we are surrounded by neurotics to claim that this is natural. I would include parents and other caregivers amongst the neighbors. Other animals spend their *extra* energy on sex and reproduction. We spend ours supporting our System of evolved ritual. Our other *Darwinian* environmental dangers are trivial by comparison to most other species, because of our tech, such as fire. Goodall shows (edit: some) neurosis in chimps, but it is not organized! Once understood, the System must be destroyed.
The causation of this neurosis was the ever present environmental dangers including nearby neighbors who would be happy to rob, kill, and rape you. The easy life is only suitable for dodo birds on isolated islands. Humans like all life have no option but to evolve.
The issue is, for other types of life, we have a sample size of zero. So until we get an example of life developing under other base conditions, or using different chemistry, etc, it’s reasonable to assume that most life is likely to be similar (likely, not sure to be). Based on carbon and liquid water, likely make use of DNA or something very similar, lipids, proteins, etc. And so it makes sense to focus the search first and foremost where we:
A) Are most likely to find something.
B) Will be able to recognize it.
Our closest/farthest examples of “different” life are extremophiles. But even they are based on carbon, liquid water, and DNA (or maybe some of them RNA). And if I’m not mistaken, they evolved from life that originated in less extreme conditions (except maybe thermal vent archaea).
Finding mentioned in article: upper limit, no lower limit specifically mentioned.
Headline of article: “As many as…”
well, technically that’s correct.
The obsession with “Habitable Planets” is a rabbit track. Most of this world is not habitable for humans unless they have technology like clothing, shoes, shelter, and food preservation and preparation techniques. Space suits, space ships, recycling is just a natural extension of this. The only context it make sense in is that of “Alien Civilizations”. But that is dubious at best, because we are assuming all the intelligent life in the universe or most of it is like us…requiring the same temperature range, solar radiation, and other factors. With a sample size of 1, we have absolutely zero basses for bounding the prospect for intelligent life like that.
There was a time when fusion was perpetually 50 years away. Now it seems perpetually 10 years away. There seems to be an asymptotic curve in place.
Cephalopods seem rather intelligent too, and they do have limbs that can manipulate tools to some degree. However, a lot of technology depends on fire, which doesn’t work well under water. And furthermore, without view of the sky, any such intelligent civilization would have a very limited world view. There would be no incentive to develop astronomy, study the far universe, nor explore beyond the local environment. So many discoveries would not occur, or would be much less likely. An interstellar species pretty requires access to the sky.
Other than that, red dwarf stars may yet surprise us. For example, an Earth-sized moon around a gas giant in a ref dwarf’s habitable zone may be tidally locked to the giant instead of the star. The giant’s magnetic field may offer some protection from the flares.
Even for Earth-like planets tidally locked to the star, it has been hypothesized that increased cloud formation on the sunny side may improve habitability. The advantage of such stars is first their sheer number, and second their long-term stability, which would give life much more time to evolve. If the flares don’t happen too often, they may actually promote evolution, as mass-extinction events have done on Earth. Over time, the local species may evolve resistance to flare events.
The bigger issue for red dwarf planets is water retention. A thick ozone layer and a strong magnetic field may help with that. Or a large sub-surface reservoir, perhaps.
The proposed exoplanet round Alpha Cen B only has a period of twenty days. To be in the habitable zone it would have to be between 0.7 and 1.2 astronomical units away. Twenty days would put it closer to its star than Mercury, with an 88 day period, is to the Sun.
Creating and containing any considerable amount of antimatter is way beyond our capability now or the foreseeable future, indeed, but fusion is not. Remember the saying that fusion is 20 years away ?, of course it will probable be twice or thrice that, but eventually we will crack that nut, I’m certain plenty of our current fellow citizens will live to see fusion power plants.
Not really, you are talking about Proxima Centauri b, a confirmed exoplanet, but I was talking about Alpha Centauri B b, as I said, an exoplanet candidate.
http://www.openexoplanetcatalogue.com/planet/Alpha%20Centauri%20B%20b/
The earthlike planet was not discovered around Alpha Centauri B, but Proxima Centauri, also known as Alpha Centauri C. This is not a yellow star like the sun, but a much smaller red star, orbiting well away from the other two in the system.
Quite possibly true. Though that underlines that SENS won’t be enough to reverse that – at least not to the point that we reach 1 trillion sooner than with today’s growth rate. SENS can only add ~0.8% to the growth rate, but the decline in birth rates can remove more than that.
I agree that it is very difficult to imagine how you could develop a technological civilization under water, especially in the early stages.
