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TimberWolf
2004-Jul-28, 01:07 AM
Hello,

From what I have gathered, astronomers looking for exo-planets have seen many planetary systems that would be inhospitable to life as we know it. ( Planets with very elliptical orbits and other oddities of orbital mechanics. ) Is there any planetary systems out there that are remotely Sol-like? If so, how close are these systems?

Cordially,

TimberWolf

Manchurian Taikonaut
2004-Jul-28, 01:45 AM
I've read that Upsilon Andromedae could be a little sol like, unlike some of the others it had a family of many planets.



http://www.solstation.com/stars2/upsandc2.jpg

http://antwrp.gsfc.nasa.gov/apod/image/planets_marcy.gif




Unfortunately most of the worlds discovered so far have been giant hot-jupiter like planets

http://cfa-www.harvard.edu/afoe/espd.html

If we are to find more then we need to improve our detections and build super scopes for finding these Earth like planets. NASA's TPF the Terrestrial-Planet-Finder, Gaia, or Darwin might be able to do this. Some day we could be able to view Earth's twin out there in space what an amazing alien world it would be, such a wonderful thing it would be to see an Earth like world.

http://t2wesa.r3h.net/esaCP/SEMBVPWLDMD_FeatureWeek_0.html

Brady Yoon
2004-Jul-28, 01:57 AM
The solar systems that have been discovered are different from our own solar sytem. However, there is a bias. Only very massive planets that are close to their parents stars can be detected. That will change as more sensitive equipment is developed.

RoboSpy
2004-Jul-28, 03:57 AM
Somebody please correct me if I'm outdated, but to my knowledge only two methods have even proven themselves in finding exoplanets, and neither is sensitive enough to find most terrestrial-sized worlds.

One method is watching the stars wobble. Very large planets, at least as massive as Jupiter, but often far larger exert such a gravitational influence on their primary that we can observe the star "wobbling" as this massive planet orbits it. I think only very large planets can be found this way, and usually in very close orbits, often as close to their primary as Mercury is to Sol.

The other method I know of is to watch for the star to dim as the planet passes in front of it, like observing an eclipse from several light-years away. By measuring how much a star dims, the size of the planet can be inferred. I think many of the planets we know about now have been found by combining data from both these methods, since either one used alone will provide incomplete data about the planet being observed.

The obvious way to find planets is of course to point telescopes at them, but the problem is that the primary star usually washes out any light relfected by the comparitvely small, dark planets we're trying to observe. Maybe I just haven't been reading the right articles, but I still haven't heard of any direct telescopic observations being successful. If anyone knows better, please correct me.

Kullat Nunu
2004-Jul-28, 07:05 AM
I've read that Upsilon Andromedae could be a little sol like, unlike some of the others it had a family of many planets.

Unfortunately Upsilon Andromedae is very different from our Solar System. It has one hot Jupiter and two eccentric Jupiters which prohibit any Earth-like planets.

55 Cancri d has very Jupiter-like orbit but the two inner planets are also poorly located.

47 Ursae Majoris system is probably most Solar System-like. It has two relatively distant, massive planets orbiting in circular orbits.

Some giant planets, like HD 27442 have very Earth-like orbits. If a such planet has giant moons, they might harbor life.

There are few other systems which allow Earth-like worlds to form (i.e. the known planets don't perturb possible inner planets).

Kullat Nunu
2004-Jul-28, 07:21 AM
Somebody please correct me if I'm outdated, but to my knowledge only two methods have even proven themselves in finding exoplanets, and neither is sensitive enough to find most terrestrial-sized worlds.

You're correct. Although radial velocity method may become sensitive enough to detect terrestrial planets, motions on the star's surface drows the signal out.


One method is watching the stars wobble. Very large planets, at least as massive as Jupiter, but often far larger exert such a gravitational influence on their primary that we can observe the star "wobbling" as this massive planet orbits it. I think only very large planets can be found this way, and usually in very close orbits, often as close to their primary as Mercury is to Sol.

The other method I know of is to watch for the star to dim as the planet passes in front of it, like observing an eclipse from several light-years away. By measuring how much a star dims, the size of the planet can be inferred. I think many of the planets we know about now have been found by combining data from both these methods, since either one used alone will provide incomplete data about the planet being observed.

Actually so far only three planets have been found and one confirmed by this method. After all, it is very unlikely that the planet crosses the surface of the star. But if we watch continuously hundreds of thousands of stars, we can detect the transists. However, signal from terrestrial planet transits is lost because of unquiet atmosphere. If we put the telescope in space, we can detect even the terrestrial planets. Kepler mission is going to do just that.

It is a little awkward but the actual detection of the transit is not the most difficult part of the search; more difficult is to eliminate the false positives like brown dwarfs, small companion stars and grazing eclipses by normal stars.

More promising discovery method is the gravitational microlensing method. Deviations from the normal brightening curve can reveal even terrestrial planets already now. One giant planet is already found by this way. Major problem in this method is that the microlensing events are very rare and unique. We cannot detect these planets again.


The obvious way to find planets is of course to point telescopes at them, but the problem is that the primary star usually washes out any light relfected by the comparitvely small, dark planets we're trying to observe. Maybe I just haven't been reading the right articles, but I still haven't heard of any direct telescopic observations being successful. If anyone knows better, please correct me.

