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Starfury
2009-Jan-28, 08:59 PM
For a space-based RPG campaign I'm working on I'm planning on having the players start on a world orbiting Tau Ceti. Here's the particulars:

Name: Lilith
Position in System: 2nd
Distance from Primary: 0.68 AU
Diameter: 10040 mi.
Density: 4.8
Surface Gravity: 1.1G
Orbital Period: 228 Earth days
Rotational Period: 30 hours
Axial Tilt: 24 degrees
Atmospheric Pressure: 12.3 psi at sea level
Surface Water: 46%
Moons: 1 (Dia. 1706 mi., Density 2.6, 0.11G, Dist 191,400 mi.)

Lilith is a cool world with more extensive ice caps than Earth. Due to the large amount of asteroidal and cometary material in the Tau Ceti system (10 times more than our solar system), it is more subject to impact events. Because of these impacts, the most advanced forms of life are rodent-like mammalian creatures about the size of housecats.

Having a lower metal content as compared to the Sun, Tau Ceti's planets are not as dense as their Solar counterparts. The system boasts three asteroid belts as well as a Kuiper Belt.

cjameshuff
2009-Jan-28, 10:21 PM
One thing I notice...using Wikipedia's value for Tau Ceti's luminosity, that planet receives about 1.1 times as much insolation as Earth. There's plenty of room for other factors to make it cooler, of course.



Because of these impacts, the most advanced forms of life are rodent-like mammalian creatures about the size of housecats.

How do you define "advanced"? For an environment subject to constant bombardment and resulting climate variations, an advanced species would be a generalist, capable of inhabiting a wide range of habitats, possibly with the ability to go long periods in a dormant state. They could have more efficient metabolisms and more effective and flexible digestive systems than anything on Earth, but they needn't even be multicellular. Given that Tau Ceti appears to be significantly older than the sun, they may also have been evolving for a much longer time than life on Earth.



Having a lower metal content as compared to the Sun, Tau Ceti's planets are not as dense as their Solar counterparts. The system boasts three asteroid belts as well as a Kuiper Belt.

Also note that "metals" in the astrophysics sense are anything other than hydrogen and helium...though it's measured using iron.

timb
2009-Jan-28, 10:35 PM
Seems rather lacking in density. Any sort of mammalian is advanced.

RalofTyr
2009-Jan-28, 10:54 PM
I doubt there's any habitable worlds at Tau Ceti.

Tau Ceti is 8 to 10 Billion years old. Given the system's low metallicity, The worlds are probably averaging Mars size or smaller. We know there's no large gas giants orbiting, so the system is probably full of terrestrial worlds. However, given the age of the system, plate tectonics and the magnetic dynamos of any world there, even Earth sized, would have stopped or slowed down. Giving Tau Ceti's solar winds a chance to strip down the atmospheres. Any advanced life that evolved on any world in the Tau Ceti system is probably long dead, it's heyday was billions of years ago.

Most of the worlds in the Tau Ceti system would probably resemble Mars. Either hot Mars or cold Mars. There might be a few that resemble the outer moons of our solar system. Frozen balls with thin atmospheres.

EDG
2009-Jan-29, 08:59 AM
Why should lower metallicity than Sol mean the planets are smaller? It's not a subdwarf, after all (it's got 25-75% of Sol's metallicity, which is still quite significant), and there'd still be plenty of dust in the proto-planetary nebula (in fact, dust has been detected there: http://www.solstation.com/stars/tau-ceti.htm )

If there's as much material in the 10Ga old dust disk as the solstation article implies, I'd imagine there was a LOT more of it when the planets were forming, so I don't think there'd be any reason for them to be smaller.

eburacum45
2009-Jan-29, 12:16 PM
A recent run of simulations I have seen somewhere (blast, should have bookmarked it) created a wide range of planetary systems with a wide range of metallicities; the low metallicity systems often had few or no gas giants, but there were sometimes quite a few terrestrials, of all sizes. I think that low metallicity systems might make lots of smaller planets, even if the larger ones are absent.

eburacum45
2009-Jan-29, 12:24 PM
Try moving the planet to about 0.75 AU. You'll get a cool world, but with a respectable atmosphere it will warm up enough to suit your purposes.

