Kepler finds a mini solar system!

By Phil Plait | February 2, 2011 11:07 am

Using NASA’s orbiting Kepler observatory, astronomers have found a complete solar system of six planets orbiting a sun-like star… and it’s really weird: five of the six planets huddle closer to their star than Mercury does to the Sun!

None of them is what I would call precisely earth-like — they’re all more massive and much hotter than Earth — but their properties are intriguing, and promise that more wonderful discoveries from Kepler are coming.

The star, called Kepler-11 for convenience, is very sun-like, with a mass, size, age, and temperature very close to that of our own Sun. Located 2000 light years away, it’s way too faint to be seen without a good telescope, though. The planets, called Kepler-11 b, c, d, e, f, and g, were detected using the transit method: their orbits are edge-on as seen from Earth, so when they pass in front of (transit) their star they block a bit of its light. That dip in brightness is what Kepler detects. Over time, multiple dips are seen and those are used to determine the periods of the planets. The amount of light blocked by the planet gives you its size; the bigger the planet the more light it blocks. So the transit method tells you how many planets there are, how long it takes them to orbit, and their size.

But in the case of the minisystem Kepler-11, the planets are huddled together near their star like a close-knit family. And like any such family, the kids poke and prod each other: the planets are so tightly packed near the star that they all interact gravitationally! Each one pulls and tugs on the others, subtly changing their orbits, which in turn affects the timing of the transits. Applying a complex mathematical model to the timings allowed the astronomers to actually calculate the masses of the planets, something usually not possible in systems with fewer planets or more widely spaced orbits. What they found was that the five inner planets have periods from 10 to 46 days (Mercury, by comparison, takes 88 days to orbit the Sun), meaning they lie from 13.6 million to 37.4 million km (8.4 million to 23 million miles) from the star. The planet sizes range from twice the Earth’s size to 4.5 times, and have 4 to 14 times the Earth’s mass.

Once you have the size and mass, you can calculate the density, and that’s where things get interesting. The density of an object is a strong clue to its composition; low density objects may be mostly gas (meaning a thick atmosphere) whereas high densities imply a rocky and metallic object like the Earth. These planets are all over the place, from fairly dense to fairly puffy:

That graph shows the size of the planet (vertical axis) versus its mass (horizontal axis). Earth and Venus, being small and dense, are at the lower left. Neptune and Uranus, which are more larger and puffier, are to the upper right. The ellipses represent the planets of Kepler-11; uncertainties in the sizes and masses mean the real values could be anywhere in those ellipses (for example, Kepler-11 c has the least well-known mass, so its ellipse is quite long horizontally). The lines represent where you’d see planets of certain compositions, so a planet anywhere on that bottom solid line could be Earthlike in its makeup — though I’ll point out that doesn’t mean it is, just that it could be.

All of the newly found planets are more massive than the Earth, but Kepler-11 b has the highest density, putting it very near the Earth’s density, about 60% of ours, in fact. This means that it most likely does not have a super-thick atmosphere like a gas giant, but may be rocky with lots of water.

Again, none of these planets is earth-like in the sense of really being like our home planet; they are far too hot and massive for that. But the important thing to note here is that this system is, apparently, stable over long periods of time. The star is billions of years old, and assuming the planets formed at the same time as the star (which is kinda how it has to go) then the system must be stable. That doesn’t mean it hasn’t changed since it formed, however. Most likely the planets formed farther out and migrated inwards over time, as detailed physical models have shown can happen. Most likely the inner planets were also more massive when they formed, and lost a lot of their atmospheres as they got closer to their star.

I’ll admit, I was pretty skeptical when I first heard of this system. With planets packed that closely together they can interact pretty strongly, and I’d think that would be unstable over billions of years. At some point one of the planets would get a bit too close to another and get chucked out of the system or dropped into the star, making a hash of the other orbits at the same time. However, after reading the paper, the detection of these planets looks really solid, so clearly nature is telling us we have to be careful — once again — with our assumptions and prejudices.

But this news is very cool. Kepler will probably find quite a few systems similar to this one, and this new technique of using the slight changes in the orbital periods to get the planets’ masses is really amazing and useful. I’ll note that finding a solar system like ours, with well-spaced-out planets, will take years to confirm since the planets take much longer to orbit their star; we see fewer transits from them. But this shows that Kepler can easily see planets not too much different than Earth when they transit a star very much like our Sun, and that stokes my hope that soon, maybe very soon, we’ll detect a truly earth-like world orbiting another star.