Not yet, but I am sure that the first direct extrasolar planet observations will be done soon.

Tranquility
2004-Jul-28, 05:39 PM
Also the Kepler mission (http://www.kepler.arc.nasa.gov/) seems to be underway.

Kullat Nunu
2004-Jul-28, 06:18 PM
Also the Kepler mission (http://www.kepler.arc.nasa.gov/) seems to be underway.

Yes, it was selected as a Discovery-class mission with the Dawn asteroid probe. Too bad ESA had to cancel their own Eddington (http://astro.esa.int/SA-general/Projects/Eddington/) project, which would have been similar to Kepler. COROT (http://www.esa.int/export/esaSC/120372_index_0_m.html), a French transit search telescope will be launched earlier, in 2006. It however lacks the required sensitivity to detect another Earths, altough it can find many gas giants and possible terrestrial planets few times larger than the Earth.

Tranquility
2004-Jul-28, 06:21 PM
altough it can find many gas giants and possible terrestrial planets few times larger than the Earth.

I wonder what is the limiting size of a terrestrial planet for it to be able to support the evolution of intelligent life forms. Seems like a terrestrial planet several times the size of the Earth and hence having a high density would provide a gravitational force that hinders the growth of intelligent life, or would the shortness of creatures on such a planet not present a hindrance?

rleyland
2004-Jul-28, 06:39 PM
The solar systems that have been discovered are different from our own solar sytem. However, there is a bias. Only very massive planets that are close to their parents stars can be detected. That will change as more sensitive equipment is developed.

Isn't there another bias inherent in both of the methods mentioned.

That is, the orbits of the planets must be somewhat lined up with our orientation to the star. This is obvious for the transit detection methods, and I thought the wobble methods use doppler spectrum shifts.

If not, could someone enlighten me?

If so, do we have enough information to project the probable incidence of planets around nearby stars?

thanks,
Robbo

CaptainToonces
2004-Jul-28, 08:15 PM
I read on the Kepler page the chances of the transit being inside our line of sight are about 0.5%. So if we surveyed enough systems that wouldn't actually be a problem (as far as answering questions about the "average" solar system).

As for stronger gravity hindering evolution of intelligence, I don't see why it would. The hard part i think is getting to the point of molecular replication, after that intelligence seems a natural progression of evolution regardless of the strength of gravity. After all, life on this planet moved from the sea (where buoyancy weakens the effect gravity) on to land where gravity is stronger. And it seems intelligence developed on land.

TimberWolf
2004-Jul-29, 12:54 AM
Hello,

Actually, intelligence is not limited to land creatures. Maybe not the intelligence that humans possess, but some aquatic species do possess some measure of intelligence. These species include dolphins, whales and octopi, believe it or not. From what I gather about current means of planet detection, it sounds like it's mostly mathematical.

In other words, the planet hunters look for the effect on the parent star that the mass of the smaller planet exerts. At least in one method. I'm no mathematician, so they're the better judges on the merits of the different methods.

Cordially,

TimberWolf

scourge
2004-Jul-29, 03:27 AM
I read an article in the New York Times ‘Science Times’ section about three or four weeks ago that was very exciting regarding this topic. A team is developing a computer program that will ‘black out’ the starlight of a target system, revealing the reflected light off of orbiting planets. This will help us find terrestrial-style planets, because we’re very bright for our size ;)

If I recall correctly, we can then run a spectral analysis on the light reflected from the orbiting planet, and determine some information regarding the atmospheric and surface composition.

I can hardly wait til that system is up and running! (but it's tricky because the computer has to be meticulously programmed to recognize microscopic imperfections in the lens, to get the clarity required)

Brady Yoon
2004-Jul-29, 05:39 AM
Isn't there another bias inherent in both of the methods mentioned.

That is, the orbits of the planets must be somewhat lined up with our orientation to the star. This is obvious for the transit detection methods, and I thought the wobble methods use doppler spectrum shifts.

If not, could someone enlighten me?

If so, do we have enough information to project the probable incidence of planets around nearby stars?

thanks,
Robbo

That's not really a bias, because any kind of planet, whether it be large or small, can be lined up. Or am I wrong... #-o

Kullat Nunu
2004-Jul-29, 06:02 AM
Isn't there another bias inherent in both of the methods mentioned.

That is, the orbits of the planets must be somewhat lined up with our orientation to the star. This is obvious for the transit detection methods, and I thought the wobble methods use doppler spectrum shifts.

If not, could someone enlighten me?

If so, do we have enough information to project the probable incidence of planets around nearby stars?

thanks,
Robbo

That's not really a bias, because any kind of planet, whether it be large or small, can be lined up. Or am I wrong... #-o

Well, inclination of the planet's orbit does affect the radial velocity. Less inclined the orbit is, smaller is the amplitude in radial velocity. That's why the masses measured by radial velocity are always minimum masses(* (inclination 90°, a transiting orbit). If the inclination becomes near zero ("bullseye view") the companion may not be a planet but a brown dwarf or even a small red dwarf. However, probability for that many of the found planets really are more massive objects is very low, and moderate inclinations don't change the predicted mass much. Another proof for this is the fact that there are very few brown dwarfs in small orbits (the brown dwarf desert). The reason for this is unknown.

