Earths may be common in the galaxy

By Phil Plait | February 18, 2008 9:40 am

One of the biggest questions in astronomy today is, are there any other Earths out there, and if so, how many?

We know that there are over 100 billion stars in the galaxy, and that a lot of them have planets. But we’re still groping round trying to nail down the frequency of planets, and how many are rocky bodies like Earth (as opposed to gas giants like Saturn and Jupiter).

A new study using the Spitzer Space Telescope has revealed that planets like Earth may be common! The procedure was actually rather simple. They looked at over 300 sun-like stars, and grouped them by ages (very young, young, middle aged, etc). They then used Spitzer to look for the presence of dust around the stars; dust glows in the infrared when warm, and the temperature (and thus the distance of the dust from the star) can be found.

What they found was striking: young stars had lots of dust that was at about the same distance from the star as the Earth is from the Sun, but as stars get older, the amount of dust drops. The timescale for the dust to disappear — a few hundred million years — is roughly the same timescale it takes planets to form. The obvious conclusion is that as time goes on, planets are forming around those stars, and they hoover up the dust. This process would make rocky planets, much like the Earth!

In other words, this study did not directly detect planets, but it found that planet-making material disappears with age. That’s incredibly provocative.

They found that 10-20% of young stars had these disks of dusty debris around them. As it happens, about 10% of the stars in the Milky Way can be categorized as sun-like, which is about 10 billion stars. If 10% of them have rocky planets, as this study indicates, then there may be a billion Earths orbiting stars in our galaxy alone! And that’s only for stars like the Sun; lower mass stars also can form planetary systems, and there are far more of them then stars like the Sun. It is entirely possible that there are many billions of terrestrial planets in the galaxy… and there are hundreds of billions of galaxies in the Universe.

The Universe may be buzzing with life. We still don’t know, but this is another big step forward.

CATEGORIZED UNDER: Astronomy, Cool stuff, NASA, Science

Comments (47)

  1. Now we just have to start aiming some ET detecting telescopes at those “less dusty suns” and start decoding their 100 year old satellite TV broadcasts. :o)

  2. Ken_g6

    I see how this could mean there are a billion stars with planets. But does this mean that all or most of those systems will form rocky planets?

    Or could a “hot jupiter” form in many cases by spiraling in toward the star, eating all the inner rocky planets along the way?

  3. matt E

    Absences of evidence is not evidence of absences. We will find life given enough time

  4. Thomas Siefert

    Now we just have to start aiming some ET detecting telescopes at those “less dusty suns” and start decoding their 100 year old satellite TV broadcasts. :o)

    Let’s not, you know it’s only going to be reruns of The Simpsons.

  5. Starring Kang and Kodos?

  6. Radwaste

    Say, Dr. Phil – how do stars in the local group stack up with this observation? Since the observation deals with such a large number, there should be another observation that makes this relevent to the first places we’d want to send probes.

  7. Would Venus and Mars count as “Earths” within the parameters of the study? Not all terrestrial planets are necessarily nice places to live.

    Did they look at other distances? For example, did they look for evidence of Jupiter-like, Mercury-like, and Neptune-like planets? Having a Jupiter-like planet at a comfortable distance in your system could be a good thing; it could sweep up a lot of the debris that would otherwise keep falling on your Earth-like planet while life is trying to evolve. (And yes, my use of the word “trying” is shamelessly anthropomorphic.)

  8. Thomas Siefert

    Starring Kang and Kodos?

    Yes, and Serak.

  9. zer0

    How far are these 300 or so stars including in the survey, more specifically the 10% that showed the potential for a rocky planet like ours?

    Also, on an unrelated note, is there currently any research that you know of concerning cheaper methods of launching into space? I remember reading some 10 years ago some proposal for an underground “railgun” or maglev type launching apparatus. Is this a feasibility, and if not what other methods are being pursued to get us into space faster, more often and cheaper?

