List of possible systems containing life includes Alpha Centauri B but not A???

[RichF]

Alpha Centauri B??? I'm curious as to why Alpha Centauri A is not included in this list since it is the same spectral type as our sun. B is a K1 class star, cooler than our sun, while A is a G2 class star, the same as our sun. Both stars are thought to be able to sustain planets with in the orbit of Jupiter; so why would they pick B over A?

 

http://msnbc.msn.com/id/11427824/

 

Top 5

Beta Canum Venaticorum, Turnbull's top prospect. It's a sunlike star about 26 light-years away in the northern constellation Canes Venatici. Astronomers have been looking for planets around the star but have found none to date.

 

HD 10307, another sunlike star about 42 light-years away. It has nearly the same mass, temperature and metal content as our sun — plus a companion star.

 

HD 211415, which has about half the metal content of the sun and is a bit cooler.

 

18 Scorpii, a popular target for proposed planet searches. The star is almost an identical twin of the sun, Turnbull says.

 

51 Pegasus, which was the first normal star beyond our solar system known to have a planet. The Jupiterlike planet was detected in 1995, and Turnbull believes 51 Pegasus could harbor Earthlike planets as well.

 

 

Not too dim, not too bright

Turnbull said the top five prospects for the Terrestrial Planet Finder mission were chosen a bit differently, because the TPF's instruments would look for the signature of planets circling around the target stars. The star couldn't be too bright — otherwise the planets themselves would be lost in the star's glare. It couldn't be too dim — otherwise there wouldn't be enough energy to sustain life as we know it. Here are the TPF prospects she came up with:

 

Epsilon Indi A, about 11.8 light-years from Earth, leads Turnbull's list. It's a star somewhat cooler and smaller than our sun, and was recently found to have a brown-dwarf companion. "Star Trek" fans consider it the home of the Andorian race. In the original "Star Trek" series, it was the base of operations for an evil entity called "Gorkon."

 

Epsilon Eridani, 10.5 light-years away, is a star somewhat smaller and cooler than our sun, and is already known to have at least one planet. By some science-fiction accounts, Epsilon Eridani is the parent star for Vulcan, Mr. Spock's home planet on "Star Trek." However, Trekkers have come to favor another star in the same constellation....

 

Omicron 2 Eridani, also known as 40 Eridani, is now cited in most "Star Trek" literature as Mr. Spock's home turf. It's a yellow-orange star about 16 light-years away, and is roughly the same age as our sun.

 

Alpha Centauri B is part of the triple-star system closest to our own sun, just 4.35 light-years away. It's long been considered one of the places in the Milky Way that might offer terrestrial conditions — and it's often cited in science-fiction tales, including Isaac Asimov's Foundation series.

 

Tau Ceti is in the same brightness category as our sun. It's metal-poor, compared to the sun, but long-lived enough for life forms to evolve. It has also served as a locale for science-fiction works ranging from Ursula Le Guin's "The Dispossessed" to the TV show "Earth: Final Conflict."

 

 

For referance...

1. http://en.wikipedia.org/wiki/Alpha_Centauri

2. Isaac Asimov's "Alpha Centauri"

 

Why

 

Edited due to poor English.

[[Tycho?]]

Thats a good question. Any star orbiting either Centauri Alpha or Beta would be recieving lots of light from both of them, so its not like you could have one star over another. I really dont know why they phrased it that way, quite strange.

[Klaynos]

This is just a stab in the dark, but if B had a lower mass than A it would orbit "outside" of A, and they may pick the "outer" most star...

[[Tycho?]]

This is just a stab in the dark, but if B had a lower mass than A it would orbit "outside" of A, and they may pick the "outer" most star...

 

Well B does have a lower mass than A, but why woud they pick the outermost star?

[RichF]

These stars are close, but not that close. They are not very far off in size and they do a little dance with their orbits. Good stats here....

 

http://homepage.sunrise.ch/homepage/schatzer/Alpha-Centauri.html

 

The two brightest components Alpha Centauri A and B form a binary. They orbit each other in 80 years with a mean separation of 23 astronomical units (1 astronomical unit = 1 AU = distance between the Sun and Earth).