Except for transiting planets, only the orbital inclination (84°) of Gliese 876 b has been measured astronomically with the Hubble Space Telescope Fine Guidance Sensors. Similar measurements for other stars like Upsilon Andromedae are under way.

*) Actually M = M_min/sin(i), where i is the orbital inclination.

sol_g2v
2004-Jul-29, 06:12 AM
I think the most sol-like planetary "system" found so far is HD70642 (http://www.solstation.com/stars2/hd70642.htm)

Kullat Nunu
2004-Jul-29, 06:18 AM
I think the most sol-like planetary "system" found so far is HD70642 (http://www.solstation.com/stars2/hd70642.htm)

I counted as a "system" only the known multiple-planet systems. :oops: :wink:
But you're right.

sol_g2v
2004-Jul-29, 07:18 PM
I think the most sol-like planetary "system" found so far is HD70642 (http://www.solstation.com/stars2/hd70642.htm)

I counted as a "system" only the known multiple-planet systems. :oops: :wink:
But you're right.

Hence "system" with quotation marks. Including only multiple-planet systems the most sol-like is probably the previously mentioned 47 Urs Maj. There are stars extremely similar to the sun like 37 Geminorum and 18 Scorpii, no planets yet, hopefully only because the detection capability is not there. Here (http://www.lowell.edu/users/jch/workshop/drs/drs-p1.html) is a good discussion of the search for solar twins.

rleyland
2004-Jul-29, 08:09 PM
Isn't there another bias inherent in both of the methods mentioned.

That is, the orbits of the planets must be somewhat lined up with our orientation to the star. This is obvious for the transit detection methods, and I thought the wobble methods use doppler spectrum shifts.


That's not really a bias, because any kind of planet, whether it be large or small, can be lined up. Or am I wrong... #-o

Well, inclination of the planet's orbit does affect the radial velocity. Less inclined the orbit is, smaller is the amplitude in radial velocity. That's why the masses measured by radial velocity are always minimum masses(* (inclination 90°, a transiting orbit). If the inclination becomes near zero ("bullseye view") the companion may not be a planet but a brown dwarf or even a small red dwarf. However, probability for that many of the found planets really are more massive objects is very low, and moderate inclinations don't change the predicted mass much. Another proof for this is the fact that there are very few brown dwarfs in small orbits (the brown dwarf desert). The reason for this is unknown.

Except for transiting planets, only the orbital inclination (84°) of Gliese 876 b has been measured astronomically with the Hubble Space Telescope Fine Guidance Sensors. Similar measurements for other stars like Upsilon Andromedae are under way.

*) Actually M = M_min/sin(i), where i is the orbital inclination.


I am having a problem with notation here... surely inclination should be measured relative to the "line of sight" from Earth, and as such a 0 degree inclination would be parallel, and an orbit that is perpendicular ("bullseye") would be 90 degrees.

Imagine we're looking at a disc in 3-D, our location (Earth) is a point, and the target system is a disc tilted in some random way. It could be tilted in any axis, but only one that really matters is along the line of sight from us.

If the plane is tilted close to 90 degrees, we aren't going to see any spectral shifts (or transits), but we might see astrometric (position) changes if the star is relatively nearby.

If the plane is tilted closer to 0 degrees then we would see spectral shifts, but our astrometric shifts would tend towards linear motion (i.e. the star would move back and forth in a straight line, if the planes were parallel)

What I am getting at is, that I expect there to be a selection bias in the observations. It will be easier to detect planetary systems around stars if the orbital plane is more closely oriented to the line of sight from Earth.

Only for relatively nearby stars can we measure their positions accurately enough for detection of planets, and even then knowing the orbital inclination would be pretty darned handy.

So what are our estimates of fraction-of-stars-with-planets? Have these taken the selection bias into account? (and how?)

thanks,
Robbo

Kullat Nunu
2004-Jul-30, 07:17 AM
I am having a problem with notation here... surely inclination should be measured relative to the "line of sight" from Earth, and as such a 0 degree inclination would be parallel, and an orbit that is perpendicular ("bullseye") would be 90 degrees.

It is not, altough in my opinion it would be easier to understand this way.


Imagine we're looking at a disc in 3-D, our location (Earth) is a point, and the target system is a disc tilted in some random way. It could be tilted in any axis, but only one that really matters is along the line of sight from us.

If the plane is tilted close to 90 degrees, we aren't going to see any spectral shifts (or transits), but we might see astrometric (position) changes if the star is relatively nearby.

If the plane is tilted closer to 0 degrees then we would see spectral shifts, but our astrometric shifts would tend towards linear motion (i.e. the star would move back and forth in a straight line, if the planes were parallel)

Close to 0 degrees ;). Otherwise you're right, the inclination of the planet does not affect the astrometric detection.


What I am getting at is, that I expect there to be a selection bias in the observations. It will be easier to detect planetary systems around stars if the orbital plane is more closely oriented to the line of sight from Earth.

Sure, but does it matter? We only find less planets, but because we know that the inclinations are randomly distributed, we can take it into account. Massive planets are found easier, but they are already biased by the RV method.


