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Post Tau Ceti probably has five planets, one in habitable zone
Tau Ceti's planets nearest around single, Sun-like star

Potentially Habitable Planet Detected Around Nearby Star

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Astronomers have detected five possible alien planets circling the star Tau Ceti, which is less than 12 light-years from Earth — a mere stone's throw in the cosmic scheme of things. One of the newfound worlds appears to orbit in Tau Ceti's habitable zone, a range of distances from a star where liquid water can exist on a planet's surface.

With a minimum mass just 4.3 times that of Earth, this potential planet would be the smallest yet found in the habitable zone of a sun-like star if it's confirmed, researchers said.

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These periodicities could be interpreted as corresponding to planets on dynamically stable close-circular orbits with periods of 13.9, 35.4, 94, 168, and 640 days and minimum masses of 2.0, 3.1, 3.6, 4.3, and 6.6 M⊕, respectively.


Time to update the info on the 6 worlds of 'humaniti'? :)


Wed Dec 19, 2012 3:55 am
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Post Re: Tau Ceti probably has five planets, one in habitable zon
Unfortunately, a 4.3 Earth mass planet would probably not be habitable by humans; an Earth-composition atmosphere with those gravity levels would probably be dense enough to be toxic.

Luckily, the details of the Earth settlements are largely inconsequential to the Outsider story (being leftovers from the ancestor story that preceded it).

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Post Re: Tau Ceti probably has five planets, one in habitable zon
Arioch wrote:
Unfortunately, a 4.3 Earth mass planet would probably not be habitable by humans; an Earth-composition atmosphere with those gravity levels would probably be dense enough to be toxic.


Tau Ceti has lower metallicity than our Sun, so it is conceivable that the planet (if it really exists) has a much lower average density (smaller/lighter core) than Earth. It would probably be covered by a global ocean considering how much cometary debris there is in the system, but with some luck, it could be colonizable...? :)


Wed Dec 19, 2012 12:14 pm
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Post Re: Tau Ceti probably has five planets, one in habitable zon
gravity is not linear with mass, nor volume.

the moon is not 1/6th the mass or earth after all.


Wed Dec 19, 2012 12:45 pm
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Post Re: Tau Ceti probably has five planets, one in habitable zon
"A lot of cometary debris" is likely to mean a lot of impacts to the planet, which in turn means a bad candidate for colonization for a very long time.


Thu Dec 20, 2012 3:09 am
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Post Re: Tau Ceti probably has five planets, one in habitable zon
I had an idea - what if the 4.3 Earth-mass planet isn't one planet, but a binary super-terrestrial planet? Say 2.5 + 1.8 planets orbiting a common centre of gravity?

That's what I'd call 'winning a jackpot' :D

Absalom wrote:
"A lot of cometary debris" is likely to mean a lot of impacts to the planet, which in turn means a bad candidate for colonization for a very long time.


Not for a civilization capable of interstellar travel. Even if large impacts occurred 10-times as often as here (say, every 5 million years on average), the likelihood of it occurring in a timespan sentients can comprehend is low. And if it were to occur, then they'd simply deflect the comet (or capture it and turn it into an in-orbit refuelling station).


Thu Dec 20, 2012 3:28 am
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Post Re: Tau Ceti probably has five planets, one in habitable zon
Victor_D wrote:
I had an idea - what if the 4.3 Earth-mass planet isn't one planet, but a binary super-terrestrial planet? Say 2.5 + 1.8 planets orbiting a common centre of gravity?


Interesting thought. :mrgreen:
But with the other planet candidates around Tau Ceti having large masses as well, I'd doubt it. :(

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Thu Dec 20, 2012 9:42 am
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Post Re: Tau Ceti probably has five planets, one in habitable zon
Victor_D wrote:
Tau Ceti has lower metallicity than our Sun, so it is conceivable that the planet (if it really exists) has a much lower average density (smaller/lighter core) than Earth. It would probably be covered by a global ocean considering how much cometary debris there is in the system, but with some luck, it could be colonizable...?

Yes, this is the other disquieting fact that I have discovered since the creation of the Humanity worlds -- both Tau Ceti and 82 Eridani are ancient stars with very low metallicities. Some had even speculated that such systems might not have terrestrial planets (due to a lack of elements heavier than H and He). The good news about the Tau Ceti finding is that it does appear to have terrestrial-size planets, but it's not clear to me what the compositions of such planets would be like. My idea when I learned about the metallicity issue was that Aldea (at Tau Ceti) might have very deep oceans... like, deeper than the mantle.