(according to this calculator anyway)
http://www.geocities.com/albmont/hard-sf.htm

Starfury
2009-Jan-29, 01:52 PM
Thanks!

Lots of good input to consider. I can't wait to play with the calculator and see what I can do. :)

Fiery Phoenix
2009-Jan-29, 04:07 PM
Sorry, but what exactly am I looking for in here? Are we talking about imaginary planets and making them up? If so, I actually wrote an article describing an entire imaginary star system last weekend. I would be happy to post it here, if anyone cares.

EDG
2009-Jan-29, 05:08 PM
A recent run of simulations I have seen somewhere (blast, should have bookmarked it) created a wide range of planetary systems with a wide range of metallicities; the low metallicity systems often had few or no gas giants, but there were sometimes quite a few terrestrials, of all sizes. I think that low metallicity systems might make lots of smaller planets, even if the larger ones are absent.

I think that's Sean Raymond's work. I have notes from his papers and PhD thesis that say the same thing - they'd have fewer GGs (and the ones that are there would be be smaller) but IIRC there was nothing that particularly limited the formation of rocky planets there.

Starfury
2009-Jan-29, 06:19 PM
Sorry, but what exactly am I looking for in here? Are we talking about imaginary planets and making them up? If so, I actually wrote an article describing an entire imaginary star system last weekend. I would be happy to post it here, if anyone cares.

By all means. :)

RalofTyr
2009-Jan-29, 10:28 PM
Sorry, but what exactly am I looking for in here? Are we talking about imaginary planets and making them up? If so, I actually wrote an article describing an entire imaginary star system last weekend. I would be happy to post it here, if anyone cares.

Seconded.

Fiery Phoenix
2009-Jan-31, 01:30 PM
Here:


A Look into an Imaginary Star System; The Amazon System

http://i43.tinypic.com/2upbxvs.jpg
Borr, the gas giant nearest Amazon and the second closest planet to it, and its largest moon, Yuto, appear in this picture.


The Universe is a big and beautiful place. In fact, it is so beautiful that it almost surpasses all human comprehension. But it is always easy to simply imagine things. In this article, a star system is imagined -- the Amazon System. The Amazon System is an imaginary star system of the Milky Way Galaxy located 2,350 light years from the Solar System. Amazon is a brilliant star that burns at half again the temperature of Sol and is several times more massive. It has a total of seven planetary companions with one asteroid belt; a small band of drifting rocks between the orbits of the planets Opkel and Thessa. The seven planets are, in order from closest to farthest from their host star, Terbin, Borr, Opkel, Thessa, Minov, Woller, and Rignant. Three of which are terrestrial planets; while the other four are gaseous. Amazon’s closest companion is the massive terrestrial world of Terbin, which is twice as massive as Earth. Lying at a distance of two astronomical units from giant Amazon and having a thick atmosphere of carbon dioxide and water vapor, Terbin has an average surface temperature of 2,200 C! This alone makes it by far the most hostile world in the system. Not to mention the burning planet makes one complete orbit about Amazon in just a month period. Borr, the next planet out, separated by a distance of approximately five astronomical units from Terbin, is a standard hydrogen and helium gas giant a little smaller than Saturn with 28 satellites orbiting it; most of which are very small and insignificant. Its largest satellite, as large as Earth’s moon, is Yuto -- which is very similar to Jupiter’s Io, especially in its great and increasing volcanic activity. Further still is the rocky world of Opkel; a desert planet about the same size as Venus, with a fairly thin atmosphere of carbon dioxide and argon, mixed with some trace amounts of sulfur dioxide and ammonia. Opkel is orbited by three small moons; one of which is so tiny that it has not achieved hydrostatic equilibrium to be in a round shape. One interesting fact about Opke is the existence of some form of primitive life beneath its relatively cold surface. About 40 astronomical units beyond Opkel is Thessa; the smallest planet in the Amazon System. Thessa is a frozen world only one-third the size of Earth. It has an extremely thin atmosphere of oxygen and nitrogen along with a thin pair of ice rings. One giant, icy satellite that is roughly half the size of Thessa itself orbits very close to the planet. The average surface temperature on Thessa can reach a freezing -105 C. From this point onward, Amazon’s awesome influence starts to decrease considerably. A few tens of millions of kilometers after Thessa lies Minov; a standard Neptune-like gas giant with a few dozen moons circling it. Minov’s glorious dark-blue color can be easily glimpsed from the clear skies of Thessa; as the two planets are not too far away from each other. Next planet, Woller, another hydrogen and helium gas giant, is the largest planet in the entire system; with a radius two and a half times that of Jupiter. It orbits two times Minov’s distance from Amazon, and is orbited by at least 100 moons. Its immense gravity played a major role in capturing such a great number of satellites. The last planet, Rignant, is a relatively small gas giant lying at an inconceivable distance of 710 astronomical units from Amazon! This locates this planet in the outer limits of the system, where Amazon’s influence is essentially non-functional. Rignant’s most remarkable feature is its spectacular ring system; as well as its green atmosphere. It is also the only gas giant in the system which does not have moons whatsoever. A hypothetical astronaut looking at Amazon from near Rignant would not know or even realize that the planet right next to them actually orbited that star; it would just seem as if they were looking at a very bright but insignificant star between thousands of other stars!