Only for relatively nearby stars can we measure their positions accurately enough for detection of planets, and even then knowing the orbital inclination would be pretty darned handy.

If we could detect them astrometrically, inclinations and true masses would be measured. GAIA astrometric mission will do just that.


So what are our estimates of fraction-of-stars-with-planets? Have these taken the selection bias into account? (and how?)

Only estimation I've found is from California & Carnegie Planet Search (http://exoplanets.org). According to them metallicity of the star affects the occurence of planets. Only ~3% of stars with metallicity one third of Sun's have planets, whereas stars with 3 x Sun's metallicity the occurence rises to 20%. I don't know answer to the latter question.

eburacum45
2004-Jul-30, 05:27 PM
You mean that only 3% and 20% respectively have detectable planets. It is likely that almost every non-binary star has planets of some sort, even if they are only little ones as big as (say) Mercury or Ceres;; several of the binaries probably have them as well.

rleyland
2004-Jul-30, 07:10 PM
KN:

Thanks for the answers...

I personally think both methods for choosing inclination are hopelessly earth centric, we should be measuring inclination against the local galactic reference frame. At least the current method is so hard to follow that we are more likely to be forced to change to a galactic centric system sooner rather than later ;-)






So what are our estimates of fraction-of-stars-with-planets? Have these taken the selection bias into account? (and how?)

Only estimation I've found is from California & Carnegie Planet Search (http://exoplanets.org). According to them metallicity of the star affects the occurence of planets. Only ~3% of stars with metallicity one third of Sun's have planets, whereas stars with 3 x Sun's metallicity the occurence rises to 20%. I don't know answer to the latter question.

Interesting site!


The next obvious question (at least to me) is:

Are planetary systems more commonly aligned with the galactic plane, or are they essentially random? i.e. is there a "normal" distribution curve w.r.t. to galactic plane, or a flat one?

Sol and the Solar system are relatively close to the galactic plane (i.e < 45 degrees), right?

cheers,
Robbo

Kullat Nunu
2004-Jul-31, 05:16 PM
Are planetary systems more commonly aligned with the galactic plane, or are they essentially random? i.e. is there a "normal" distribution curve w.r.t. to galactic plane, or a flat one?

They are randomly distributed.


Sol and the Solar system are relatively close to the galactic plane (i.e < 45 degrees), right?

Not close at all, ecliptic plane is almost perpendicular to the galactic plane.

rleyland
2004-Aug-01, 07:04 PM
Are planetary systems more commonly aligned with the galactic plane, or are they essentially random? i.e. is there a "normal" distribution curve w.r.t. to galactic plane, or a flat one?

They are randomly distributed.


Cool. *

So where I was going earlier... Around what percentage of stars do we expect to find planet? and what are the error bars?

(As we find more of course the error bars get smaller! :-) )

I have my own humble opinion (lots and lots), but evidence is better.

cheers,
Robbo

* Do you have a reference or link, I googled but didn't find anything other than the link about metallicity.

[edit:for content]

Kullat Nunu
2004-Aug-01, 07:31 PM
Do you have a reference or link, I googled but didn't find anything other than the link about metallicity.

No, but so I've been told. ;)

There is no reason why stars should prefer any axial tilt relative to galactic plane as they form from collapsing interstellar cloud fragments.

Kullat Nunu
2004-Aug-01, 09:14 PM
Found it :) This is the answer for your question (from exoplanets.org (http://exoplanets.org)): Are the "Planets" Merely Stars and Brown Dwarfs in Face-on Orbits? (http://exoplanets.org/face_on.html)

polytropos
2004-Aug-01, 10:31 PM
Excellent thread, let's hope I don't have my usual cause/effect of being the post de grace.
Here is another good site, NASA's Planet Quest (http://planetquest.jpl.nasa.gov/index.htmlurl) with both a list of systems with exoplanets and explanations of detection methods, complete with cheerfully simple animated MPEGs.

rleyland
2004-Aug-01, 11:07 PM
Found it :) This is the answer for your question (from exoplanets.org (http://exoplanets.org)): Are the "Planets" Merely Stars and Brown Dwarfs in Face-on Orbits? (http://exoplanets.org/face_on.html)

Ah ha! excellent, quoting from the link:


1. There is no way to bias star selection toward extreme face-on orbits, especially for unseen companions. The target stars are located all over the sky, and they are selected with no prior knowledge about the existence of any low-mass companions. Thus no information about orbital inclinations exists. Stellar and brown dwarf companions reside in randomly oriented orbits.


-> confirms KN's point about random orbits, and:


Clearly, the probability that most of the orbits of the 50 extrasolar planets are viewed nearly face-on is negligible. Indeed, there is a bias toward detecting orbits that are nearly edge-on, because the Doppler variations are greater for such orientations.


-> Which confirms what I was saying earlier, about selection bias.

What are the odds of planets around any arbitary system, finding any planet is pretty significant but of course, being human, finding terrestrial analogs are most interesting, like finding Solar analogs.

Our self-centered view assumes that these will have the best prospects for finding life comparable to ourselves.


I am a bit more interested in the nearby stars... and which if them harbour prospects for life.

So what now are the odds of, say, Alpha Centauri having planets?

