Q&BA: How many exoplanets have been discovered?

By Phil Plait | April 26, 2012 12:52 pm

[Q&BA is a live video chat session I do every weekend, more or less, on Google+ where people can ask me questions about space and astronomy.]

I’m very excited about all the news we’re getting of planets orbiting other stars. For Q&BA I got a good question about them: How many exoplanets are there?

[Note: the aspect ratio on this video is messed up a bit, like it was on the last one. I understand the problem now, but cannot fix it in this video. They should be back to normal next time!]

I wonder how many of the thousands of candidate planets known will turn out to be real? Probably most of them. And there are billion, hundreds of billions, of planets in our galaxy alone! How many of those are like Earth? Maybe soon we’ll know.

I don’t care if it’s a curse or not. We do live in interesting times.

I have an archive of Q&BA links and videos. Take a look and see if there are other ones that tickle your imagination.

Related Posts:

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Q&BA: Can we build a space habitat?
Q&BA: The Science of Science Fiction
Q&BA: How does a gravity slingshot work?
Q&BA: Why spend money on NASA?
Q&BA: What happens if you are exposed to the vacuum of space?

CATEGORIZED UNDER: Astronomy, Cool stuff, Q & BA

Comments (21)

Links to this Post

  1. Science Tidbits for June 20, 2012 « Teaching Sapiens | June 20, 2012
  1. chief

    Dang. Born to soon to take the trip out to see them.

    I agree Phil, It is a remarkable time to be here on the threshold of discovery of new worlds. It will never happen again. Kids will grow up knowing there are other earths out there and something to reach for in their future.

    Thanks for the information and clarification of how detection is done through three axis of detection methods.

  2. Chris

    A green tennis ball representing a star. Phil, there are no green stars! :-)

    But if you want a list of all the confirmed exoplanets and properties look at The Extrasolar Planets Encyclopaedia

    More fun than planning your vacation!

  3. Let’s do the math the easy way. 400,000 Earth like planets. Milky Way radius is 50,000 light years. 3.14 * 50^2 / 400000 = 19,635 square light years per earth. They should be about 140 light years apart? OK, so the thickness of the galaxy matters, and they’re farther apart.

  4. Thanks for your Q & BA, Phil I am enjoying your point of view. I would love to get you to come on my weekly radio program devoted to astronomy and science every Friday night from 8-10PM EDT. Please and thank you! ~ Francis

  5. Again, something pretty radical to think about too. We have some very big biases in our ability to detect these extra solar planets. For the “wobble” method, the bias is towards planets that are large and/or close. For the transit method, we have the bias that the planet and star must align from our point of view (where only 1% or so are actually going to be detectable according to this page: http://certificate.ulo.ucl.ac.uk/modules/year_one/NASA_Kepler/character.html ).

    I think we are actually under estimating.

  6. andy

    What’s interesting is how the different databases define their cutoffs. Wikipedia seems to work on the basis of a hard 13 Jupiter masses cutoff based on the deuterium fusion criterion for solar-metallicity objects. This has the interesting result for Upsilon Andromedae that the second object from the star (Upsilon Andromedae c) is not considered a planet but a brown dwarf, despite the fact that it appears to be part of a non-hierarchical multiple system (more like a planetary system than a stellar system which tend to pair off).

    The Extrasolar Planets Encyclopaedia takes a criterion of 25 Jupiter masses based on the shape of the mass distribution which falls off above ~20 Jupiter masses (but they admit that this is a rather arbitrary distinction). This allows for the possibility of giant planets whose evolutionary history includes a stage of internal deuterium fusion. EPE generally does not consider free-floating “planetary-mass” objects, but there are already surveys of clusters which suggest the stellar mass function extends below the deuterium fusion threshold, i.e. there are objects which form like stars that never ignite fusion reactions.

    At the moment the evidence of the currently-known exoplanetary systems strongly suggests to me that using fusion as a criterion to distinguish planets from low-mass stars (brown dwarfs) is the wrong way to go… the universe is perfectly happy to build both fusing planets and non-fusing stars.

  7. SLC

    I have been having a running discussion over at Larry Moran’s blog with an old earth creationist, who calls himself Denny, and who claims that planetary systems can only be formed in spiral galaxies. I would greatly appreciate it if Dr. Plait could comment on this claim as I fail to see why it should be so.


  8. Rayceeya

    I was so happy when the existence of extra solar planets was confirmed. Since then the data has only gotten better.

    At this point I’m more interested in estimates for the total number (in our galaxy).

    We’re actually narrowing down on some of those Drake equation factors.

    It’s a good time to be alive.