I wanted to add it via attachments, where it looks much better. But I couldn't due to the invalid extension.

Anyhow... Have at it.

cjameshuff
2009-Jan-31, 10:34 PM
Lying at a distance of two astronomical units from giant Amazon and having a thick atmosphere of carbon dioxide and water vapor, Terbin has an average surface temperature of 2,200 C! This alone makes it by far the most hostile world in the system.

That's probably excessively hot. Water and CO2 will probably both undergo some degree of thermal decomposition. Even without this, I have doubts Terbin would have any significant water. Amazon's hotter, putting out more UV with which to photodissociate upper atmosphere hydrogen-containing compounds...combined with the temperature, I think Terbin would be dryer than Venus. I would also expect it to lose the rest of its atmosphere rather quickly, given that temperature and that much sunlight, though it might simply be sufficiently volcanic to maintain an atmosphere.

As for the source of the heat...half again as hot and "several solar masses" makes it a heavy class A. Fomalhaut, surface temperature 8751 K, 2.1 solar masses, luminosity of 17.66 times that of the Sun, would be somewhat similar. Taking the different distance and possible higher mass into account, Terbin would be receiving about 2-3 times as much insolation as Venus (2.2 if orbiting Fomalhaut). Scaling temperature with that as a very rough and probably high estimate, I would expect it to be no more than 1400-1900 C.



A few tens of millions of kilometers after Thessa lies Minov; a standard Neptune-like gas giant with a few dozen moons circling it. Minovís glorious dark-blue color can be easily glimpsed from the clear skies of Thessa; as the two planets are not too far away from each other.

Unless they are orbiting each other at this distance, they will be up to 80 AU apart...

Fiery Phoenix
2009-Feb-01, 06:40 AM
That's probably excessively hot. Water and CO2 will probably both undergo some degree of thermal decomposition. Even without this, I have doubts Terbin would have any significant water. Amazon's hotter, putting out more UV with which to photodissociate upper atmosphere hydrogen-containing compounds...combined with the temperature, I think Terbin would be dryer than Venus. I would also expect it to lose the rest of its atmosphere rather quickly, given that temperature and that much sunlight, though it might simply be sufficiently volcanic to maintain an atmosphere.

Regardless. Lying at such a relatively short distance from Amazon, any lost atmosphere would probably be replaced with a brand new one within a fairly short span of time.