 

Below are Uranus's statistics; given that it has about the same distance and orbital period to the sun as Alpha and Beta have to each other, I thought it might make a good comparason of what each star would do to the other's planets.

 

Orbital Semimajor Axis: 19.19 AU

Revolution Period about the Sun: 84 years.

Temperature at Cloud Tops: -200o C ( -328o F)

Average Cloud Top Temperature (K): 73K

 

The two stars probably have an impact on the other's inner planets (if they exist) but from the naked eye the companion star would look like a really really bright star....nothing else. From Uranus, the Sun is a hell of long ways off.

 

 

Please feel free to correct me.

[Molotov]

Possibly because the habitable zone of B is further away from the instability zone giving it a better chance of containing a planet within the habitable zone.

 

 

I think both A and B should top the list because of the close proximity of the system to our own. If we actually find a habitable planet around either one it wouldn't be an impossibility to send probes and eventually go there.

[Ophiolite]

Here is the abstract of the relevant paper:

 

Margaret C. Turnbull, Jill C. Tarter Target Selection for SETI. I. A Catalog of Nearby Habitable Stellar Systems The Astrophysical Journal Supplement Series, 145:181-198, 2003 March

 

In preparation for the advent of the Allen Telescope Array, the SETI Institute has the need to greatly expand its former list of 2000 targets compiled for Project Phoenix, a search for extraterrestrial technological signals. In this paper we present a catalog of stellar systems that are potentially habitable to complex life forms (including intelligent life), which comprises the largest portion of the new SETI target list. The Catalog of Nearby Habitable Systems (HabCat) was created from the Hipparcos Catalogue by examining the information on distances, stellar variability, multiplicity, kinematics, and spectral classification for the 118,218 stars contained therein. We also make use of information from several other catalogs containing data for Hipparcos stars on X-ray luminosity, Ca II H and K activity, rotation, spectral types, kinematics, metallicity, and Strömgren photometry. Combined with theoretical studies on habitable zones, evolutionary tracks, and third-body orbital stability, these data are used to remove unsuitable stars from HabCat, leaving a residue of stars that, to the best of our current knowledge, are potentially habitable hosts for complex life. While this catalog will no doubt need to be modified as we learn more about individual objects, the present analysis results in 17,129 Hipparcos "habstars" near the Sun (75% within 140 pc), 2200 of which are known or suspected to be members of binary or triple star systems.

 

There are plenty of possible reasons suggested by their criteria for why B was chosen over A. Unfortunately, I do not have access to the full paper, which might give better insight to the specific rationale.

[RichF]

Thanks for the replies; the location of the habitable zones makes since. Alpha has 9% more mass than the sun which would push the habitable zone outward a bit. On the other hand, Beta has 10% less mass than the sun so it's habitable zone would be closer to the star.

 

Soooo Beta's radiation would have a greater impact on Apha's habitable zone than Alpha would have on Beta's habitable zone. Correct

[Molotov]

True about the radiation but I don't think it would be that big of deal. What it boils down to is that in a binary system the closer you get to the star the greater chance there is of finding a planet that hasn't been flung out of the system.

[[Tycho?]]

Thanks for the replies; the location of the habitable zones makes since. Alpha has 9% more mass than the sun which would push the habitable zone outward a bit. On the other hand' date=' Beta has 10% less mass than the sun so it's habitable zone would be closer to the star.

 

Soooo Beta's radiation would have a greater impact on Apha's habitable zone than Alpha would have on Beta's habitable zone. Correct [/quote']

 

Look up Alpha Centauri on wikipedia. It includes a section about the possibility of life supporting planets, and where they would have to be located.

[RichF]

True about the radiation but I don't think it would be that big of deal. What it boils down to is that in a binary system the closer you get to the star the greater chance there is of finding a planet that hasn't been flung out of the system.

 

Yes but it is believed that both stars formed at the same time. In the case of Alpha Centauri it is more of a question of whether planets would be able to form due to the gravitational forces and solar winds.

 

I hope they exist......32 year round trip at 1/4 the speed of light. I hope it's possible sometime but I don't think it'll happen in my life time.

 

Edit: and yes Wikipedia is awesome! Great taste & less filling; try it.