But, again, it's mostly irrelevant to the story. Even in a potential sequel, I don't think these particular worlds would be visited.

Science fiction universes have to exist in kind of an "alternate future" from ours, as discoveries and the advance of history will inevitably contradict whatever an author establishes. Bradbury and Heinlein don't have to rewrite Martian Chronicles or Stranger in a Strange Land just because we've discovered that there isn't an alien civilization on Mars, and we to don't have to amend Star Trek's history to compensate for the fact that there wasn't a Eugenics War in the 1990's.

Victor_D wrote:
I had an idea - what if the 4.3 Earth-mass planet isn't one planet, but a binary super-terrestrial planet? Say 2.5 + 1.8 planets orbiting a common centre of gravity?

It's a good point that the masses of any such companions would include any moons. I don't see any reason why a planet might not have a very large moon; the collision that formed our moon as with a Mars-sized planet; a slightly different impact might have resulted in a much larger moon. Planets (or at least dwarf planets) do exist with very large moons; Charon is something like 12% the mass of Pluto.

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Post Re: Tau Ceti probably has five planets, one in habitable zon
Arioch wrote:
But, again, it's mostly irrelevant to the story. Even in a potential sequel, I don't think these particular worlds would be visited.

Science fiction universes have to exist in kind of an "alternate future" from ours, as discoveries and the advance of history will inevitably contradict whatever an author establishes. Bradbury and Heinlein don't have to rewrite Martian Chronicles or Stranger in a Strange Land just because we've discovered that there isn't an alien civilization on Mars, and we to don't have to amend Star Trek's history to compensate for the fact that there wasn't a Eugenics War in the 1990's.


Of course. I just thought that since the human worlds haven't yet appeared 'officially' in your work, it's still time to adjust the 'canon' so that it reflects the reality better :) I am fine with it either way, it's a minor and unimportant detail as you said :)


Thu Dec 20, 2012 12:11 pm
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Post Re: Tau Ceti probably has five planets, one in habitable zon
Arioch wrote:
Victor_D wrote:
I had an idea - what if the 4.3 Earth-mass planet isn't one planet, but a binary super-terrestrial planet? Say 2.5 + 1.8 planets orbiting a common centre of gravity?

It's a good point that the masses of any such companions would include any moons. I don't see any reason why a planet might not have a very large moon; the collision that formed our moon as with a Mars-sized planet; a slightly different impact might have resulted in a much larger moon. Planets (or at least dwarf planets) do exist with very large moons; Charon is something like 12% the mass of Pluto.


You could turn it around, and let Aldea be the "moon" of a 3 Earth-mass ocean world or something.
Besides, this 4.3 E-m number is the minimum mass. The real mass depends on the angle on which those planets orbit around Tau Ceti as seen from Earth. So probably the masses of those planets will turn out to be higher.

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Fri Dec 21, 2012 11:37 am
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Post Re: The Astronomy Thread
An interesting blog post at systemic: The Tau Ceti Six.

In addition to reminding the reader that the prediction by Tuomi of that particular arrangement of planets around Tau Ceti is still highly speculative... it reminded me that they had earlier discovered what they think to be a Kuiper belt some ten times more massive than our own, which had always seemed strange to me considering that Tau Ceti is so much older than the Sun... I would expect such a belt to diminish over billions of years as objects slowly coalesced into Plutoids.

But the author points out that it might make sense for a low-metallicity star like Tau Ceti to have no Jupiter-mass planets (with less metal to form the prot-cores of more massive planets), and so what one might expect in that case would be lots of inner terrestrial planets, some Neptune-size ice planets, and then a larger than normal Kuiper belt. Which so far is what Tau Ceti appears to have.

There is also a nice visualization of just how far away the predicted planets are from the Goldilocks zone.

Image

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Mon Dec 31, 2012 11:57 pm
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Post Re: The Astronomy Thread
Taken this image into account, and the data turns out to be correct, it starts to look like the 4,3 E-m planet might turn out to be Venus-like in its atmospheric density.

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Tue Jan 01, 2013 3:13 am
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Post Re: The Astronomy Thread
Arioch wrote:
An interesting blog post at systemic: The Tau Ceti Six.

In addition to reminding the reader that the prediction by Tuomi of that particular arrangement of planets around Tau Ceti is still highly speculative... it reminded me that they had earlier discovered what they think to be a Kuiper belt some ten times more massive than our own, which had always seemed strange to me considering that Tau Ceti is so much older than the Sun... I would expect such a belt to diminish over billions of years as objects slowly coalesced into Plutoids.