  10. zer0

    Disregard 2nd question, unless you really want to talk about it, the power of google led me to some interesting web pages talking about very recent prototypes for these types of launch systems.

  11. zeb

    The ulimate irony of course would be if we found a whole bunch of those planets but they ended up being like Venus!

    Also, there are probably other ways an Earth-sized planet can evolve into than are presented in our solar system.

    Still, it makes me wonder and hope that some day we will find another true Earth.

  12. biologist

    Well, then,

    “where is everybody?”

    The universe teeming with life, and nobody stops by to say hello?

  13. One thing I’m skeptical about is the method they used to determine the age of the stars. For the most part the stars and the disks cannot be resolved. They get information about the disk from the IR spectrum of the star/disk combination. They get the age of the stars (according to the actual paper) from evolutionary tracks on the HR diagram – which rely upon the spectrum of the star/disk combination. Isn’t there some circular reasoning there? Or are the evolutionary tracks based upon the visual spectrum and we’re able to assume the visual wavelengths are entirely uncontaminated by the disk?

  14. Psychman

    This is very exciting stuff. But what interests me even more is that, despite the obvious enthusiasm and excitement, the vast majority of scientists would still accept later research that offered a different explanation. Yes, we can be as excited as kids on Christmas morning about a discovery, but it doesn’t blind us to the facts; we see the evidence and move on. Too bad the creationists cannot see this, or refuse to see it.

  15. andy

    Another report spun so that it seems the only interesting thing in extrasolar planet research is to find an Earth-clone with Earthlike life on it.

  16. Ozymandis

    O.K. We have proven that there is an infra-reddish glow around some distant stars. We assume that it comes from dust, but we don’t know – because, not only don’t we see any planets, we don’t see the dust either! If planets are too small, then dust is too!

    Next crazy leap of “logic”: all of this dust means that there are earth-type planets around these stars.

  17. Well, if you get right down to it, life is nothing but a series of chemical reactions. Complicated, yes, but still chemical reactions nonetheless.
    If you doubt me, I suggest finding an evangelical, grabbing him by the throat, and carefully observing the results. Note how the lips begin turning blue? That is due to lack of oxygen; in other words, an interruption of the ‘normal’ series of chemical reactions that make life possible.

    Of course, the last time I tried this experiment, a bolt of lightning came out of a clear blue sky and knocked my butt three blocks down the street while leaving the evangelical completely unharmed.

    Probably just a coincidence….

  18. Jack

    Um… isn’t what you’re saying exactly Drake’s argument for computing the likelihood that intelligent species will emerge given the right circumstances?

    I’m convinced there is other intelligent life out there. If the statistics already indicate a high incidence of Earth-like planets, although the sample is very small, this would suggest that there is a very high likelihood of an abundance of planets in Earth-type orbits around their primaries.
    Even if we then took the most scrupulously conservative estimate of the likelihood that life will eventually emerge on these planets [without counting the emergence of life on planets that were in less-than-ideal orbits], that has to translate into hundreds of billions of life-bearing planets throughout the universe.

    It’s very likely that most of that life will be of the plentiful-but-weird variety and not much fun to talk to. However, we also see, on Earth, that when the conditions for life are favorable, it will emerge in great abundance.
    Somewhere in there intelligent life is going to emerge. Even with a very conservative estimate put into all the factors of the Drake equation, that has to amount to a great multitude of intelligent civilizations. They may be too far away to be of any good to us [or, we may be lucky they can’t get to us] but there must be vast amounts of ‘others’ looking up into their skies and wondering about the very same thing.

    Here’s my question: supposed they were inclined to try and talk to us at all, what would they be using as a means to convey their message in an unmistakble way?

    On a practical level I’d very much like to know what their music will sound like.

  19. Ozymandias, perhaps you should do a little reading before you make claims like that. Look around for articles on protoplanetary disks, IR excess, and dust around nearby stars. We have an excellent grasp of this topic these days.