  9. Torbjörn Larsson, OM

    @ SLC:

    That seemed like an odd claim, so I checked it out. A very confused subthread seemed to have started with this:

    “First, there are not going to be any aliens, as you think of them. Ask Lawrence Kraaus, Victor Stenger, Neil deGrasse Tyson (“universe on a one way journey to oblivion”) and many more atheistic naturalistic cosmologists, astrophysicists, etc. As I have said before, roughly 90% of the universe’s galaxies are no longer producing stars (no stars = no planets). Goldilocks planets have shown no realistic promise for life. Roughly 75% of the remaining galaxies are completely inhospitable to life. We’re all on our way to cold death. (Go check this with your cosmologist friends) Science confirmed the beginning and end of the universe.”

    While I’m not Plait, the astrobiology side interests me. So FWIW:

    This is, not surprisingly, fractally wrong.

    – It is most biologists that claim evolution is too contingent to have odd traits like elephant trunks or hominin linguistic ability repeated often. Putting this on Stenger et al is misdirection at best, perhaps misquoting.

    – Habitable planets, “Goldilocks” vs liquid water, are looked at because they are, well, habitable.

    – It is a fact that for instance Carroll describes that structure formation and so universal complexity starts simple, are roughly the most complex it will be (witness us!) and in the far future will dilute as the universe expands. In the creationist picture that wouldn’t happen as they put a complex creator first and the usual creationist doomsday cults puts a complex catastrophe last.

    That doesn’t mean there is necessary a beginning of the universe. If eternal inflation is correct it may well be future, and perhaps past, eternal in the inflationary parts, our universe being a non-inflating pocket universe in the larger universe.

    – Galaxy development is neither here nor there for life. Even if all galaxies were no longer star producing, there would be plenty of remaining biospheres. Let us say that the average star production ended halfway to today. That means most stars would be ~ 7 billion years or older.

    Our planet is ~ 4.5 billion years, and the biosphere will last until our sun matures toward a red giant stage and become too hot. Some predicts heating will speed geological processes and bury too much carbon for having plants using carbon dioxide in ~ 0.5 – 1.5 billion years. Else the oceans will start to boil in perhaps 3-4 billion years. So our planet will have a biosphere for ~ 5 – 9 billion years.

    But our star is much larger than most stars. And smaller stars seems to have at least as many terrestrials (and less giants) as per Kepler et cetera. So most habitable planets will last longer, with or without even running into putative problems of tidal lock of M stars. M stars can be 100s of billion of years old, and superEarths may retain their atmospheres for ~20 billions of years or more according to modeling papers. (You can google this.)

    Hence there will be plenty of biospheres around for over 10 billion years, even in the pessimistic scenario of galaxies.

    And really, I don’t think galaxy evolution is a closed issue as of yet. The “75% of the remaining galaxies are completely inhospitable” seems to be a typical creationist invented claim. We can see that all galaxies are still hospitable, even those who closed down star formation after a few billion years. This is the problem of discussing with crackpots, astrologers can’t tell you anything about astronomy and creationists can’t tell you anything about cosmology or biology but confuse the issue.

  10. SLC

    Re Torbjörn Larsson @ #9

    I am afraid that Mr. Denny is a textbook example of someone who has received his education from the University of Go0gle.

  11. Satan Claws


    Although it wasn’t part of the question I think it would have been worthwhile to mention that the “pictures” that often show up on mainstream media (newspaper, news websites) of extrasolar planets are not actual pictures. I’ve watched people asking questions in forums, somehow implying that they *think* that those pictures is what you’d actually see when looking through a “big enough” telescope. In the brief show of the iPad app, there was a glimpse of such “picture”, and I’m afraid that people don’t often read the caption, particularly the part that says “artist’s depiction”. The picture captures a lot more attention than a lengthy text, and I suspect that’s the final impression (the one that those paintings are what you actually see on a telescope’s ocular) that the readers of those media might get.

    In my humble opinion, it would have been a great opportunity to mention that in your explanation.

  12. Kappy


    You are in luck. Sure your “Old Earth Creationist” friend is unlikely to believe any source you send him, but just the other day I read an interesting article about habitable zones in other galaxies, and this one specifically talks about habitable zones in elliptical galaxies. http://www.centauri-dreams.org/?p=22613

    If you don’t want to read the whole blog, the short answer is that the metal levels are right that earth like planets can form in the particular elliptical galaxy they looked at, as long as the star in question is sufficiently far from the jet caused by the supermassive black hole at the center.

  13. SLC

    Re Kappy @ #12

    Thanks for the information. I was unaware that the presence of heavier elements was required for planet formation, although that makes sense as a solid core would seem to be required to start the process.