Such debris discs have been observed around other stars as well. It may be that our Solar System is atypical in the sense that it has been stripped of a large part of its debris by a close fly-by by another star.

Quote:
There is also a nice visualization of just how far away the predicted planets are from the Goldilocks zone.


GeoModder wrote:
Taken this image into account, and the data turns out to be correct, it starts to look like the 4,3 E-m planet might turn out to be Venus-like in its atmospheric density.


Looking at the picture, there is a conspicuous gap between the planet candidates' 4 and 5 orbits. Basically the whole of Tau Ceti's habitable zone lies within its gap (at least according to SolStation's data). Since smaller planets would not be detected, it leaves open a room for a relatively small planet (below 3 Earth masses) orbiting within the habitable zone.


Tue Jan 01, 2013 4:50 am
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Post Re: The Astronomy Thread
Victor_D wrote:
Looking at the picture, there is a conspicuous gap between the planet candidates' 4 and 5 orbits. Basically the whole of Tau Ceti's habitable zone lies within its gap (at least according to SolStation's data). Since smaller planets would not be detected, it leaves open a room for a relatively small planet (below 3 Earth masses) orbiting within the habitable zone.


There's such a gap as well in the Solar System between the orbits of Mars and Jupiter: the area of the main asteroid belt. :P
IOW, don't get you hopes up yet.

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Post Re: The Astronomy Thread
GeoModder wrote:
Taken this image into account, and the data turns out to be correct, it starts to look like the 4,3 E-m planet might turn out to be Venus-like in its atmospheric density.

I would think that such a massive planet would have a very dense atmophere just from the higher gravity alone. Unfortunately, even though super-Earths appear to be very common in other systems, we don't have any in our solar system for reference.

Victor_D wrote:
Such debris discs have been observed around other stars as well. It may be that our Solar System is atypical in the sense that it has been stripped of a large part of its debris by a close fly-by by another star.

Yes, a Kuiper belt appears to be a standard feature of solar systems. Epsilon Eridani also has a very large Kuiper belt, but it also appears to have a Jupiter-mass planet (with an asteroid belt to match), so I guess that's a strike against the idea that Jupiter might be responsible for our smaller Kuiper belt. But then again, Epsilon Eridani is estimated to be only a billion years old, so one would expect more debris.

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Post Re: The Astronomy Thread
GeoModder wrote:
There's such a gap as well in the Solar System between the orbits of Mars and Jupiter: the area of the main asteroid belt. :P
IOW, don't get you hopes up yet.


I know, but AFAIK the main asteroid belt is only possible due to the disruptive influence of Jupiter. Tau Ceti doesn't seem to have one, and the super-earths likely don't have enough pull to prevent the material between them from coalescing. I hope :) The model they used to sift through the noise to find something that might be candidate planets means the farther out from the star you go, the heavier they must be to be detected. So it is at least plausible that a smaller Earth-sized planet in the right orbit between the last two 'detected' planets might remain undetected for the time being. (We really need space interferometric telescopes, thank you NASA/ESA for cancelling them :evil: At least the ESO extremely large telescope which has recently been approved may improve the resolution of our observations of the nearby stars.)

I admit I just want us to find an Earth-like candidate within some reasonable distance from Sol, and the list of candidate G or K type stars isn't that big.


Wed Jan 02, 2013 2:31 am
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Post Re: The Astronomy Thread
Arioch wrote:
GeoModder wrote:
Taken this image into account, and the data turns out to be correct, it starts to look like the 4,3 E-m planet might turn out to be Venus-like in its atmospheric density.

I would think that such a massive planet would have a very dense atmophere just from the higher gravity alone. Unfortunately, even though super-Earths appear to be very common in other systems, we don't have any in our solar system for reference.
I've been told that it's thought that if Earth had more time before the Sun swept the nebula away, then it would have become a gas giant. I wouldn't be surprised if the details are so finicky with these things that you can only know how it'll turn out be looking at the results.


Wed Jan 02, 2013 4:05 am
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Post Re: The Astronomy Thread
Absalom wrote:
Arioch wrote:
GeoModder wrote:
Taken this image into account, and the data turns out to be correct, it starts to look like the 4,3 E-m planet might turn out to be Venus-like in its atmospheric density.

I would think that such a massive planet would have a very dense atmophere just from the higher gravity alone. Unfortunately, even though super-Earths appear to be very common in other systems, we don't have any in our solar system for reference.