As for the source of the heat...half again as hot and "several solar masses" makes it a heavy class A. Fomalhaut, surface temperature 8751 K, 2.1 solar masses, luminosity of 17.66 times that of the Sun, would be somewhat similar. Taking the different distance and possible higher mass into account, Terbin would be receiving about 2-3 times as much insolation as Venus (2.2 if orbiting Fomalhaut). Scaling temperature with that as a very rough and probably high estimate, I would expect it to be no more than 1400-1900 C.


Well, I assumed Amazon's mass to be 5-8 Solar masses, if I'm not mistaken. I have no idea if such a star would have the same physical properties mentioned in the article, though.


Unless they are orbiting each other at this distance, they will be up to 80 AU apart...

You mean "a few tens of millions of kilometers" isn't enough to separate a gas giant and a terrestrial planet? I was kind of thinking the same thing, actually; Thessa would probably end up being captured by Minov at one point and therefore being a moon of the planet. But as soon as I remembered the case of Neptune and Pluto, I kind of talked myself out of it.

Thanks for your comments, cjameshuff.

cjameshuff
2009-Feb-01, 08:24 PM
Regardless. Lying at such a relatively short distance from Amazon, any lost atmosphere would probably be replaced with a brand new one within a fairly short span of time.

Replaced from where?



You mean "a few tens of millions of kilometers" isn't enough to separate a gas giant and a terrestrial planet? I was kind of thinking the same thing, actually; Thessa would probably end up being captured by Minov at one point and therefore being a moon of the planet. But as soon as I remembered the case of Neptune and Pluto, I kind of talked myself out of it.

No, I mean they will sometimes be on opposite sides of the system, and usually at widely separated positions in their orbits, so unless Thessa is orbiting Minov, "the two planets are not too far away from each other" is only occasionally correct. But yes, Thessa would at the very least be thrown into a very different orbit the first time it and Minov passed each other, assuming circular coplanar orbits with the radii you describe.

Real orbits would be eccentric and inclined: Neptune's distance from the sun varies by just over 100 million km, Pluto's orbital distance varies by about 3 billion km, the directions of the eccentricity and planes of their orbit are different, all resulting in them never getting particularly close to each other.

Fiery Phoenix
2009-Feb-01, 09:20 PM
Replaced from where?

Amazon; particularly the limitless stuff it throws off into space, which would keep getting recaptured by Terbin's supposed strong gravity, recreating the planet's atmosphere. Anyway, that was only in response to what you said; it's not how I intended it. :)

As for the case of Thessa and Minov, I understand what you mean now.

cjameshuff
2009-Feb-03, 08:05 PM
Amazon; particularly the limitless stuff it throws off into space, which would keep getting recaptured by Terbin's supposed strong gravity, recreating the planet's atmosphere. Anyway, that was only in response to what you said; it's not how I intended it. :)

It doesn't work. Surface gravity's not going to be anywhere near strong enough to capture enough solar wind to make an atmosphere...assuming a similar density to Earth, surface escape velocity will only be about 1.3-1.4 times higher than it is on Venus. And if that were the source, it wouldn't be CO2/H2O. Some water would be formed by reactions between protons from the stellar wind and oxygen in the unprotected rock surface, but it would be quickly lost.

Tom Mazanec
2009-Feb-03, 08:06 PM
Did you mean Woller was 2 1/2 times as massive as Jupiter? GGs don't get much larger than Jupiter, unless they are "Hot Jupiters", and even then the heat does not bloat them up that much. They pretty much just get denser.
Rignant is another problem..."spectacular ring system" and "no moons whatsoever" don't go along together... such a system would need at least small shepherd moons to keep the rings stable.

RalofTyr
2009-Feb-03, 08:42 PM
Here's a diddy I made about the Tau Ceti system. It's a future Caldarian settled system (The Caldarians are originally from Mars(colony) in which a large number of Japanese and Finnish cultures and people merged into one society).