Image credits: transit art: ESO/L. Calçada; graphs: Nature magazine.

Related posts:

- Gallery of exoplanets: real pictures of alien worlds
- Big news: first solid exoplanet found!
- Exoplanet found from another galaxy!
- Kepler works!

Comments (52)

  1. Very cool, but shouldn’t it be “mini star system” instead of “mini solar system”? Or “mini Kepler-11ar system”?

  2. Ron1

    Welcome to the new Golden Age of Exploration.

    or,

    to quote Dory the fish, ” Keep on funding, keep on funding …”
    Cheers

  3. Happen to know the KIC number for Kepler-11? I think it might be 006541920. If so, it’s still available for adoption on Pale Blue Dot.

  4. Chief

    Reading the description of the orbits and planets in it, I am aware of how big our own solar system is just out to mercury. I have no doubt that the next few years will bring the news of a close match to our own system.

  5. Sansuki

    Wait, did I see that right? A 0.9-Earth mass planet with a blackbody temperature in the liquid-water range? Was I hallucinating in this press conference!?

  6. That dip in brightness is what Kepler detects.

    Ah, Kepler can tell when I’m in a meeting.

  7. Or “mini Kepler-11ar system”?

    Demonstrating why such pedantry is silly.

  8. @NBwaW

    Yeah. I forgot to use the humor font.

  9. Ben

    That was only half the announcement:

    “Of the 54 new planet candidates found in the habitable zone, five are
    near Earth-sized. The remaining 49 habitable zone candidates range
    from super-Earth size — up to twice the size of Earth — to larger
    than Jupiter. “

  10. andy

    Certainly looking like many “super-Earths” are actually “mini-Neptunes”. That may have negative implications for habitability: if they have too much hydrogen then the high pressures and temperatures may prevent oceans from forming. Even with smaller quantities of hydrogen you may end up with an “ocean planet”: the liquid water ocean would be separated from the the rocky core by thousands of kilometres of high-pressure ices, which could lead to waterworlds too nutrient-poor for life.

  11. bk89

    Wonder why they’re huddled so “close” together… low mass?

  12. Gus Snarp

    “more larger”? In a hurry, Phil?

  13. for example, Kepler-11 c has the least well-known mass, so its ellipse is quite long horizontally

    Wouldn’t the large horizontal ellipse of an error indicate that we know it’s radius quite well, but are uncertian about it’s mass? Or am I reading the graph incorrectly? As I see it, C has a radius fairly well established at 3.2, but has a mass anywhere form 7-18 earths? Or are the axis on the graphic misslabeled?

  14. Dean

    Just watched the news conference. The thing that I can’t figure out is how they’re expected to announce a planet rather than a planetary candidate of an Earth sized body with a period of about a year.

    It’s not likely to have a multiple star system, so that technique of confirmation won’t be available. It’s not big enough or close enough to use doppler measurements.

  15. Erwan

    @ #3
    Correct, the paper’s figure 1 gives that KIC id.
    Only a few lightcurves released so far, but you can see the many little dips!
    http://archive.stsci.edu/kepler/data_search/search.php?action=Search&ktc_kepler_id=6541920

  16. CB
    for example, Kepler-11 c has the least well-known mass, so its ellipse is quite long horizontally

    Wouldn’t the large horizontal ellipse of an error indicate that we know it’s radius quite well, but are uncertian about it’s mass? Or am I reading the graph incorrectly?

    You’re reading the statement you quoted incorrectly. Yes, it’s the mass that’s most uncertain, as Phil said. :)

  17. sHx

    Briefly,

    A star like our Sun…

    A system with six planets…

    Each planet bigger and more massive than the Earth…

    The system slightly larger than Mercury’s orbit…

    Does the Kepler 11 data rule out the possibility of more planets at longer distance and/or outside Kepler 11′s orbital plane?

    I just would like to be the first to speculate that we may well find systems with a hundred planets within twenty years with the current speed of discovery.

    I can’t wait for James Webb launch.

  18. Steven Spray

    I’m amazed at the speed with which Phil releases stories like these while the announcement is still in progress!

  19. Can someone tell me how much (if anything) we can learn about an exoplanet’s atmosphere and even the presence of life with our current technology? What about technology projected into the near future? Thank you.

  20. Gonçalo Aguiar

    Is there anyway you could provide the light curve in which these discoveries were made?