I've been told that it's thought that if Earth had more time before the Sun swept the nebula away, then it would have become a gas giant. I wouldn't be surprised if the details are so finicky with these things that you can only know how it'll turn out be looking at the results.


Erm, no. As far as we know Earth has never been far enough from the Sun to give the lighter elements a chance to coalesce on its proto-planetary atmosphere before the solar wind blew them off. Besides, the composition of our current atmosphere is sufficient prove for this. If not, Earth's atmosphere would've been mostly hydrogen.
For instance, most of the water Earth holds is said to come from cometary impacts.

Victor_D wrote:
I know, but AFAIK the main asteroid belt is only possible due to the disruptive influence of Jupiter. Tau Ceti doesn't seem to have one, and the super-earths likely don't have enough pull to prevent the material between them from coalescing. I hope :) The model they used to sift through the noise to find something that might be candidate planets means the farther out from the star you go, the heavier they must be to be detected. So it is at least plausible that a smaller Earth-sized planet in the right orbit between the last two 'detected' planets might remain undetected for the time being.


Well, as I said before, the masses we know of are minimum masses. Granted this outer planet of Tau Ceti won't have dozens of Earth masses, but its orbit is way closer to Tau Ceti, so it makes more sweeps (and consequently gives more gravitic "pushes") to whatever is approaching the inner system. Also, as Arioch's linked picture shows, the orbit where at Tau Ceti a planet would receive the equivalent of Earth's received sunlight is not that far out of the 4,3 E-m planetary orbit. So to have an Earth-like habitable planet on an orbit between f and e, we need a planet with more then one E-m. And the higher the mass, the more likely it would already have been spotted in the data. Especially since five more "signals" have already been detected.

Arioch wrote:
I would think that such a massive planet would have a very dense atmophere just from the higher gravity alone. Unfortunately, even though super-Earths appear to be very common in other systems, we don't have any in our solar system for reference.


Yes, of course. But to give you a comparison from our system, Uranus (the lightest giant) has a total mass of about 14.5 E-m. Only about 0.55 E-m of that is thought to be part of its solid core. It all depends on composition of the planetary core (and subsequently in what part of a star system it formed).

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Wed Jan 02, 2013 2:16 pm
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Post Re: The Astronomy Thread
Interesting that we just happened to be talking about this very subject.

This is an artist's rendering of the proplyd around HD 142527, a young white giant (F6 III) some 450 light years away.

The proplyd features a small inner disk, a large gap from ~10 to ~140 AU (presumably swept out by large Jupiter-type planets), and a massive outer disk extending out to ~300 AU. But the most interesting features are the long streamers of gas that appear to be drawn in from the outer disk by the massive planets in the gap. Some of the material then overshoots the large planets and then is funneled into the inner disk, and perhaps into the star itself.

Image

Which goes to the discussion of why the Kuiper belts of systems with very larger planets might be much less massive than those of systems with no Jupiter-size planets; the large planets may funnel much of the outer disk material into the inner solar system.

And once again, we see that the planetary systems of massive stars are huuuge. A proposed mass for one of the outer planets is 10 Jupiter masses and an orbital radius of 90 AU (3 x Neptune distance).

---------------

Unrelated: there appear to be huge jets of gamma ray radiation extending out both poles of our galactic center. Apparently the galaxy itself has a very strong magnetic field. http://www.usatoday.com/story/tech/scie ... c/1800581/

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Post Re: The Astronomy Thread
GeoModder wrote:
Well, as I said before, the masses we know of are minimum masses. Granted this outer planet of Tau Ceti won't have dozens of Earth masses, but its orbit is way closer to Tau Ceti, so it makes more sweeps (and consequently gives more gravitic "pushes") to whatever is approaching the inner system. Also, as Arioch's linked picture shows, the orbit where at Tau Ceti a planet would receive the equivalent of Earth's received sunlight is not that far out of the 4,3 E-m planetary orbit. So to have an Earth-like habitable planet on an orbit between f and e, we need a planet with more then one E-m. And the higher the mass, the more likely it would already have been spotted in the data. Especially since five more "signals" have already been detected.


As we know from Kepler-11,

Spoiler: show
Image


planetary orbits can be extremely tightly packed, yet stable. Even the proposed planets around Tau Ceti are pretty close to each other - and then there is a conspicuous gap right where we'd expect a habitable planet.