Tau Ceti (Kujira)

The worlds,

A Vulcan in a torch orbit.[0.1AU]

A Mercurian/Moon further out.[0.23AU] Greyscale

Another Mercurian/Moon, a bit larger or smaller. Greyscale

A Hot Mars, with normal atmosphereic pressure, but little oxygen and little to no water, having lost it all from an eon of life. [0.5AU] Barren Terrain with 0% water

A Blue Planet, with virtually no land, except a few small continents near the equator, just waking up through stellar evolution. This world was ice and was just outside the HZ for 8 billion years. It has shallow, warm seas.[0.7AU] Barren with 100% water.

A Mars-like world.

Ice world, outside the HZ and utterly frozen with an atmosphere. It has a small band of broken ice at the equator. It spins rapidly and is more massive than the Earth. Barren with 100% and 100% ice.

Ice world, twice as massive as the Earth with volcanism still going on. The atmpshere is thick, however cloudless. Vulcan with 0% water.

Ice world, dead world.

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

TCet 1 (Ichi): In a "Torch" orbit, this world is stressed by the tidal forces of Tau Ceti and has a molten surface, like Io.

TCeti 2 (Ni) : A small Mercury/Moon like world

TCeti 3 (San) : Another small Mercury/Moon like world

TCeti 4 (Shi) : A hot Mars. Started out with liquid water, but it was too close and the water boiled away. creating a Venus like world. After billions of years of solar winds, the atmosphere has been reduced enough to prevent a run away greenhouse. Any water on this world has been brought be cometary impacts over the last few billion years.

TCeti 5 (Go) : A blue planet. 100% water, (well, 99.9%). It has an unusually active core that is slowly turning off. The magnetic field is rather weak here. It is full of life, Cambrian-like that haven't evolved much in the billions of years. Everytime it does, one of the many comets in the skies will cash causing a mass extinction.

TCeti 6 (Roku) : An ice sea world. 100% water as well. However, it is frozen, except for the equator that has some liquid water.

TCeti 7 (Nana) : A Mars like world. A cold desert with ice caps.

TCeti 8 (Hachi) : A Mercurian/Moon like world.

TCeti 9 (Kyuu) : A super earth. It's very volcanically active, however, it has a thin atmosphere and most of the C02 falls back as snow.

TCeti 10 (Juu) : A snowball world. Thin atmosphere, like one of Jupiter's moons. In fact, any other TCeti worlds will resemble the moons of the outer system.

A few more Titan like worlds.





Why should lower metallicity than Sol mean the planets are smaller? It's not a subdwarf, after all (it's got 25-75% of Sol's metallicity, which is still quite significant), and there'd still be plenty of dust in the proto-planetary nebula (in fact, dust has been detected there: http://www.solstation.com/stars/tau-ceti.htm )

If there's as much material in the 10Ga old dust disk as the solstation article implies, I'd imagine there was a LOT more of it when the planets were forming, so I don't think there'd be any reason for them to be smaller.

So each world should be 25 to 75% less massive. I suspect that in you eliminate the gas giants, like in our system, you'd have a lot more terrestrial worlds in the outer Tau Ceti System.

10Ga old dust disk is probably cometery material. Mostly ice. Any worlds were or are probably very wet.



A recent run of simulations I have seen somewhere (blast, should have bookmarked it) created a wide range of planetary systems with a wide range of metallicities; the low metallicity systems often had few or no gas giants, but there were sometimes quite a few terrestrials, of all sizes. I think that low metallicity systems might make lots of smaller planets, even if the larger ones are absent.

*rubs eburacram45's head so he remembers the site.

I used Accrete, but it keeps crashing my computer.

EDG
2009-Feb-04, 06:00 AM
So each world should be 25 to 75% less massive.

I don't think that follows at all. If you look at the metal content of our own solar system and reduced it to 25-75% of what it is, I think you'd find that there's still more than enough metals to account for the formation of the planets we see today.

Fiery Phoenix
2009-Feb-04, 06:25 AM
Did you mean Woller was 2 1/2 times as massive as Jupiter? GGs don't get much larger than Jupiter, unless they are "Hot Jupiters", and even then the heat does not bloat them up that much. They pretty much just get denser.
Rignant is another problem..."spectacular ring system" and "no moons whatsoever" don't go along together... such a system would need at least small shepherd moons to keep the rings stable.

