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. …
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.
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.
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.
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
"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..
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?
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.
...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.
"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.
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"
"they don't pass in front of their star when we're looking": that could of course be, although the missions are planned so that they look at the same stars frequently. The real problem is that the plane of the planet's orbit has to make the planet pass in front of the star from our perspective. Putting it differently, we need to be in their ecliptic. The problem is that most planets don't orbit in such a plane, so we'll never see them pass in front of their parent star, even if we watch continually.
By the same token, an alien living on one of those planets would only see the Earth pass in front of the Sun if they happened to be on our ecliptic.
> 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. :(
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.
> 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.
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.
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.
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
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