  20. Crux Australis

    To biologist, that’s the Fermi paradox, dealt with very very well by Stephen Baxter in his sci-fi novels (Time, Origin, etc). Best. Author. Ever. Present. Company. Excluded.

  21. Arthur Maruyama

    biologist:

    You asked:
    “where is everybody?”

    Some possible explanations:
    1) We’re first. Given that the universe started with the Big Bang and turned into mostly hydrogen, some helium and a tiny trace of everything else, it has to take some time for stars to produce enough of the “everything else” that life can develop in the first place. Somebody HAS to be the first technological species–no reason to suppose so, but no reason to suppose otherwise.

    2) We’re first locally. Given a 100-year “detection” limit for Earth (radio being about 100 years old), there are some 100,000 stars (by my back-of-the-envelope estimate) within that sphere. Even if there were 5 million technological species in the Milky Way alone, there would be only about 4 technological species within our detection limit (which is about 1-millionth the number of stars in the Milky Way). Of these species, we may be first.

    Note that this ignores 99.99999999% of the stars which are outside of the Milky Way.

    3) Life may be common, but technology may be rare. One of the factors in the Drake Equation is the number of intelligent species that develops technology–it isn’t a given. There are species on Earth which are arguably intelligent–chimps and other primates, cetaceans, octopi–but are not technological. Having no other models to check upon, this factor can only be guessed.

    4) Technology may be common, but surviving technological species are rare. One of Carl Sagan’s fears expressed in his Cosmos series was that technology may be self-limiting factor in detecting intelligent life. Technological species may be–by their nature–self-destructive, so there may be only a short time-window through which a technological species may be detectable. Before that window, there is no way of detecting it short of going there by some means; after that window, there may be no one left to contact or they may be incapable of radio-contact. While humanity’s situation has considerably improved when Sagan postulated this (mutually-assured destruction no longer being the nuclear weapons policy between the US and USSR/Russia), we still are not out of possible technological self-destructions (overpopulation, pollution, runaway greenhouse effect, probably others I cannot think of at the moment).

    5) Non-self-destructing technological societies may be common, but surviving ones may not be. Rather than ennumerate the number of possibilities myself, we should all await BA’s next book, “Death From the Skies!”

  22. SLC

    The notion that only earthlike planets can allow the evolution of life is too restrictive. How about moons revolving around Jupiter like planets? For instance, the moon Europa.

  23. Ian Kemmish

    Dr Iain Stewart’s excellent recent BBC TV series “The Power of the Planet” devotes one program to why it’s not enough just to look for rocky planets of approximately one earth mass.

    They need to have been hit at just the right time in their early history by a big enough rock. This gave us the moon (which I already knew) and our oceans (which I didn’t.). The odds against this one event alone seem staggering.

    They need (reasonable) plate tectonics. It’s easy to see why Mars doesn’t have any at all (too cold) but do we yet know why Venus recycles its surface so rapidly? Without plate tectonics and reasonably stable continents, life will never leave the oceans and therefore won’t ever invent radio.

    They need to go into a snowball mode at some point to nudge evolution away from the remarkably stable state of producing nothing but stromatolites forever.

    They need to come out of snowball mode some time later, in order to give all those new species a chance to develop into a Cambrian explosion.

    And so on…..

  24. Larry

    “One of the biggest questions in astronomy today is, are there any other Earths out there, and if so, how many?”

    Well of course there are. Slartibartfast and his associates on Magrathea churn them out regularly. Seems those hyper-intelligent, pan-dimensional beings really want that question about life, the universe, and everything answered.

  25. Torbjörn Larsson, OM

    The ulimate irony of course would be if we found a whole bunch of those planets but they ended up being like Venus!

    Which is why we wouldn’t want to look for Earth-sized analogues. AFAIK Earth is marginal for life – a little smaller and it would have been without plate tectonics and have a Venus-like atmosphere instead. Another benefit of plate tectonics apart from extending the habitable zone is that it gives more diverse climates and geography – more speciation possibilities.

    The good news is that simulations seems to give plenty of larger rocky bodies.

