Exoplanet news part 1: I shall call it Mini Solar System

By Phil Plait | January 12, 2012 7:00 am

There’s been so much exoplanet news this week! I was in Texas the past couple of days giving a bunch of talks, so I’m trying to catch up. All the exoplanet news is way cool, but too much for one post, so I’ve split them up. I’ll post the other parts shortly.

Part 1: A trio of hot little rocks

First up? The three smallest exoplanets found so far. I usually don’t like to write about incremental discoveries, but this one is truly cool: all three orbit the same star, and all three are smaller than Earth! Any one of these would be a record breaker, but to find all three at once, in the same place? Amazing.

They orbit the star KOI-961 (short for Kepler Object of Interest), and were observed by the Kepler Observatory (details on how that all works can be found here). They all orbit the star extremely close in: the farthest one is a mere 2.3 million km (1.5 million miles) from the star! They’re so close they all take less than two days to circle it once. And even though the star is a red dwarf, and therefore relatively cool, they are so close to it that they probably resemble airless, heat-blasted Mercury more than Earth. They are almost certainly rocky/metallic bodies, since they are so small: 0.78, 0.73 and 0.57 times the diameter of the Earth. Although we’ve been surprised before, it’s hard to imagine anything that small could hold onto much atmosphere when they are so hot.

Funny, too: the star is tiny, only a bit bigger than Jupiter. And the planets are so close in the KOI-961 system looks more like Jupiter and its moons than our own solar system! The artwork above drives that point home. Everything there is to scale: the relative size of the star, the planets, Jupiter, and its moons. [Edited to add: Note that the distances are not to scale!]

Why is this news important? Well first, it adds more weight to the idea that planets smaller than Earth exist and can be found around other stars. Second, it shows that red dwarf stars can form and hold onto planets… which itself is important because red dwarfs are by far the most common kind of star in the Universe. They make up roughly 80% of the total number of stars! So finding multiple planets around one means, once again, planets are almost certainly common in the galaxy.

And third, it just shows once again that the Universe is a surprising place. This mini-solar system proves that nature is diverse, and will fill any niche it can. It also implies, very strongly, that we need to broaden our concepts of how solar systems form, what they look like, and how they behave.

Image credit: NASA/JPL-Caltech

Related posts:

Kepler finds a mini solar system!
Another Kepler milestone: Astronomers find two Earth-sized planets orbiting the same star!
No, it’s *not* the smallest exoplanet found!
A boiling superEarth joins the exoplanet roster

CATEGORIZED UNDER: Astronomy, Cool stuff

Comments (40)

  1. Will

    Could it be?!? Have we discovered the homeworld of the Keebler Elves?!?

  2. I’m sure this is going to lead in to a post about all the other planets scientists think may be out there.

    By any chance, what is the inclination of the orbital plane of KOI-961 to earth? There may be even more planets around this star than we are detecting if the inclination deviates from zero. That in itself is cool.

    Check out this info over at the Astronomy StackExchange regarding the geometry for finding transiting planets: http://astronomy.stackexchange.com/questions/1229/what-percent-of-planets-are-in-the-position-that-they-could-be-viewed-edge-on-fr/1232#1232

  3. Eric TF Bat

    I seem to recall that the Chirpsithra have dominion over all tidally-locked red dwarf planets, so perhaps we should say hi and invite them round to the Draco Tavern…

  4. Duke York

    Just for my own curiosity, where would the “Goldilocks Zone” be with that star?

    Could a star that small even potential have habitable planets?

  5. Planets, planets, everywhere. Ain’t it cool?

  6. Hampus

    This universe of ours is just getting more and more awesome.

  7. Wzrd1

    Another report I saw yesterday was that EVERY star is thought to have planets, as reported by the BBC. http://www.bbc.co.uk/news/science-environment-16515944
    I’m dubious, as close binary and trinary systems should be disruptive enough to prevent planet formation, just as Jupiter prevented the asteroid belt from becoming a planet.

  8. Steven

    How significant are the red dwarf’s tidal forces on these planets? If it’s like the Galilean moons around Jupiter, then they’re a big deal! Are these new planets tidally locked to their star? Do we expect active volcanism on the innermost one (or two?) if tidal heating is significant?

  9. J Alexander

    Thanks for posting this. It really helps my visualization of all the exoplanets that have such short orbital periods.

  10. @#5 (Duke York), yes, a red dwarf will have a habitable zone. Although it will be much closer to the star itself, and the planet would most likely be tidally locked as well.

    Some reading on that:



  11. @Wzrd1,

    At the rate we’re discovering planets now, I wouldn’t be surprised if not having a planet was the exception rather than the rule. We’ve already found some Earth-sized planets that might be within habitable zones. I’m wondering if there’s any chance to, with improved technology, see what the atmospheric composition of these planets are. If we can do that, could we detect whether the planet is likely to have life on it.