Thanks for the correction. I'm editing the original version of the write up now.

RalofTyr
2009-Feb-04, 05:09 PM
I don't think that follows at all. If you look at the metal content of our own solar system and reduced it to 25-75% of what it is, I think you'd find that there's still more than enough metals to account for the formation of the planets we see today.

But what are the odds?

cjameshuff
2009-Feb-04, 05:57 PM
But what are the odds?

Probably not much different, assuming the Solar system is typical and considering that both Sol and Tau Ceti started with significantly more material than necessary. The Tau Ceti system might have been significantly thinner early on, but as bodies were ejected or absorbed by the sun/gas giants, I would expect it to end up in a similar state...probably a bit faster, with less material to eject/sort into stable orbits.

EDG
2009-Feb-04, 10:32 PM
But what are the odds?

Pretty high. I think you'd only have a really big difference if you had around 1% or less of Sol's metal content.

RalofTyr
2009-Feb-05, 01:06 AM
We know there's no gas giants orbiting around Tau Ceti. We would have detected them by now. There's probably a lot of terrestrial worlds in orbit around Tau Ceti...

cjameshuff
2009-Feb-05, 01:24 AM
We know there's no gas giants orbiting around Tau Ceti. We would have detected them by now. There's probably a lot of terrestrial worlds in orbit around Tau Ceti...

We know there's no brown dwarfs or close-orbit superjovians orbiting Tau Ceti. I've not seen anything excluding the possibility of smaller, further giants like those in our own system.

BigDon
2009-Feb-05, 01:42 AM
You mean "a few tens of millions of kilometers" isn't enough to separate a gas giant and a terrestrial planet? I was kind of thinking the same thing, actually; Thessa would probably end up being captured by Minov at one point and therefore being a moon of the planet. But as soon as I remembered the case of Neptune and Pluto, I kind of talked myself out of it.

Thanks for your comments, cjameshuff.

Actually I think it would more likely be ejected from the system than captured.

timb
2009-Feb-05, 02:21 AM
We know there's no gas giants orbiting around Tau Ceti. We would have detected them by now. There's probably a lot of terrestrial worlds in orbit around Tau Ceti...

How do you reach that conclusion? Probing the hot dust content around epsilon Eridani and tau Ceti with CHARA/FLUOR (http://arxiv.org/abs/0710.1731) found about as much "warm dust" within 3AU of tau Ceti as exists in the Sun's zodiacal cloud. This implies a debris disk perhaps similar to the asteroid belt to replenish the dust (dust particles that close to a star tend to drop towards it until they evaporate) or some transient event that has moved material from the outer belt into the inner system. The former possibility does not rule out the existence of terrestrial planets but does imply a planet-free zone; the latter might make the inner system an inhospitable place similar to Earth during the Late Heavy Bombardment.

timb
2009-Feb-05, 02:41 AM
Actually I think it would more likely be ejected from the system than captured.

Ejection (either from the system or into an highly eccentric and inclined orbit) is more likely than collision with the gas giant which is in turn much more likely than capture. I doubt the claim that a few tens of millions of km is not sufficient separation between a small planet and a Neptune mass planet though. I seem to recall that you can replace Earth with a Jupiter clone in OrbitSimulator and Venus remains stable. (Not that I'd risk anything I cared about on results from OrbitSimulator).

That said, I've noticed that imaginary stellar systems are usually implausibly packed with planets, especially in the HZ.

cjameshuff
2009-Feb-05, 03:06 AM
I doubt the claim that a few tens of millions of km is not sufficient separation between a small planet and a Neptune mass planet though. I seem to recall that you can replace Earth with a Jupiter clone in OrbitSimulator and Venus remains stable. (Not that I'd risk anything I cared about on results from OrbitSimulator).