    They need to have been hit at just the right time in their early history by a big enough rock. This gave us the moon (which I already knew) and our oceans (which I didn’t.).

    Why would life need a large moon – for spin stabilization? But it could be a toss of the coin if a more stable climate is beneficial or harmful for evolutionary processes.

    I’m not sure what you refer to with the oceans. Water will be plentiful, perhaps excessively so. Plate thicknesses, and possibly the likelihood for thinner oceanic plates, will AFAIU not only depend on remaining material proportions after large impactors, but planet and atmosphere sizes as well. (See above.)

    do we yet know why Venus recycles its surface so rapidly?

    Good question. My earlier readings couldn’t give a consistent answer, but Wikipedia has an account of a more reasonable hypothesis. (Cyclical plate tectonics would explain both recent recycling and the hothouse atmosphere.)

    What is the current understanding?

  26. Radwaste

    There is a limitation hinted at but not directly addressed, also, in that high speeds in “normal” space produces lots of radiation. I think I remember 40,000km/s as the region where simply plowing through interstellar H makes you shine. Of course, any decent rock will make a good lightshow out of you and an example to others. One of you geniuses has to go where the bottom quark is, get into subspace and invent us a way around the speed limit, Bussard notwithstanding.

    So, BA, sir – what do the odds look like locally?

  27. Well, if the key to Earth-like planets is dust, there are entire solar systems full of them under my bed.

  28. Dave Kary

    To the question about how the age of the star is determined, zandperl is basically right that you can use the visible spectrum to get the star, because the disk just isn’t going to produce much visible light. On the other hand, by the time you get out a ways into the IR, the light is completely dominated by the disk. In fact, a star-disk combination has a very characteristic spectral shape, and you can even start picking out gaps in the disk from deviations in that shape. While the Spitzer survey is the latest example this type of work, I know groups that have been doing these sorts of surveys since the 80’s.

  29. andy

    Torbjörn Larsson, OM: I’m not entirely sure the situation with Venusian tectonics is just a matter of the smaller mass of the planet. Venus is also an incredibly dry planet compared to Earth, which means that it is much harder to drive plate tectonics on that planet (for a start mixing water in promotes melting of the rock at lower temperatures, which makes the plates more mobile). I’ve seen papers which suggest a dry Earth would not have plate tectonics.

    In the super-Earth regime the studies come out conflicting – one recent one suggests that even dry super-Earths would have plate tectonics, another suggesting that the high gravity would compress the rocks to an extent that plate tectonics couldn’t function. Neither of the papers in question seemed to explicitly mention water content.

    So Venus may not be the best comparison for determining the lower limit of planetary mass needed for plate tectonics – we need a comparison where the planet’s crust is in contact with liquid water. Therefore it would be very interesting to find out what’s happening on the ocean floor of Europa…

  30. StevoR

    Thanks.

    How old are Vega, Fomalhaut and Beta Pictoris? Were they included in this? All three are nearby A-type main-sequence stars -hardly ‘sun-like’ but famed for their dust disks discovrered back in 1983 (?) by IRAS.

    How “sun-like” counts as being sun-like?

    Are these planets or asteroids belts or – as noted by someone else – is this dust and any proto-exo-planets being swept aside or into Hot Jupiter’s migrating inwards?

    Its interesting, its suggestive – but there are still an awful lot of questions and investigations that need to be made.

    & while there may be a few other planets similar to Earth, there’ll only ever be one Earth -and we’re standing on it .. ;-)

  31. Wouter Lievens

    We’re finding gas giants in habitable zones by the dozens, so if these have rocky moons, it’s quite possible life-friendly conditions arise there.

    Think about it: we may live in a universe where 99% of the life-friendly bodies are gas giant moons, rather than rocky planets.

  32. ken davies

    There are 1000’s of trillions of stars in the Universe.

    Approximately 25% of all those stars have one or more planets circling them.

    Approximately 1% of all those planets have some form of biological life.