- it is a binary (trinary) star system, with widely separated companions
(11-35 AU for A and B in an 80 yr orbit, 13,000 AU to Proxima)
- the expectation is that there would be no gas-giant planets (due to A - B proximity)
- there appears to be room between them for terrestrial planet-sized orbits to be stable.

61 Cygni?

- two K type stars in a 650+ year orbit?

Epsilon Indi?

- a K5

Tau Ceti ?

- G8, low metallicity
- observed cometary halo


fun speculating,
Robbo

TimberWolf
2004-Aug-02, 03:31 AM
Hello,

Interesting. But how could there be room for a stable orbit of a terrestrial planet? Wouldn't the large gravity wells of A and B play mayhem with any planets there?

Cordially,

TimberWolf

Kullat Nunu
2004-Aug-02, 06:49 AM
So what now are the odds of, say, Alpha Centauri having planets?

- it is a binary (trinary) star system, with widely separated companions
(11-35 AU for A and B in an 80 yr orbit, 13,000 AU to Proxima)
- the expectation is that there would be no gas-giant planets (due to A - B proximity)
- there appears to be room between them for terrestrial planet-sized orbits to be stable.

If terrestrial planets can form in a system like Alpha Centauri, it is probably one of the best places to search for Earth-like planets. However, we still lack the required technology to detect them.


61 Cygni?

- two K type stars in a 650+ year orbit?

The components of 61 Cygni are rather dim stars.


Epsilon Indi?

- a K5

Epsilon Indi is probably too young.


Tau Ceti ?

- G8, low metallicity
- observed cometary halo

We have no evidence how star's metallicity and occurence of terrestrial planets are related. Tau Ceti is one of the most studied system for extrasolar planets, so it probably does not have a gas giants in Jupiter-like or smaller orbits. Otherwise it is very Sun-like, albeit less luminous star.

Epsilon Eridani, which is slightly brighter than Epsilon Indi, is already known to have a eccentric Jupiter in a wide orbit. It also has a massive Kuiper Belt. Problem is that it also is young (about billion years old).

Other interesting Sun-like stars within 20 ly are
* Omicron2 Eridani (distance 16.45 ly): K1, DA4, M4.5; a K star and white dwarf+red dwarf pair. The white dwarf is a former red giant, and its transformation may have affected the possible planets of the K star.
* 70 Ophiuchi (16.59 ly): K0+K5, two K stars. Component A brighter than Epsilon Eridani or Epsilon Indi.
* Sigma Draconis (18.81 ly): K0, this star is slightly dimmer than 70 Ophiuchi A.
* Eta Cassiopeiae (19.42 ly): G0+K7, an Alpha Centauri -like system in wider orbit. The stars' metal abundance is low.
* 82 Eridani: G5, second-closest single G star (19.77 ly away); slightly dimmer than the Sun. It may be over ten billion years old.
* Delta Pavonis (19.92 ly): G5. Although this star has a later spectral type than the Sun, it is brighter, meaning it is probably soon leaving the main sequence. The star is only slightly more massive than the Sun so it must be much older.

Kullat Nunu
2004-Aug-02, 06:58 AM
But how could there be room for a stable orbit of a terrestrial planet? Wouldn't the large gravity wells of A and B play mayhem with any planets there?

Actually, if the planets orbit sufficiently close (say, within the orbit of Mars), their orbits stay stabile. So in this sense Earth-like planets could be possible around both stars. If the planets can rally form there is another matter.

ToSeek
2004-Aug-17, 04:50 PM
Sun-Like Solar Systems Undetected (http://www.spacedaily.com/news/extrasolar-04y.html)


The planets orbiting the sun may have formed differently than those in other solar systems, which is why astronomers have not yet discovered systems that resemble Earth's planetary neighborhood.

Swift
2004-Aug-17, 06:10 PM
Sun-Like Solar Systems Undetected (http://www.spacedaily.com/news/extrasolar-04y.html)


The planets orbiting the sun may have formed differently than those in other solar systems, which is why astronomers have not yet discovered systems that resemble Earth's planetary neighborhood.
A very interesting idea. In other words, both proposed mechanisms of planet formation are correct and occur in different systems. I like it, though, as always, we need more data.

George
2004-Aug-17, 06:50 PM
Was it Astronomy which had an article suggesting our history was tramatic and included a supernova and numerous others in our nursery?
(In addition to ToSeek's link, no doubt)

The supernova would have quite an impact on planetary development depending on the time of it's arrival.

ToSeek
2004-Aug-17, 06:57 PM
we need more data.

The scientist's mantra. ;)

Grand Vizier
2004-Aug-17, 07:03 PM
I'm a little sad that star systems with possible Earth-type planets seem to be getting further and further away. Traditionally, there have been four star systems within 12 light years where science fiction writers have often placed ETs or used as standard destinations for colonisation. But two of them seem to be out of the running already:

Alpha Centauri (A and B): Still something of an enigma, though it seems to be felt that binary stars do not favour the dynamics of planetary formation. But who knows?

Epsilon Eridani: 1.2 Jupiter mass companion on a highly eccentric orbit (1.3 to 5.3 AUs). Does not bode well for terrestrial planets in stable orbits.