And I’m kind of disappointed that I can’t think of SF authors who’ve tried to develop this. The only sort-of example I can think of is Harry Harrison with his West of Eden novels, where the entire technological base was genetic engineering. Starting with selective breeding (which clearly doesn’t have any restrictions underwater) and moving on from there. They still never got beyond about a 16th century level tech.
Considering the only example we have, marine ecosystems did produce very smart creatures, with big brains and complex social interactions (cetaceans).
But those were with the help of a branch of animals living on dry land for a long time (something Europa lacks), then returning to the sea with all the evolutionary learnings. And they don’t make tools, lacking limbs appropriate for that.
Truth is, we have no idea if an aquatic ecosystem can develop a tool making species, but my opinion (and it’s just that) is that ice or stone cloistered oceans are dead ends for technological civilization, even if they could produce the required brains.
Yeah, but that’s because of poor nations still keeping the population growing.
The population growth isn’t staying fixed. As developing nations do develop, they will stop having as many kids, as the Western nations have.
This is a consistent trend across all nations after some development threshold is reached, not just some wishful thinking.
Population will most likely peak a few decades from now once all the nations reach some basic level of development, and then it can remain stationary (or decline) if there is no other big societal phenomenon impacting demographic growth.
Some sort of beamed propulsion to get up to speed & electric or magnetic sail braking against the interstellar medium, seems like the least implausible means of interstellar travel to me.
Yes, that’s also what I’ve read, although I wrote 1 billion years to be on the safe side.
See also ‘Starlifting’ if you want an ambitious long term project for keeping the earth habitable.
https://www.youtube.com/watch?v=pzuHxL5FD5U
The human race will transform itself thru genetic engineering into a species that can live naturally in space. Imagine an organ that can fuse hydrogen and what kind of creature such an organ can power. Imagine the ability to navigate from star to star under your own power.
I read that the sun’s energy output will rise over the years so we have only 500 millions more years before the earth will be to hot to sustain life.
Meanwhile, on Arcturus V:
I’m not so sure about that. Alpha Centauri A & B are very close to the Sun size, and can have stable planets within their Habitable zones, in fact we already have an Earth-like candidate in Alpha Centauri B.
Four light years in less than a century is doable with fusion.
Current global birth rate is ~18 per 1000 per year, and death rate is ~8 per 1000. That comes out to ~1% growth per year. Starting from 8 billion with 1% annual growth, it would take about 500 years to reach 1 trillion. But then only another 70 years to reach 2 trillion. At 0.5% growth, it would take about twice as long, and with 2%, about half as long.
That’s a long time, sure, but wouldn’t that still qualify as “historically meaningful”? A few hundred years is just what we need to get to the level of multiple large space colonies.
Re SENS, it’s been discussed just the other day in another thread. Fully successful SENS just means reducing the death rate close to zero. That may be completely offset by reductions in birth rate.
Long before gravity tractors, star shades can easily mitigate an increase in solar output, up to a point. Either ones in Earth orbit, or bigger ones in solar orbit. Earth orbit is easier, obviously, but sun orbit can also let us move the solar system where we need it further down the line. A Shkadov thruster or smt similar.
On a smaller scale, it can even go inside Earth’s atmosphere – see J. Storrs Hall’s weather machine – https://foresight.org/the-weather-machine/
But sooner or later, we’ll still need to move Earth further out. I like ssbaker’s Moon tractor idea for that.
That’d be difficult. The Sun is too hot. If we need hydrogen, we’re much more likely to mine the giant planets.
Better yet, mass drivers flinging regolith. Cheaper, less otherwise-useful reaction mass, and higher Isp. As it happens, the side that needs to do the flinging is always the lit side.
You are so right: on the outside our HZ has plenty of room, but on the inside we are scarily close to the inner edge: the inner edge of the HZ is estimated by most astronomers at 0.97-0.99 AU, and since our sun is brightening at some 10% per gy, or 1% per 100 my, we only have about 500 my at the most, before it gets too hot for all higher life. About as much time as since the Cambrian diversity explosion.
Sobering thought.
A planet around a G8 or G9 star, or even a K0, K1, would have a much longer stable HZ lifespan, between 10 an 20 gy, and still be safe from tidal locking.
The nice thing about this study is that it considers true Earth analogs: G type stars, rocky planets between 0.75 – 1.5 Re, in the conservative Habitable Zone of their star.
I still find the 6 billion figure rather high, I myself and others, astronomers among them, came to a guesstimate of a few hundred (200-300) million in our galaxy, using more and stricter criteria, such as: being in the galactic disk (i.e. not near the core, not in the halo), only single stars (not binary), not too old or too young, suitable metalicity, not variable, etc.
But encouraging all the same.
I am afraid that the abundant red dwarfs are largely unsuitable for planets with (complex) life, because of flaring, tidal locking, etc.
