back to article Three planets and two stars adds up to one research team made very happy by Kepler's unique discovery

Binary star systems are relatively rare but astroboffins poring through data from the now-defunct Kepler telescope have found something unique - a binary system with three planets. Kepler-47 is an oddity. At the center of the system lies not one but two stars locked into a tight orbit, making it a circumbinary system. Every 7. …

  1. Doctor Syntax Silver badge

    "Although it's rocky and lies in the habitable zone ... its surface reaches scorching temperatures of about 427˚C (~800 degrees Fahrenheit)."

    Habitable?

    1. Anonymous Coward
      Anonymous Coward

      Dehydrate! Dehydrate!

    2. Loyal Commenter Silver badge

      Beat me to it!

      IIRC, the habitable zone is defiend as the area around a star where water can exist in a liquid state. Unless we're talking about stupidly high pressure, I don't think this one qualifies...

    3. cray74

      Goldilocks screwed up a good deal, too

      Habitable?

      Just because a planet shows up in a "Goldilocks" zone doesn't mean it's automatically habitable. Venus and Mars are in (or near) Sol's habitable zone but screwed up their chances to be infested with humans due to poor choices.

      1. A Non e-mouse Silver badge

        Re: Goldilocks screwed up a good deal, too

        ...but screwed up their chances to be infested with humans due to poor choices

        Sounds like they made a good choice to me.

    4. ThatOne Silver badge

      There are two of them actually

      How can something that hot pretend to be in the habitable zone? Yes, Venus, but I'm assuming those 427˚C are extrapolated from something we can reliably assess at 52 ly distance, like distance to the sun.

      Anyway, there is some confusion in the article as HD 21749b is the sub-Neptune planet, and HD 21749c is the vaguely earth-sized one. Remains to be seen to which of the two that temperature estimation applies; My guess would be HD 21749b, but the article is behind a paywall.

      1. the spectacularly refined chap

        Re: There are two of them actually

        From the paper it appears the two are indeed confused:

        HD 21749b: Semimajor axis 0.19 au, 2.61 Earth radii, 22.7 Earth masses, period 35.6 days, temperature 422K.

        HD 21749c: Semimajor axis 0.07 au, 0.89 Earth radii, less than 3.7 Earth masses (no lower limit), period 7.79 days, temperature 701K.

        1. ThatOne Silver badge

          Re: There are two of them actually

          So the earth-like HD 21749c is actually the closest one to the suns. Thanks, that explains the temperature.

          This means the sub-Neptune HD 21749b would be the one "in the habitable zone", although at still 149°C I wonder, habitable by what exactly?...

    5. Anonymous Coward
      Anonymous Coward

      427C

      Welcome to Crematoria

  2. old_IT_guy

    Binary star systems with planets may be a rarity, but binary systems (unqualified) aren't.

    1. Cuddles Silver badge

      "Binary star systems with planets may be a rarity"

      Even that isn't true. Planets seem to be less common in binary systems, but around 1/3 of them still have planets. Sure, it's less than single systems where most of them probably have planets, but it's hardly rare.

    2. Francis Boyle Silver badge

      But circumbinary systems are rare. In most binary systems the planets orbit one star and the other star is at a sufficiently great distance that it has relatively little influence on the planets. Here the planets orbit both stars which is not a way of guaranteeing a stable system. I remember trying it in a simulator once and not being able to get it to work.

  3. jmch Silver badge

    Naming conventions?

    I'm not up to speed with naming conventions but from what I can work out, 'a' is usually used for a system's star and successive letters for every newly discovered planet. But in this case of a binary star, why aren't the stars named Kepler-47-a and b, with the planets being c, d, e? Or is 'a' reserved for stars with binaries being called a1 and a2?

    And I can understand the logic of naming planets in successive order of discovery, but it could also lead to some confusing outcomes, as in this case the planets from inside to out are named b, d, c. Not that I have an easy solution for this, just curious if there is anything I'm missing.

    Also, as an aside, surely the odds are quite low that a planet's orbital plane is edge-on to Earth so that they can be seen by this method. I would hazard to guess that pretty much every star has planets, we just can't see most of them because they don't pass in front of their star

    1. Cuddles Silver badge

      Re: Naming conventions?

      "I'm not up to speed with naming conventions but from what I can work out, 'a' is usually used for a system's star and successive letters for every newly discovered planet. But in this case of a binary star, why aren't the stars named Kepler-47-a and b, with the planets being c, d, e? Or is 'a' reserved for stars with binaries being called a1 and a2?"

      Stars get capital letters, planets get lower case but starting from b. So the Kepler-47 system consists of Kepler-47A, Kepler-47B, Kepler-47b, Kepler-47c, etc..

      1. Yet Another Hierachial Anonynmous Coward

        Binary systems

        If you have a binary system, with two stars, how do the planets orbit?

        Around one star, or both stars? If both stars, does that give the planet something of a figure of 8 orbit as it gets pulled one way then the other. Even if it orbits one star, then it must get pulled towards the other on each pass.....

        Sounds to me like it wouldn't be very stable over the systems lifetime?

        Any informed commentards out there?

        1. Loyal Commenter Silver badge

          Re: Binary systems

          I think the way it works is that if the stars are co-orbiting (i.e., in the centre of the system, with planets orbiting both), they're named as above. If there are two stars far apart, these would be A and B, and if they have planets orbiting them, they would be Ab, Ac... and Bb, Bc... respectively.