Tau Ceti: It's only recently been determined that this system is full of cometary bodies, making it kind of hazardous. Doesn't bode well for ET, but I suppose if there were an Earth-type world there you could consider colonising it if you were real good at steering comets around.

Epsilon Indi: Hey - in spite of having two brown dwarf companions this one seems to be still in business, though no planets of any type have been found so far.

Beyond Epsilon Indi (11.8 lys), I'm not sure what the next good solar-type candidate is. Anyway, none of this necessarily spells doom for colonisers from Earth, but it's not looking that good for ET so far (except that advanced civilisations might prefer to migrate to brighter stars like Sirius, or something).

eburacum45
2004-Aug-17, 07:30 PM
There are plenty of reasonably sun-like stars in our neighborhood;
Eta Cassiopeia is probably the next one after Epsilon Indi.
David Nash has more here...
http://www.astronexus.com/general/50sunlike.html

What isn't known is how many have suitable terrestrial planets...

not many, by the indications I am seeing.

But perhaps a few stars per thousand might have a reasonably Earth-like world...

Grand Vizier
2004-Aug-17, 07:45 PM
There are plenty of reasonably sun-like stars in our neighborhood;
Eta Cassiopeia is probably the next one after Epsilon Indi.


Binary (36 AU - 107 AU) (http://www.solstation.com/stars/eta-cass.htm) which presumably is why David Nash does not include either it or Alpha Centauri. I note that Solstation is quite happy to discuss habitable regions around binary stars, though.

Thanks for that table, by the way...



David Nash has more here...
http://www.astronexus.com/general/50sunlike.html


Yes - he leaves out Epsilon Eridani but keeps Tau Ceti, I see - but Sigma Draconis still looks plausible.

eburacum45
2004-Aug-17, 07:57 PM
Eta Cassiopeia is binary, but the B star is 70 AU distant; surely that is far enough?

Jim Kaler on this star, also called Achird
http://www.astro.uiuc.edu/~kaler/sow/achird.html

Grand Vizier
2004-Aug-17, 08:37 PM
Eta Cassiopeia is binary, but the B star is 70 AU distant; surely that is far enough?


Yeah - you can have habitable zones, and the same is true of Alpha Centauri too. But the real reason, I think, that binaries are under a cloud is to do with our current theories of planetary formation, often the result of computer simulations. As it is, a lot of simulations seem to show that singular systems eject a fair number of their planets due to gravitational interactions early on. Binary systems seem to lead to initial planetary orbits that are even more chaotic, so the idea is that planets are likely to be either ejected or impact one of the suns.

Of course, the existence of gas giants in torch orbits was not predicted before they were found, so I'd hardly think this was written in stone. I hope that when we get instruments that can detect terrestrial-type planets, Alpha Centauri and Eta Cassiopeia will be studied very closely.

Launch window
2004-Sep-01, 05:25 AM
Try here

The first known quadruple-planet system outside the system belongs to the solar-type star 55 Cancri. Three planets, including a newly found Neptune-mass object, huddle close to the star. The fourth planet orbits much farther out, at approximately Jupiter's distance from the Sun.




Compiled by California and Carnegie Planet Search
http://exoplanets.org/planet_table.shtml

ICRS 2000.0 coordinates*
M sin i Per a K Mstar -------------+----------
(Mjup) (d) (AU) e (m/s) (suns) R.A. | Dec
-----------------------------------------------------------------+----------
0 hd73256 1.85 2.548 0.037 0.04 267.0 1.05 | 8 36 23.0 | -30 02 15
1 hd83443 0.41 2.985 0.04 0.05 58.0 0.79 | 9 37 11.8 | -43 16 19
2 hd46375 0.25 3.024 0.04 0.04 34.5 1.00 | 6 33 12.6 | 5 27 47
3 hd179949 0.98 3.093 0.04 0.00 118.0 1.24 | 19 15 33.2 | -24 10 45
4 hd187123 0.51 3.097 0.04 0.02 68.0 1.06 | 19 46 58.1 | 34 25 10
5 tauboo 4.13 3.312 0.05 0.01 471.4 1.30 | 13 47 16.0 | 17 27 24
6 bd-103166 0.48 3.488 0.05 0.07 59.9 1.10 | 10 58 28.8 | -10 46 13
7 hd75289 0.44 3.509 0.05 0.01 53.5 1.15 | 8 47 41.0 | -41 44 14
8 hd209458 0.67 3.525 0.05 0.11 86.5 1.05 | 22 3 10.8 | 18 53 4
9 hd76700 0.19 3.971 0.05 0.13 25.0 1.00 | 8 53 55.0 | -66 48 4
10 51peg 0.46 4.231 0.05 0.01 55.0 1.06 | 22 57 27.9 | 20 46 7


http://www.obspm.fr/encycl/55Cnc.html

Grand Vizier
2004-Sep-01, 05:41 AM
Do you have a reference or link, I googled but didn't find anything other than the link about metallicity.

No, but so I've been told. ;)

There is no reason why stars should prefer any axial tilt relative to galactic plane as they form from collapsing interstellar cloud fragments.

I'm curious here. I wondered whether there was any 'inheritance' of angular momentum like this many years ago. The best I could do at the time was to plot the distribution of eclipsing binaries, which can stand in just as well for, or better than, planetary systems. They did, in fact, seem to group towards the galactic equator.