  21. SC

    I’m always curious how planets could possibly migrate inwards. How has this not happened in our own solar system?

  22. @CB (#16) DOH! YOU are correct! Thanks. :D

  23. Joseph G

    Yayyyy exoplanets! I lurrrrrv me some exoplanets!!! Even better if they aren’t gas giants! Hooray for Kepler! :)

    Just curious, what exactly is the proposed mechanism for planets migrating inward toward their primary like that? And how lucky are we that Jupiter didn’t do the same thing and squash us?

  24. JohnW

    Surely that picture is an artist’s conception, or a picture of our own sun?

  25. @21. SC: it’s got something to do with the transference of angular momentum from the star to the planet….enough and the planets end up spiraling right into the star. I think by this point in the system’s development the planets have arrived at their stable orbits.

  26. Joseph G

    D’oh. I can’t believe I didn’t see your post there, SC :)

    I really need to learn to Wiki this stuff before I ask questions, too. Interesting link is interesting:
    en.wikipedia(dot org)/wiki/Planetary_migration

  27. ERK_in_AK

    Do the planets in this solar system create a problem for the definition of a planet? The requirement that the body has to have cleared its orbit? Could be interesting.

  28. #24 JohnW:
    Of course the picture is an artist’s conception! Where has anyone said or suggested otherwise?

  29. Brian Too

    @29. Neil Haggath,

    No no no! 4000 years ago we sent a probe to Kepler-11 (at the speed of light mind). 2000 years ago it sent that picture back to us (also at the speed of light) and we’re just getting the first results now.

    The mission was called Mayan Apocalypse 2012. Or something.

    Ain’t technology great?

    :-/

  30. qbsmd

    “10. andy Says:
    Certainly looking like many “super-Earths” are actually “mini-Neptunes”. That may have negative implications for habitability: if they have too much hydrogen then the high pressures and temperatures may prevent oceans from forming. Even with smaller quantities of hydrogen you may end up with an “ocean planet”: the liquid water ocean would be separated from the the rocky core by thousands of kilometres of high-pressure ices, which could lead to waterworlds too nutrient-poor for life.”

    The phase diagram for water has a negative slope on the solidliquid interface. Shouldn’t higher pressures increase the area of the habitable zone both toward and away from the star? And what ices are heavier than liquid water?

  31. Messier Tidy Upper

    Superb news. :-)

    Fascinating little system – wonder if they are more exoplanets further out that haven’t yet been detected and, if so, what the the chance’s of finding them might be?

    What *other* surprises might this tightly-packed system hold further out?

    More “gas dwarfs” – as Planet-hunter Sara Seager neatly termed the in-between mass “Mini-Neptune / Super-Earth” worlds, a Jovian or Neptunean style planet, even an Earth-like world – or two maybe?

    How interesting the skies of some these Kepler-XI (a Cricket team worth of planets?) must be – shame their surfaces are most probably well either lava or cloud and not solid.

  32. Messier Tidy Upper

    @31. qbsmd Says:

    And what ices are heavier than liquid water?

    “Hot ices” or supercritical forms of ice incl. water ice that form under high pressures and even at high temperatures.

    Also “ices” composed of other elements – most of which , if not all usually sink rather than float. Our familiar water ic eis actually a fairly bizarre substance in this regard.

    Incidentally, our normal water ice is also, I think, a form of sedimentary rock! ;-)

  33. Phiiiiiiiiilllll, CNN’s posting bad science again….

    http://www.cnn.com/2011/US/02/02/nasa.kepler.planets/index.html?hpt=C1

    I’ve already sent them an e-mail saying that no, these planets are not in the goldilocks zone, but I don’t think they’ll listen to me. They never have in the past.

  34. Messier Tidy Upper

    ^ See http://en.wikipedia.org/wiki/Ice#Phases for more on the other lesser known ices – and scrollup for the article on the familiar form of water ice.

    Or “solid dihydrogen monoxide” if you want to confuse folks! ;-)

    RE : Our solar system migration & consequences see :

    http://en.wikipedia.org/wiki/Late_Heavy_Bombardment

    for more too. :-)

  35. Messier Tidy Upper

    @23. Joseph G :

    Yayyyy exoplanets! I lurrrrrv me some exoplanets!!! Even better if they aren’t gas giants! Hooray for Kepler! :-)

    Seconded by me. :-D

    We are discovering entire strange fascinating new worlds around other stars and around our own Sun too – all the new found ice dwarfs out in the Edgeworth-Kuiper Cometary Belt. I love learning about these new discoveries. I am only (slightly) saddened that they don’t seem to get more attention and interest from more folks and the general media.