We don’t even need little asteroids. Carefully placed and times rockets on the Moon will work even better, as long as we keep the orbital distance constant by pushing the Moon back in place when it is between us and the sun, and then pushing it away to tug at the Earth when the Earth is between the sun and the Moon. Obviously, this would take place over millions of years, so it is the ultimate “slow burn” of fuel.
Removing mass from a star over a reasonable tIme can decrease the energy output and provide a source of materials. Usually is found under the term ‘Star-lifting.’
Depends on what you think the phrase “known propulsion source” means.
We can wave our hands and gesture towards known energy systems, and calculate that if we could use them efficiently then we could, in time, make a suitable propulsion system.
But that’s kind of like Archimedes pointing to an oil lamp and saying that the burning of oil provides enough energy to propel a giant ship through the skies and around the entire planet.
Well, yes, burning oil DOES provide the propulsion energy for an A380, but it’s a big stretch to say that Archimedes knew high-bypass turbofans.
I would go with the line that the ancient greeks didn’t have any “known propulsion source” that could propel intercontinental aircraft.
Indeed, gravity tractors are very effective across hundreds of thousands or even millions of years.
A lot of very carefully arranged asteroid passes near Earth, repeated across a lot of time could move Earth gradually farther from the Sun and take care of the star’s natural increase of luminosity, whenever we start to grow concerned about it.
And again, the times involved are so long that it shouldn’t be a problem for any surviving technological civilization with space launch capabilities to keep Earth livable for many hundreds of millions or even billions of years more.
This is so practically doable with enough resources and patience, that they could indeed remake the Solar System to be much different than it is now.
Well … you were fine until you stepped in the shît.
“The Drake Equation way, way, way overestimates …”
Drake’s equation only has parameters, not overesimates.
variable = low, high, log MEAN, log SD
r• (form’n rate of stars/yr) = 0.200000, 10.000000, 0.346574, 3.912023
Fp (num planets) = 0.500000, 0.990000, –0.351599, 0.683097
Ne (num earths) = 0.001000, 0.100000, –4.605170, 4.605170
Fl (having life) = 0.100000, 0.900000, –1.203973, 2.197225
Fi (having intelligence) = 0.000010, 0.003000, –8.661034, 5.703782
Fc (developing com) = 0.010000, 0.300000, –2.904571, 3.401197
L (span of civ…) = 200.0000, 20000.00, 7.600902, 4.605170
You can do the sim, yourself, with a little knowledge of Monte Carlo methods (couple of hours of Google reading), and a bit of C, C++, PERL or R programming.
After about 12 days of programming, simulation, programming, fixing, testing, confirming, and so forth, 300 civilizations came out 12 billion years. 250 million discrete simulation windows. 24 hours of runtime..
The DRAKE equation is not as you say. It does not in and of itself overestimate anything. The parameters chosen can … result in NO civilizations, some, a bunch, to bazillions.
That’s up the researcher’s basis-of-choice.
Not Drake.
⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅
What probably will change everything for real will be first contact.
This will trigger our social instincts like nothing else. Overcome the threat together or all may be lost….
Schemes have already been worked out to mitigate the increased energy output of the sun. All it takes are some asteroids and mass drivers. Slow gravity tugging will physically migrate Earth in pace with the sun. No magic.
Motivation to do this should be high enough among our descendants.
While they are at it, they can practice on Venus and make it habitable. If they also move the excess carbon from Venus to Mars, they can have a third habitable planet. All it takes is delta-V and time.
Looks like Brian is the only one around here that hasn’t. XD
If we can get to “immortal mind self-discovery” tech we will also be able to speed things up, which means getting very small, and will be out of touch with anything over a few inches away.
“complex (multi-cellular) life” I’ve heard that the move from bacteria to nucleated cells was so hard that that may be the biggie that is so rare. I would add the notion that neurosis provides he energy for tech, that without it people would have relaxed upon the easy life achieved with control of fire, and stopped evolving right there. The hard part was staying stupid enuf to not understand/cure neurosis while getting smart enuf for spaceflight. O’Neill and Janov published within a cosmic instant of each other. Wheeler?