          I think the planet naming is further complicated by the fact that they tend to be named in the order they are discovered, rather than their distance from their parent star, hence the rather confusing 'd' being between 'b' and 'c' - naming them after the distance form the star would also be problematic for objects with highly elliptical orbits that may cross over each other, objects that orbit in different planes, etc.

        2. Loyal Commenter Silver badge

          Re: Binary systems

          ...in terms of stability of orbits, if you have two widely spaced stars, planets around either are not going to be graviotationally affected very much by the distant star (think about how much Jupiter affects the Earth's orbit for example). On the other hand, if there are two co-orbiting stars surrounded by planets (such as in this system), you might expect some crazy tides (in the same way as spring and neap tides are affected by the relative positions fo the Sun and Moon). Depending on the mass of the stars, and how closely they orbit each other, such planetary orbits may be less affected than you might think.

        3. Cuddles Silver badge

          Re: Binary systems

          "If you have a binary system, with two stars, how do the planets orbit?"

          Carefully!

          Aside from that, the problem with the three body problem is that there is no general solution, so figuring out how a system with multiple stars and planets behave is a bit tricky. In the easy case, planets orbit one star while the other star(s) is far enough away that it can effectively be ignored. In an almost as easy case, if the stars are close together and the planets are much further out, the stars effectively act as a single body to orbit. Once the stars and planets start getting a bit mixed up, it's just a mess. It's entirely possible to have a chaotic system in which there doesn't appear to be any regular orbit defined, but which is nevertheless stable in the long term, but it's also possible to have something that temporarily looks sensible but which is actually unstable.

          So basically the answer is that it depends. There are less options than in a single-star system, which is why there are fewer binary systems with planets. There are a still a variety of ways planets can exist in multiple star systems, even when everything is mixed up close together. but it's generally impossible to calculate such solutions analytically, so you have to rely on tracking simulations which are inherently less accurate and depend heavily on how well you know the parameters (mass, distance, etc.) of all the bodies involved.

    2. Sean o' bhaile na gleann

      Re: Naming conventions?

      Your statement; "... I would hazard to guess that pretty much every star has planets, we just can't see most of them because they don't pass in front of their star" is almost certainly correct, but I'm going to add a few words: "...because they don't pass in front of their star when we're looking"

  4. ProperDave
    Coat

    > Located about 3,340 light-years away in the Cygnus constellation

    These figures always make my heart sink, given that vast distance.

    Whats the top speed of our best spacecraft right now? Something like 36,000years/per light year?

    It'd take almost the entirety of known human existence to reach our closest neighbour in Alpha Centauri, just 4 light years away. :(

    1. Crisp Silver badge

      It'd take almost the entirety of known human existence

      Depends on your reference frame.

      For a human accelerating at 1g for half the trip and decelerating at 1g for the rest of the trip the travel time can be measured in years.

      For a human sitting on earth watching all of this, it's going to take a lot longer.

      1. DougS Silver badge
        Devil

        Re: It'd take almost the entirety of known human existence

        Still going to be a boring trip, with shitty internet latency.

      2. ProperDave

        Re: It'd take almost the entirety of known human existence

        > Depends on your reference frame.

        Reality was my reference frame. Last time I checked we weren't capable of reaching 1g. I'd wildly speculate the Voyager probes' on-board clock still keeps accurate Earth time and hasn't slowed despite travelling as fast as we can possibly travel right now.

        1. Carpet Deal 'em
          Boffin

          Re: It'd take almost the entirety of known human existence

          That depends on how you define "capable". In terms of anything flown or on the drawing table, you're more-or-less right. However, going reeeaaalllyy fast is largely a solved problem, in terms of the engineering involved; the biggest hurdle left is the political issues nuclear explosions in space raise.

  5. Peter Prof Fox

    Counting assumption

    You can only see a transit if the plane of a planet's orbit intersects (approximately) with the axis of viewing from Earth. Surely when 'counting', if you get say three hits from star A that's a good count, but if you get none that's an absence of data. Somebody must have an idea (roughly) what proportion of stars have a suitably aligned planetary disc.

    1. Paul Kinsler

      Re: what proportion of stars have a suitably aligned planetary disc.

      Apparently, the alignment is pretty random:

      http://curious.astro.cornell.edu/about-us/159-our-solar-system/the-sun/the-solar-system/236-are-the-planes-of-solar-systems-aligned-with-the-plane-of-the-galaxy-intermediate

  6. Mike Shepherd
    Meh

    Weasel words

    Binary star systems are relatively rare...

    Relative to 100%, perhaps, since most stars are binary: https://www.atnf.csiro.au/outreach/education/senior/astrophysics/binary_intro.html

  7. jonfr

    Good chance for Alpha Centauri planets

    There is a good chance that Alpha Centauri has plants based on this discovery even if none has been found today.

    https://en.wikipedia.org/wiki/Alpha_Centauri

  8. ZanzibarRastapopulous

    Meh

    Really, who cares?

    Too much money is wasted on this distant bollocks. You don't see Musk dicking about with telescopes.

  9. Anonymous Coward
    Anonymous Coward

    based upon current data, it seems relatively rare for stars to form without having some planets - now which ones of them are going to avoid the EU? That is the question.

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