    This is quite an interesting time and it is only going to get more interesting.

  12. Firemage

    “This mini-solar system proves that nature is diverse, and will fill any niche it can.”

    Sorry to be nitpicky, but stars and solar systems don’t exactly evolve in the biological sense. There is no selection at work and thus they can’t “fill a niche”.
    It’s more like “The universe is so big and old that the law of large numbers applies: Anything that is even remotely physically possible happens or happened somewhere in the universe. “

  13. lunchstealer

    The orbital distances are NOT to scale, are they? I mean, the Jupiter diagram shows Io orbiting at an altitude that is significantly less than Jupiter’s diameter. Quick mental calculation seems like Io should be much farther from Jupiter at that scale.

  14. @ lunchstealer (#13), the picture is basically showing the radius of the orbit from the center of Jupiter (and same for the KOI-961 planets). Hope that helps. :)

  15. David K


    Planets around close binaries and triple stars? Easy — they orbit far out around both/all stars in the system. The hypothesis is that all or nearly all stars have planets, not that they all have planets in their habitable zones.

  16. The artwork above drives that point home. Everything there is to scale: the relative size of the star, the planets, Jupiter, and its moons.

    Is everything really to scale, or just the relative sizes? Cause it looks to my untrained eye like the planets are really close to the star, and that the moons are too close to Jupiter.

    Oops… Just noticed lunchstealer beat me to this. Oh well, I already posted it, so I guess I’ll leave it up.

  17. amphiox

    Everything there is to scale: the relative size of the star, the planets, Jupiter, and its moons.

    The relative sizes and the relative distances are to scale, but surely the distances and sizes are not to scale with each other, right? Io does not orbit less than a Jupiter-diameter away from Jupiter’s surface, does it?

  18. amphiox

    I’m dubious, as close binary and trinary systems should be disruptive enough to prevent planet formation, just as Jupiter prevented the asteroid belt from becoming a planet.

    Perhaps, but Jupiter didn’t prevent the formation of the dwarf planet Ceres. So if you’re of the opinion that dwarf planets should be considered planets too….

  19. amphiox

    Well, Fatboy, it looks like you have company!

  20. Chris A.

    Jupiter’s radius R(J) = 71,492 km

    Average orbital radii:
    Io: 422,000 km = 5.9 R(J)
    Europa: 671,100 km =9.4 R(J)
    Ganymede: 1,070,000 km = 15.0 R(J)
    Callisto: 1,883,000 km = 26.3 R(J)

    So, no, the sizes and distances are not to the same scale.

  21. Kurt

    It’s a shame. As near as I can tell Jupiter is only about four times larger than it should be at the scale of its moons’ orbit. It would have been recognizable at the correct scale. The moons might just be dots at that scale though.

    Diagrams invariably inflate the size of things so they are visible as more than dots, but that throws off the layman’s sense of scale. So I love to see astronomical diagrams done at correct scale. So many people have no concept of the actual distances involved and how much empty space is out there.

  22. andy

    One interesting point here is that the star is a near-twin of the somewhat infamous (at least in exoplanets terms) Barnard’s Star. The fact the two stars have very similar properties and that Barnard’s Star is very well studied allowed for much better estimates of the KOI-961 system parameters. The star is both low mass and low metallicity, which puts it in an interesting region of parameter space: at least for giant planets, both factors are unfavourable.

    The resemblance between so many of these super-Earth systems and the Jupiter system is interesting, even though there’s obviously a detection bias involved which favours finding these types of systems over analogues to our much more widely-spaced inner solar system.

  23. I do not understand. How is the Red Dwarf only a bit bigger than Jupiter? Is it diameter, or mass? The news articles I have seen state that the Red Dwarf is 1/6, or 1/7 the mass of the Sun. Jupiter is about 1/1000 the mass of the Sun, which means that Jupiter is still less than 1/100 the mass of the Red Dwarf. The diameter of Jupiter vs. the Sun is smaller than 1/10. I am unsure of the diameter of the Red Dwarf.

  24. Torbjörn Larsson, OM

    “Jupiter didn’t prevent the formation of the dwarf planet Ceres. ”

    As far as I know no one thinks today the asteroid belt would have aggregated to a planet. (Wasn’t that the idea of one early astronomer all by himself, btw?)

    What happened was that no planet kicked it sufficiently clean after planet/planetesimal formation, same as the Kuipers are left and so on.

  25. Ah, sorry for the confusion. No, the distances are not to scale, just the sizes. I’ll amend the text. Note: I had the distance of the outer planet as 2 million km; I updated that to 2.3. Just to be accurate. :)

  26. @14 Firemage: Sorry to be nitpicky, but stars and solar systems don’t exactly evolve in the biological sense. There is no selection at work and thus they can’t “fill a niche”.