    @30.Brian Too :

    No no no! 4000 years ago we sent a probe to Kepler-11 (at the speed of light mind). 2000 years ago it sent that picture back to us (also at the speed of light) and we’re just getting the first results now.

    Ah, if only *that* were true. 8)
    Sigh.

    @21. SC Says:

    I’m always curious how planets could possibly migrate inwards. How has this not happened in our own solar system?

    Apparently it has. My understanding is that Jupiter migrated inwards slightly and the outer gas giants migrated outwards quite a way. Neptune’s shift outwards may well have been responsible for the Late Heavy Bombardment episode responsible for creating the Lunar maria. (See comment #34 awaiting moderat’n above.)

    Moreover, this quote* :

    ” ..we now realise that if Saturn were about twice as massive, it would gravitationally interact with Jupiter on a short timescale. One of them would get thrown in, and the other one would get thrown out – and we wouldn’t be here having this conversation.”
    – Exoplanet hunter, Paul Butler quoted on page 41, Nigel Henbest & Heather Couper, ‘Extreme Universe’, Channel 4 Books, 2001.

    Shows we indeed got very lucky with our solar system and the planetary formation and migration processes NOT going awry. :-D

    ——

    * Also quoted earlier in my comment # 87 on the Blastr: Other than that, Spock, how was the movie? thread (January 27th, 2011 2:37 PM) here but worth reposting in this context methinks. :-)

  36. Messier Tidy Upper

    @1. Todd W. Says:

    Very cool, but shouldn’t it be “mini star system” instead of “mini solar system”? Or “mini Kepler-11ar system”?

    Yup. Pedantic and silly perhaps but I do think there’s a need to give these worlds – or at least the more significant and interesting of them proper names rather than dull, hardtorecall, hard to pronounce catalogue designations. its a pet gripe of mine but I think it does impede communicating astronomy and the wonder of it to the public – at least a little.

    Given that these worlds are packed in like sardines might I suggest the Sardines System for the worlds of Kepler-11? ;-)

    @28. ERK_in_AK Says:

    Do the planets in this solar system create a problem for the definition of a planet? The requirement that the body has to have cleared its orbit? Could be interesting.

    Very. Good. Point.

    When do plenats get so close together they stop being panets? As long as they orbit independently rather than as moons or at least as double planets perhaps its better we just call them planets – along with the ice dwrafs in our own solar system.

    Curiously enough, the IAU definition apparently strictly applies only toour own Sun. Yet another flaw in it and sign it should be scrapped and the issue recrtified by a change to a more reasonable definition methinks.

  37. ellie

    Is it possible that one or more of these are moons of one of the bigger planets?

    I’m so anxious for the results once Kepler has been observing for a few years! This is great – hooray science!!

  38. andy

    Do the planets in this solar system create a problem for the definition of a planet? The requirement that the body has to have cleared its orbit? Could be interesting.

    Only if you interpret the clearing the orbit requirement at a pedantic level. Since it is actually interpreted as “not a member of an asteroid belt” (which is a far more sensible definition) there is no particular problem.

    (Incidentally the upper limit of planetary mass under the IAU definition, which is set by the deuterium fusion criterion, is far more problematic if generalised to other planetary systems – there are several cases which clearly show its inadequacy.)

  39. Joseph G

    MTU: I am only (slightly) saddened that they don’t seem to get more attention and interest from more folks and the general media.

    I think it’s ridiculous.
    “In other news, scientists have announced the discovery of hundreds of new planets, more then tripling the number of known exoplanets overnight – findings that challenge many assumptions that we have about our place in the universe.
    Now that your 15 seconds of science is over, here’s 45 minutes of Lindsay Lohan in court, and people yelling at each other.”

  40. MaDeR

    @Messier Tidy Upper :
    “Curiously enough, the IAU definition apparently strictly applies only toour own Sun. Yet another flaw in it”
    Stop lying. This restriction was completely deliberate, not a oversight. Reason was exactly this fact that we do not know what is possible out there. thsi definition was from beginning temporaty, to cover our US only. It will be extended and changed after accumulating sufficient data.

    No, Pluto will not come back. Stop being kid and get over this.