    God told me. (seriously)
    Not the God of the Bible — but the Supreme Being who created the Universe.

  33. ken davies writes:

    [[God told me. (seriously)
    Not the God of the Bible — but the Supreme Being who created the Universe.
    ]]

    They’re the same God, ken.

  34. Tom Marking

    “They need (reasonable) plate tectonics. It’s easy to see why Mars doesn’t have any at all (too cold) but do we yet know why Venus recycles its surface so rapidly? Without plate tectonics and reasonably stable continents, life will never leave the oceans and therefore won’t ever invent radio.

    They need to go into a snowball mode at some point to nudge evolution away from the remarkably stable state of producing nothing but stromatolites forever.

    They need to come out of snowball mode some time later, in order to give all those new species a chance to develop into a Cambrian explosion.”

    These are arguments straight out of Ward and Brownlee’s book “Rare Earth” so I’m assuming you’ve read that book. Of course, the thesis of the book is controversial and it’s still being debated. I’m not sure where I stand on the issue. I guess I would be more on the skeptical side of these claims that plate tectonics is essential, a large satellite is essential, a large Jupiter in the right place is essential, etc., etc. If taken to the extreme such arguments would suggest that Homo sapiens is the only intelligent organism in the entire universe, which seems patently ludicrous to me.

  35. Saw this posting from digg yesterday – what do you think of the logic of applying the 80/20 rule to where and how much intelligent life there is in the universe and galaxy. Conceptially makes sense……if you’re not religious.

    http://www.alienreviews.com/b/life-on-other-planets.htm

    Also, heard that there are religions that believe in life outside of earth – mormons?? Scientollogy???

  36. Tom Marking

    The 80/20 rule or so called Pareto Law seems to be a variation of Zipf’s Law which is used in linguistics:

    http://en.wikipedia.org/wiki/Zipf%27s_law

    This is nothing but a power law distribution which frequently occurs in nature in a variety of settings. Not sure how it applies to extraterrestrial life or how it helps us find it.

  37. Snake

    Its not whether our galaxy and others are teaming with life,its about interstellar travel.Hell even i have to stop for coffee when i travel by car.

  38. Thohry

    OK, there may be numerous Earth like planets out there, but it is a long way until we can ‘see’ those planet and even very very long way before we can see if there are any sign of life over there.

  39. Torbjörn Larsson, OM

    Old thread, no time. FWIW., @ andy:

    Sure, dry Venus. I’ve seen the idea, but IIRC a geologist I discussed it with was skeptical. And rightly the dryness can be an effect of the lack of plates, and the atmosphere that gave.

    But I see your point about the lack of answers (or even correct questions) all around. Thanks for the answer!

  40. khrystal

    i was just wondering. what if the earth is the most backwards of all life in the universe. What if “the others” can see us but we cant see them. They may be trying to contact us but we cant hear them. Very interesting topic.

  41. leah

    if thr r other “earths” then thr has to be water and land. if you think back to us, humans, then it goes back to the question how we were formed. was it some chemical or was it god himself?i do believe in god but not him entirely about creating us.That goes back to the question of other life forms. is it really possible that could happen, or is it just an illusion about there being more earths?perhaps in the future they may be more research, but i don’t think that anyone could prove how we were actually formed.

  42. Guest User

    there have to be more planets out there like earth we still dont know even a full 1% of our existance.take a good look at the planets its like a rose tree they bloom at a time when one go another comes, we see them all as dead planet some may have florish and died while the others are know building,earth took millions of man years to develope.so know one now or for millions of years will be able to solve the universe mystry .we are a dot in the universe microscopic from certain distance. we mankind is a virus to the planet,we will suck it dry and move on.

  43. Marty

    There may be life out there, but we are far from discovering it yet. Our technology is not that advanced. Sorry to quote from contact “if there isn’t any other life out there, it would be an awefull waste of space.”

  44. charlie

    is thare ether planets like your planet?
    is thare in on ether planets?

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