Antimatter is known and will do the job. Fusion might also be up to the task of ~.05c, under 100 years to Alpha C. Both are known, do not violate physics but are just way beyond our engineering capability now.
not convinced people will want to stay as weak, fragile, bi-pedal entities seeking out gravity, atmospheres, etc., when minds and sensory apparatus reach a certain level of complexity and durability. I doubt people will want to even stay in communities or families, much less cities or the space-station equivalent – a bit too Heinlein and Edgar Rice Burroughs and Ray Bradbury – very sentimental-60s-famili-ocentric. People will unplug, pick a spacecraft, hollowed-out-asteroid, etc., and disperse. People will care far too little about sunsets, authentic nature, and germo-phobic fellow human beings when a vast universe of immortal mind self-discovery is possible. O’Neils, Dyson spheres, toroidal/ annular habitats, etc., are all huge resource monstrosities and individual-freedom inhibiters. There will probably even become a time when the vast majority of human entity-minds are so distant from each other and spread out that they will hardly recognize nor acknowledge a common ancestry. That being said there will probably be a century of LEO and GEO space yards and habitats as humans start to mobilize their space-faring ways – likely 2050 – 2150 maybe. Less romantic sure, but we are all info-craving introverts at heart.
We will soon be living in O’Neill settlements, which can stay livable with mirrors and shields far outside the “habitable zone”, which is a limit on planets only.
“Highly developed nations all show a reduced birth rate.” While I agree with your points, I have encountered far too many young people who state that they do not want children for “small World” reasons, the sort of things only O’Neill can solve. This shows how critical understanding O’Neill is. What if the Japanese, the usu example, knew from childhood and thru society that the physical limits of Earth/Japan were illusory?
“Maybe people in the future will . . . make . . . kids their topmost priority”. Sorry for the deceptive edit, but the result states the end result of the Primal Revolution, which has begun.
Not convinced that the number of Earth-like planets (or red dwarf system equivalents) helps much statistically if we care about complex or even intelligent life. The Drake equation way, way, way over-estimates the likelihood of a technological society. I have no doubt that simple life is everywhere. I have very much doubt that the many, many of 100s of millions of ‘continuous goldilock’ years that it takes to develop complex (multi-cellular) life and even more for intelligent-ish life (simple day-to-day tools) is enough. The global seismic structures, geo-chemicals, macro-living areas of land-water, dynamic but still relatively safe asteroid/ meteor/ moon interactions with the planet surface, ideal solar protections offered by our VA belt, are so absurdly rare, emergent, and asymptotic-circular that if there is even a handful of electromagnetic-generating societies in our galaxy, I will be gob-smacked. And even then… I hyper-very-much doubt that a space-faring technological civilization will stay together as a community and reach out in a coordinated way to other systems. I figure from the time of 0.01c travel with easily-obtained ex-solar system fuel, near-immortal intelligent entities (if not bodies), and a limitless ability to store data and sense the universe around us — we will scatter. We all get our individual winnebago-hollowed-out-asteroids and out we go! There may be trillions of us, but we will not settle on the harsh and unpredictable surface of a planet.
thanks for the Isaac Asimov boil down… none of us knew those theories, read those books…
And no known propulsion source powerful and reliable enough to get us near a single one within 100 years…
You are very optimistic thinking that Earth will sustain superior life for another 5 billion years. That’s going to be when the Sun will become a Red Giant and engulf Earth, killing any lifeform, indeed. The problem is that we don’t need to wait that long, Earth will become unlivable much, much sooner.
Earth is right in the middle of the habitable zone of our system, but those habitable zones are not static, they move while its star ages. While the Sun depletes its hydrogen, it burns heavier elements, making it brighter every year. Around 1 billion years from now that increased radiation will wipe the water out of Earth’s surface, making the planet unlivable. Mankind would probably have disappeared way sooner than that, hopefully to go to Mars that it will then be in the middle of the Sun’s habitable zone (while the Earth has been becoming the inner edge and finally out).
We are not in the middle of Earth’s livability, we are in its final stages, fortunately final stages in planetary metrics are still an awful lot of time, hundreds of millions of year, so there is no need to panic … yet.
I don’t believe we will approach that many people alive in the Solar System in any historically meaningful date.
Highly developed nations all show a reduced birth rate. And a space civilization will need to be very developed just to exist.
What might change that is:
The first two seem more realistic to me, with the last one sounding like the inspiration for a future dystopia for liberals.
Most likely flare stars and tidally locked planets aren’t very good for the prospects of Earth-like life. Red Dwarf planets will probably be more interesting as resources for an interstellar civilization.
But as the Solar System has shown, liquid water always finds surprising places to be and in great amounts, only requiring to be protected from space. So probably there are a bazillion planets and moons with oceans, which may have simple or even quite complex life.
But these oceans will be entombed in ice and rock, not a great prospect for civilizations and even less interstellar ones. The greatest hope of finding another Earth still is in star systems with similar overall parameters, with planets in similar orbital distances, temperatures and attributes. The classic Goldilocks planets.
And we are just getting a faint idea of how common those might be.
So as we approach trillions of people living in this solar system, in O’Neill settlements, we can find other gravity wells to look at for life.