But this is anecdotal now, and I certainly did not run it through any thorough statistical analysis. I have wondered about such questions ever since, however...

Manchurian Taikonaut
2005-Jan-16, 01:07 PM
There are plenty of reasonably sun-like stars in our neighborhood;
Eta Cassiopeia is probably the next one after Epsilon Indi.
David Nash has more here...
http://www.astronexus.com/general/50sunlike.html

What isn't known is how many have suitable terrestrial planets...

not many, by the indications I am seeing.

But perhaps a few stars per thousand might have a reasonably Earth-like world...

NASA's TPF and ESA's Gaia may be able to tell us more, Gaia will provide unprecedented positional and radial velocity measurements with the accuracies needed to produce a stereoscopic and kinematic census of about one billion stars in our Galaxy. Estimates suggest that Gaia will detect between 10 000 and 50 000 planets beyond our Solar System. It will do this by watching out for tiny movements in the star's position. NASA's TPF has a chance of finding Alien worlds. The TPF mission is in development stages, and hopefully will be launched within the next decade. Once operational, this space-based telescope system will revolutionize planet hunting


http://sci.esa.int/science-e/www/area/index.cfm?fareaid=26
http://planetquest.jpl.nasa.gov/TPF/tpf_sample.cfm

:D

Kullat Nunu
2005-Jan-16, 02:18 PM
Do you known what is going to happen to the Sloan Sky Survey telescope after it has completed the survey?

Here's one plan (from a January 2005 AAS meeting abstract (http://www.aas.org/publications/baas/v36n5/aas205/1259.htm)):


We present a plan for a decade-long, all-sky, extrasolar planet survey at the Sloan 2.5 meter wide field telescope for monitoring 1,000,000 stars in the solar neighborhood to detect 100,000 of extrasolar planets between 2005-2020. This survey is enabled by a new generation multiple object Doppler radial velocity instrument based on new Doppler technology called dispersed fixed-delay interferometry pioneered by Ge’s group over last four years. The instrument capable of observing hundreds of stars in a single observation increases planet survey speed by more than two orders of magnitude over current single object echelle instruments. One new instrument is designed for monitoring F,G,K type stars in the visible and looking for planetary systems similar to our own. The other one is designed for monitoring very low mass M type stars in the near infrared aiming at detection of earth-like planets in a habitable zone, which may host life. In the end of the survey, the total numbers of known extrasolar planetary systems will be increased by hundreds of times over current total number of known planets. This survey will provide a powerful statistical base for understanding different kinds of extrasolar planetary systems, their formation and evolution.

Microlensing Planet Finder proposal (http://www.aas.org/publications/baas/v36n5/aas205/1548.htm) sounds interesting too:


The Microlensing Planet Finder (MPF) is a proposed Discovery mission that will complete the first census of extrasolar planets with sensitivity to planets like those in our own solar system. MPF will employ a 1.1m aperture telescope, which images a 1.3 sq. deg. field-of-view in the near-IR, in order to detect extrasolar planets with the gravitational microlensing effect. MPF's sensitivity extends down to planets of 0.1 Earth masses, and MPF can detect Earth-like planets at all separations from 0.7AU to infinity. If the planet:star mass ratios and planetary semi-major axes of our own Solar System are typical, MPF will detect 66 terrestrial planets (Venus/Earth/Mars analogs), 3300 gas giants (Jupiter/Saturn analogs), and 110 ice giants (Uranus/Neptune analogs). Thus, MPF will be able to be able to find analogs to our own Solar System's planets even if planetary systems like ours are not common. MPF's extrasolar planet census will provide critical information needed to understand the formation and frequency of extra solar planetary systems similar to our own.

As well as OBSS (http://ad.usno.navy.mil/OBSS/), although it may overlap GAIA (So what? ;) We haven't got decent trigonometric parallax missions since Hipparchos).

Kullat Nunu
2005-Jan-16, 02:29 PM
But probably the first mission to discover Earth-like planets is the Kepler Mission (http://www.kepler.arc.nasa.gov/).

Doodler
2005-Jan-16, 03:06 PM
altough it can find many gas giants and possible terrestrial planets few times larger than the Earth.

I wonder what is the limiting size of a terrestrial planet for it to be able to support the evolution of intelligent life forms. Seems like a terrestrial planet several times the size of the Earth and hence having a high density would provide a gravitational force that hinders the growth of intelligent life, or would the shortness of creatures on such a planet not present a hindrance?

It shouldn't. Intelligence is the result of survival adaptations (problem solving and logic to overcome obstacles, language for advanced communication and social activity) that didn't have a whole lot to do with gravity. It might be a different type of intelligence with focus of different elements than our own (which tends to be very visual, aural and tactile), but it should still be possible. It will grow from the advanced use of the senses available.

There might be physical hindrances. One of the reasons we're as far along as we are is our thin skulls, which allows our brain to run "hotter" than other animals. A higher gravity environment might make this a taller hurdle (thin bones in hi gee is not a great adaptation, you might imagine), but again, life could find a way, there are other means of radiating heat away from the body that do not require thin bones or skin. Protomammals like dimetrodon used the fins on their back as a radiator, something similar may not be outside the realm of possibility somewhere else, but this is starting to slip into handwaving, so I'll stop. Needless to say, gravity is just another obstacle, if intelligence becomes a valuable survival train in a species, then that species will find a way to adapt and use it regardless of the environment.