Come on, have a little faith :lol:


Thu Jan 03, 2013 4:03 am
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Post Re: The Astronomy Thread
Arioch wrote:
Which goes to the discussion of why the Kuiper belts of systems with very larger planets might be much less massive than those of systems with no Jupiter-size planets; the large planets may funnel much of the outer disk material into the inner solar system.

And once again, we see that the planetary systems of massive stars are huuuge. A proposed mass for one of the outer planets is 10 Jupiter masses and an orbital radius of 90 AU (3 x Neptune distance).

---------------

Unrelated: there appear to be huge jets of gamma ray radiation extending out both poles of our galactic center. Apparently the galaxy itself has a very strong magnetic field. http://www.usatoday.com/story/tech/scie ... c/1800581/


Interesting find on the funneling. Didn't know that.
10 Jupiter masses? Wow, that's IIRC borderline brown dwarf.
I thought the gamma ray "jets" were caused by a black hole in the center of our Galaxy?

Victor_D wrote:
planetary orbits can be extremely tightly packed, yet stable. Even the proposed planets around Tau Ceti are pretty close to each other - and then there is a conspicuous gap right where we'd expect a habitable planet.

Come on, have a little faith :lol:


Heh. I hope you realize the only reason some of those planets don't orbit one another is that they skim so close to their parent star? The "stabilizer" in this case is merely the star's gravity pull.

Faith? Me? I'm pretty much agnostic, tyvm.

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Thu Jan 03, 2013 9:53 am
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Post Re: The Astronomy Thread
GeoModder wrote:
I thought the gamma ray "jets" were caused by a black hole in the center of our Galaxy?

That was one of the the initial theories when the jets were first discovered by the Fermi gamma ray telescope, but the more recent news from the radio telescopes that have been observing it since is that it's probably not created by the black hole (which is not currently active), but rather some kind of burst of star formation activity in the galactic core, shaped by the galaxy's magnetic field:

Spacedaily.com wrote:
Previously it was unclear whether it was quasar-like activity of our Galaxy's central super-massive black hole or star formation that kept injecting energy into the outflows.

The recent findings, reported in Nature today, show that the phenomenon is driven by many generations of stars forming and exploding in the Galactic Centre over the last hundred million years.

"We were able to analyse the magnetic energy content of the outflows and conclude that star formation must have happened in several bouts," said CAASTRO Director Professor Bryan Gaensler.

Further analyses of the polarisation properties and magnetic fields of the outflows can also help us to answer one of astronomy's big questions about our Galaxy.

"We found that the outflows' radiation is not homogenous but that it actually reveals a high degree of structure - which we suspect is key to how the Galaxy's overall magnetic field is generated and maintained," said Professor Gaensler.

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Post Re: The Astronomy Thread
Interesting indeed.
Now I'm happy the Solar System doesn't have a highly inclined orbit around the Galaxy. :D

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Thu Jan 03, 2013 10:12 am
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Post Re: The Astronomy Thread
More interesting exoplanet news.

First up: NASA Spitzer and ESA Herschel observations indicate the dust disk around Vega has a gap dividing the inner asteroid belt from the outer Kuiper-type belt, presumably cleared out by planets. This is becoming common theme. Note that the Vega system is about four times larger than ours (Vega being A0V and about 2.1 solar masses).

Image

Next: Strange things are going on around Fomalhaut. Fomalhaut is a white A3V 1.92 solar mass star that's very similar to Vega and about the same age (~450 million years). Hubble previously detected a distant (~115 AU) planet (Fomalhaut b) that appears to be sheparding a large outer Kuiper-type belt, both of which appeared to follow the same elliptical orbit (scale and previous orbit estimate). New Hubble obervations indicate that the disk is wider than previously thought, and the b planet's orbit is much more elliptical, and may pass through the disk.

Image

Infrared images from Herschel last year show a concentration of cometary dust elsewhere in the disk, indicating that there's a lot of disruption throughout the system.

Image

Here's a new proposed schematic of the the Fomalhaut system, though it doesn't seem right, since it depicts the disk as circular, when it seems pretty clearly in the photos to be both elliptical and off-center. Kind of wonder how accurate the new calculated Formalhaut b orbit is. They somehow went from an 875-year orbit to a 2000-year one with the addition of just one more data point (which doesn't appear to deviate significantly from the previous three).

Image

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Post Re: The Astronomy Thread
Well, if the datapoint depicted excentricity... :ugeek:

There's indeed weird stuff going on regarding what we think to see at Fomalhaut. I wouldn't be surprised to see a revision or two more to come.

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