  14. Kappy


    I don’t think heavier elements are required for planet formation (you could have a gas giant without them). Habitable planets, as we know it, would require elements such as oxygen, iron, etc. I think there was some questions if elliptical and other galaxy types outside of spiral, would have similar heavy metal content to our Sun in their various stars (and subsequently planets), and it looks like the answer is YES!

    The blog was based on an article published in what I believe is a reputable scientific journal: http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=8490356

    Oh and for those interested, the galaxy they examined was M32: http://en.wikipedia.org/wiki/Messier_32

  15. SLC

    Re Kappy @ #14

    The link seems to indicate that a rocky core is required to accumulate the outer gas layers of Jupiter type planets. I have embolden the relevant text. I must admit that that came as a surprise to me as it indicates that such planets must have a solid core, i.e. they can’t be entirely gas.

    The answer is yes, based on the authors’ comparison of metallicity — in nearby stars and stars with known planets — to star clusters in two elliptical galaxies. While many factors have been considered that could affect a galactic habitable zone, Suthar and McKay focus tightly on metallicity, with planet formation dependent upon the presence of elements heavier than hydrogen and helium. Results from Kepler have backed the notion that metallicity and planets go together, although it’s a correlation that has so far been established only for large planets like the gas giants that are the easiest for us to detect. We can’t be sure that the correlation goes all the way down to planets the size of the Earth, but the idea seems logical.

    After all, we think Jupiter-class planets formed by the accretion of gases around a rocky core, and the formation of that core may be similar to the formation of Earth-sized planets. The authors think the correlation between stellar metallicity and planets will extend to smaller worlds, while recent exoplanet discoveries help us explore the relationship and extend our thinking to elliptical galaxies. The galaxies in question are M87 and M32, and the investigation is one that invokes the history of star-forming materials. After all, heavy elements are produced inside stars, so the concentration of metals depends critically on the generations of earlier stars.

  16. andy

    That paper is particularly interesting because of the differences between the two galaxies studied: M87 is a giant elliptical at the heart of the Virgo cluster, while M32 is a dwarf elliptical that is a satellite galaxy to M31, a.k.a. the Andromeda galaxy.

  17. Dean

    Wait a second. I thought ‘planetary candidates’ WERE those that they’ve seen at least 3 transits? And that they only confirm planets using other methods?

  18. @ ^ Dean : That’s not my understanding which was that a planetary candidate with 3 transits is considered as being confirmed and one with less transits (two you’d have to say minimum surely!) would be a candidate exoplanet. But I could be mistaken. Further studies such as those done by other groups and instruments and by noting gravitational pertubations by each planet acting on others in multiplaneatary systems where such worlds are sufficiently close – such as for some of the pulsar planets round PSR 1257+12 and some of the crowded Kepler stars add to our understanding and confirmation of these worlds.

    @2. Chris : “A green tennis ball representing a star. Phil, there are no green stars! “

    Well, actually there is Zubeneschamali (Beta Librae) which is often said to be green to the unaided human eye. (Click my name for one source.) Soemtiems it looks green to me too although how much of that is my own imagination and atmospheric conditions is an open question.

    There’s also some like Antares B, the red supergiants companion star which may also be green although it’s also possibly just an optical illusion created by contrast with its larger primary.

    @ 7. SLC :

    I have been having a running discussion over at Larry Moran’s blog with an old earth creationist, who calls himself Denny, and who claims that planetary systems can only be formed in spiral galaxies.

    A lot of ellipticals formed from merging spiral galaxies – and in the future our Milky Way barred spiral & spiral galaxy M31 will collide and form such an elliptical – so I’d say that can’t be right!

    Think I read somewhere online that one recent study has suggested that some stars still form inside elliptical galaxies contrary to previous thinking so that, again, would seem to indicate planets could well form in such ellipticals too.

  19. Messier Tidy Upper

    @ ^ See :


    For one article on how elliptical gaxies are still forming stars – and presumaby planets too. Okay they may be relatively metal poor but there are still probably planets forming.

    Of coures given their vast distance from us it is going to be incredibly hard to detect anyexioplanets in Elliptical galaxies.Far as I know only one exoplanet has ever been found in another galaxy :


    and that was in the Andromeda galaxy.

    Although some exoplanets may have been born in dwarf galaxies that were absorbed into our MilkyWay suchas the Saggittarius and Canis Major dwarfs.

    Finally this item :


    may be of interest here – a solar twin that’s more exact than any yet found and a promising target for future study.

  20. Dean

    No, I’m correct.


    “Many scientists are doing follow-up observing with ground-based telescopes to confirm discoveries. Confirmed planets are announced as discoveries.”


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