Tobin Dax
2005-Jan-16, 04:58 PM
KN:

Thanks for the answers...

I personally think both methods for choosing inclination are hopelessly earth centric, we should be measuring inclination against the local galactic reference frame. At least the current method is so hard to follow that we are more likely to be forced to change to a galactic centric system sooner rather than later ;-)

Oh my word, no. The current system works wonderfully well for our current needs. When we observe, we observe from Earth. We therefore can most easily find the orbital inclination with respect to our line of site. The information we have from observations gives this to us automatically. To have to convert it back and forth, as need be, to some galacto-centric value is just another step I'd have to do whenever I need an inclination. No thanks. The system ain't broke, and earth-centric ain't hopeless--it's wonderfully convenient.

As for the possible counter-intuitiveness of the inclination system (0 deg is "bull's eye," 90 deg is line-of-sight orbit), inclination is by definition measured using the perpendicular to the motion. (It's measured along the angular momentum vector.) For rotating planets, we measure the tilt of their axes. We do the same for revolving orbits. So if the "axis" of the orbit is directly along the line of sight, it has zero inclination to the line of sight. The angle off of the line of sight of the perpendicular to the orbit is, by definition, the inclination. That system makes perfect sense. The only flaw could be in the definition of the inclination (being measured by the perpendicular, not the orbital plane), but even that has a physical reason behind it.

[editted for spelling and then to add this comment 8-[ ]

Plat
2005-Jan-16, 10:38 PM
Alot of these planets will "pop out" since technology is getting better and better

Ilya
2005-Jan-17, 01:23 AM
But probably the first mission to discover Earth-like planets is the Kepler Mission (http://www.kepler.arc.nasa.gov/).

There is a small possibility that Canadian MOST mission (http://www.astro.ubc.ca/MOST/), which is already in orbit, will beat Kepler to it. Like Kepler, MOST is a space telescope which measures minute changes in the stars' luminosity, so if a planet passes in front of the star while the telescope is watching, it will register. However, Kepler is designed specifically to look for planet transits, and will watch FAR more stars at a time. That's why I said "small possibility".

Manchurian Taikonaut
2005-Jan-18, 09:18 PM
Also the Kepler mission (http://www.kepler.arc.nasa.gov/) seems to be underway.

Yes, it was selected as a Discovery-class mission with the Dawn asteroid probe. Too bad ESA had to cancel their own Eddington (http://astro.esa.int/SA-general/Projects/Eddington/) project, which would have been similar to Kepler. COROT (http://www.esa.int/export/esaSC/120372_index_0_m.html), a French transit search telescope will be launched earlier, in 2006. It however lacks the required sensitivity to detect another Earths, altough it can find many gas giants and possible terrestrial planets few times larger than the Earth.

Indeed so it looks like Corot might be one of the first, but NASA and the ESA also have many future missions in fact their designs seem to be so similar & that it is thought they maybe soon combine efforts in a joint missions to find twin Earth worlds

http://sci.esa.int/science-e/www/area/index.cfm?fareaid=26
http://planetquest.jpl.nasa.gov/TPF/tpf_sample.cfm
http://www.esa.int/esaCP/SEMBVPWLDMD_FeatureWeek_0.html
http://origins.jpl.nasa.gov/library/ipff/ipff17.html
http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=36090
http://planetquest.jpl.nasa.gov/TPF/tpf_team.cfm#reviewPane
http://www.esa.int/export/esaSC/SEMZ0E1A6BD_index_0.html

Manchurian Taikonaut
2005-Feb-10, 06:09 AM
Alot of these planets will "pop out" since technology is getting better and better

Did you see the latest on NASA and the USA budget, the cash for NASA is 2.4 % up but still faces bad news, about $500 million less than what the agency had been expecting. Project Constellation, which is intended to develop a Crew Exploration Vehicle that can transport astronauts to the Moon has seen a cut down a bit. The budget request includes no money for any service to the Hubble Space Telescope, so there are no shuttle missions & the HST might be gone soon. The launch date for the Kepler mission, designed to look for extrasolar planets was postponed for at least a year due to budget trims. However the money in the budget appears solid, so perhaps Corot - a French transit search in a co-operative effort with the ESA will get the go-ahead before NASA gets Kepler going ? Overall they say the budget isn't bad, NASA has done well with Oil prices and rising debts the agency came out much better than others who had to face up to the Bush budget for the year.

Launch window
2005-Jun-20, 10:14 AM
the most prized real estate is advertised as "Earth-like."

scientists raced to plant their flag on a burning hunk of rock orbiting a red star.
G-876 also known as Ross 780, IL Aquarii, Hip 113020
this dim star lies about 15 light-years from Sol

This newly discovered planet is about seven times the mass of Earth, and therefore the smallest extrasolar planet found to orbit a main sequence, or "dwarf" star (stars, like our sun, that burn hydrogen).

Gliese 876 M dwarf ( Ross 780)-

Gliese 876 is thought to be about 11 billion years old, making it more than twice as old as our sun.