  41. Messier Tidy Upper

    @ 45.MaDeR :

    @Messier Tidy Upper : “Curiously enough, the IAU definition apparently strictly applies only to our own Sun. Yet another flaw in it.”
    Stop lying. This restriction was completely deliberate, not a oversight.

    It is NOT a lie as you’ve noted there. Please don’t insult me remembering that I treat you with equal courtesy. Whether it was an oversight or deliberate doesn’t matter.

    According to the IAU there can be no planets other than those in our solar system, no planets orbit other stars because the IAU says so. That sounds silly, downright ridiculous even – and it is but the silliness is coming from the IAU not me.

    Reason was exactly this fact that we do not know what is possible out there. thsi definition was from beginning temporaty, to cover our US only. It will be extended and changed after accumulating sufficient data.No, Pluto will not come back. Stop being kid and get over this.

    So the IAU definition is inadequate and *will* be changed at some point. Seems like we agree on that. ;-)

    I will go an extra step and suggest it needs to be changed to something that recognises that planets come in a range of sizes from Superjovian gas giants larger than Jupiter at one extreme through intermediate ice giants like Neptune and gas dwarfs like Kepler-11e and earth-like terrestrial planets, all the way down the size-mass range to (& including) ice dwarfs smaller than Pluto. The smallest planet is probably something around Ceres or Vesta in size and mass. Yes, that means there are many more planets incl. Pluto, Eris & Ceres in our solar system and the cosmos beyond it. That’s a good thing not a bad one.

    If its gravitationally rounded, never self-luminous by nuclear fusion and NOT a moon then it counts as a planet in my book and I think that’s the simplest and best definition out there. I hope the IAU eventually sees reason and adopts something like that because if they don’t they will become ever more irrelevant and held in well-justified contempt – much as they currently are over the current issue of planets being found around other stars. Where, again, the IAU says that by definition these objects cannot be termed planets but everyone ignores them – as they rightly should! ;-)

  42. AndyG

    Am I missing something? Wouldn’t a body of double the radius (given that the density doesn’t change) be 8 times the mass? Similarly wouldn’t a world of 20 times the mass of the earth be 2.7 times the radius?
    If this is the case, those curves don’t look right …

  43. andy

    If its gravitationally rounded, never self-luminous by nuclear fusion and NOT a moon then it counts as a planet in my book and I think that’s the simplest and best definition out there.

    I raise you the cases of Upsilon Andromedae c, HD 16760 b, BD+20 2457b, HW Virginis c and Nu Ophiuchi b,c that show that the nuclear fusion criterion for planethood does not actually work. These objects show evidence of having formed as planets: with the exception of HD 16760b they are all in multiplanet systems, and HD 16760b is on a circular orbit which is very difficult to produce via stellar-type formation processes. These objects give us strong evidence that the universe does build planets with masses above the fusion limit.

    As for the low mass end, the question is fairly simple: is the object dynamically dominant, or is it a member of an asteroid belt? The eight major planets orbiting the Sun are all several orders of magnitude more massive than the population of nearby small bodies. Pluto, Ceres and the rest do not meet this criterion and are clearly members of belt populations. Excluding them from planethood gives a clearer picture of the structure of the planetary system that would be unnecessarily confused by cherry-picking a selection of objects from each planet belt. This was recognised with the case of the asteroids, so applying it to the populations of trans-Neptunian objects is not without precedent. Furthermore there is a clear dynamical distinction between a pair of planets in a closely-packed (or even co-orbital) resonant configuration and a belt containing large numbers of objects. These closely packed exosystems have little bearing on what’s going on out beyond Neptune.

    Yes the wording of the IAU definition is not particularly good, but arguing over semantics misses the point that the exclusion of Pluto from planethood does rest on sound principles, and is not just done because the IAU says so.

  44. matt

    I sent an email along time ago refering to measuring light dimming to find ET instead of seti watching for traffic. It seems it is getting easier now but as the publication mentioned its useless for planets because its not certain.

    However it would be perfect for finding other civlisations with traffic going between stars.

    Although i dont mind this endlessly increasing number of planets total Id prefer if they used the technology to locate civilsations rather than rocks or dust giants that mean nothing for our civilisation.

    Only when we have the technology to confirm a planets habitability are we going to go any where near it so telling me there are a billion planets out there is as useless a piece of information as if youd said a thousand.

    2000 light years away is no drop in the ocean so I cannot believe the stupidity of even bothering to say it has planets….Please put technology to more useful endevours or rate your research to star systems under 50 years away.

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