    I second that. I didn’t want to nitpick either, but then, what is science about but nitpicking everything? 😛
    But yeah, (very) large numbers! I wonder if somewhere out there is a “smoke ring” as in Larry Niven’s “The Integral Trees”?

  27. Mea culpa, seems I was totally wrong on one of my posts (#16).

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  29. Messier Tidy Upper

    Superluminous (beyond brilliant) discovery – Congrats to the Kepler team once again! :-)

    The exoplanet low mass limbo just keeps getting lower! 😉

    Down to the mass of Mars – wow. 😮

    Surely we must be pushing the limit of what they can detect now – but does have me wondering how much lower (mass~wise) they can go. All the way to Pluto-mass & beyond? Also wonder if further study could use perturbations of these rock dwarfs to detect any higher (or even equally low) planets further out?

  30. Just discovered that red dwarves can live for hundreds of billions of years. We may be the unlucky ones in this Universe with our remaining 5 billion years…

  31. andy

    Previously material being evaporated from hot Jupiters has been detected. Now we’ve got something that may be a Mercury-like planet being disintegrated by its parent star…

    Possible Disintegrating Short-Period Super-Mercury Orbiting KIC 12557548

  32. Joel

    Can I be extra picky here and point out that the distances do appear to be to scale *with each other*, just not the same scale as the size of the bodies. So, for instance, the outermost planet here would indeed be just a bit further out from the centre of the system than Callisto is from Jupiter, but not by too much.

  33. FeRD

    Given the accelerating pace of new exoplanet discoveries, and the resultant expansion of many estimates regarding how common these bodies are and how many we expect to eventually locate, I’d very much like to see a minor — but symbolic — change in the language used. It’s time that we amend the vocabulary to reflect our more mature understanding (speaking of evolution, above), and stop calling them “exoplanets“! The term is Earth- and Sol-centric, and is beginning to feel embarrassingly provincial in light of recent discoveries.

    The more of these newly-detected planets we catalog, the less appropriate it will be to use distinct terminology. From a scientific standpoint, what justifies a vocabulary for discussing planetary bodies which implicitly views the ones in our own system as somehow inherently special or different?

  34. Messier Tidy Upper

    Second, it shows that red dwarf stars can form and hold onto planets… which itself is important because red dwarfs are by far the most common kind of star in the Universe. They make up roughly 80% of the total number of stars!

    Plus while they used to be considered unlikely hosts for habitable planets as Ken Croswell points out here :


    & here :


    Red dwarfs may be a lot better suited to hosting life than we used to think. :-)

    Mind you, coping with red dwarf flares – and many of them really do flare dramatically – could present life on planets necessarily tucked in close with some major issues. As kaler notes on page 31, “The Faintest Stars” article by James B. Kaler , in ‘Astronomy’magazine, August, 1991 :

    “Imagine M-dwarf [red dwarf – ed.] bathing on the beach and having your star suddenly -with no warning at all – become ten times brighter.”

    Yikes! 😮

    Can anyone imagine our Sun becoming even twice as bright abruptly?

    As always, there’s just so much still to learn.

    Wondering more than ever before if the nearer red dwarf stars – eg. Proxima Centauri, Barnard’s Star, Wolf 359 – have planets, what they’re like and when, if ever, we’ll know? A twin of Barnard’s Star eh? :-)

    Ah, for FTL travel, sigh.

  35. Dragonchild

    I nominate calling the planets Lister, Kryten and Cat. A hypothetical fourth planet would be on the outer Rimmer.

  36. Drunk Vegan

    Why does every blog post and news article about the discovery of a slightly smaller planet always trumpet it as “the smallest planet found yet!” ?

    These are *not* the smallest exoplanets found yet. That honor goes to the pulsar planets that were discovered in the early 1990’s. Their maximum masses derived from radial velocity were near or well below the mass of Earth, including this one:


    Which is, in fact, the smallest exoplanet ever discovered (in 1994), weighing in at only twice the mass of Earth’s *Moon*, making it much smaller than all other exoplanets discovered so far, and in fact smaller than any of the 8 planets in our solar system.

    I do occasionally see articles that give a nod to this by adding “around any main-sequence star” after “smallest yet discovered,” which is at least technically accurate.

    It seems like a disservice to those early planet hunters to always overlook that achievement, especially because most articles on exoplanets also indicate the “first” exoplanet discovered was 51 Pegasi b in 1995, which is also completely untrue.

  37. andy

    @Drunk Vegan: it seems that pulsar planets are routinely ignored by the exoplanet community. Another example is the Kepler team’s claim that Kepler-16 (AB) b is the first discovered circumbinary exoplanet – that honour actually goes to the PSR B1620-26 system (a pulsar+white dwarf binary located in a globular cluster) discovered in 1993.


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