Earth and Venus are separated by a much larger distance in proportion to their orbital radii. Venus is moving at a much higher velocity relative to Earth as a result...a bit over 5 km/s at closest approach, compared to about 17-18 m/s for a planet at 40 AU and another at 40.3 AU. Their relative velocity would be far less than the orbital velocity of Neptune's moons at a similar distance (390 m/s for a circular orbit).

The difference does go up with the mass of the star, but at 8 solar masses, the relative velocity is still only 50 km/s. It wouldn't stay in its circular orbit to reach this position and velocity, clearly.

timb
2009-Feb-05, 03:14 AM
Earth and Venus are separated by a much larger distance in proportion to their orbital radii. Venus is moving at a much higher velocity relative to Earth as a result...a bit over 5 km/s at closest approach, compared to about 17-18 m/s for a planet at 40 AU and another at 40.3 AU. Their relative velocity would be far less than the orbital velocity of Neptune's moons at a similar distance (390 m/s for a circular orbit).

The difference does go up with the mass of the star, but at 8 solar masses, the relative velocity is still only 50 km/s. It wouldn't stay in its circular orbit to reach this position and velocity, clearly.

You're right there. I was a little confused about which imaginary planets we were discussing.

RalofTyr
2009-Feb-06, 07:37 AM
How do you reach that conclusion? Probing the hot dust content around epsilon Eridani and tau Ceti with CHARA/FLUOR (http://arxiv.org/abs/0710.1731) found about as much "warm dust" within 3AU of tau Ceti as exists in the Sun's zodiacal cloud. This implies a debris disk perhaps similar to the asteroid belt to replenish the dust (dust particles that close to a star tend to drop towards it until they evaporate) or some transient event that has moved material from the outer belt into the inner system. The former possibility does not rule out the existence of terrestrial planets but does imply a planet-free zone; the latter might make the inner system an inhospitable place similar to Earth during the Late Heavy Bombardment.

Then I guess you're right and there's no planets around Tau Ceti, despite 10 billion years. It just hasn't happened yet.

cjameshuff
2009-Feb-06, 03:04 PM
Then I guess you're right and there's no planets around Tau Ceti, despite 10 billion years. It just hasn't happened yet.

I think you need to re-read the text you quoted.

The dust is evidence of a debris-filled planet-free zone, like the asteroid belt of the Solar system. Velocity measurements rule out hot Jupiters, which is a good thing for one looking for Earth-like terrestrial planets, and make it unlikely that there's a Jupiter-sized body in less than a 15 year orbit. Considering the lower mass of the star, that means a semi-major axis of at least 5.6 AU, compared to 5.2 for Jupiter. You could very nearly duplicate the Solar system out to Jupiter around Tau Ceti and still be consistent with observations. The dust disk cutoff at 10 AU would seem to imply no Neptune or Uranus equivalents, but there's probably still room for 2 gas giants and nothing to rule out a handful of terrestrials.

timb
2009-Feb-06, 10:24 PM
I think you need to re-read the text you quoted.

The dust is evidence of a debris-filled planet-free zone, like the asteroid belt of the Solar system. Velocity measurements rule out hot Jupiters, which is a good thing for one looking for Earth-like terrestrial planets, and make it unlikely that there's a Jupiter-sized body in less than a 15 year orbit. Considering the lower mass of the star, that means a semi-major axis of at least 5.6 AU, compared to 5.2 for Jupiter. You could very nearly duplicate the Solar system out to Jupiter around Tau Ceti and still be consistent with observations. The dust disk cutoff at 10 AU would seem to imply no Neptune or Uranus equivalents, but there's probably still room for 2 gas giants and nothing to rule out a handful of terrestrials.

For sure you could have a fair few terrestrial planets there. I don't think anyone can say they know there are many terrestrial planets there, as the previous poster did. For reasons I couldn't see the author of the paper I cited seemed to favor the idea that the dust was the result of a transient event rather than constantly replenished by a debris disk. A transient event could be triggered by interactions between the outer belt and a planet or a passing star. Lower metallicity systems seem to have fewer large gas giants on average, and smaller stars are also less lkely to host Jovian+ planets.