Starship Asterisk*

Possible life form on Alpha Centauri

APOD Robot wrote: Alpha Centauri: The Closest Star System

By an exciting coincidence, Alpha Centauri A is the same type of star as our Sun,
causing many to speculate that it might contain planets that harbor life.

Beyond wrote:
Get sirius neufer.

IF that's a possible lifeform on Alpha Centauri, Alpha Centauri is in B-I-G trouble

nstahl wrote:
There's speculation in one or more of the links about inhabitable planets orbiting Alpha Centauri, but wouldn't Beta orbiting at the distance of Uranus from the sun make the chance of a stable elliptical orbit around either one nil? And I suppose there could be orbits about the center of mass of the two but my intuition (fwiw) says such an orbit would mean a lot of time spent too far from either to be comfortable. I suppose there could be an interesting accumulation of matter at a stable Lagrange point in such a system.

http://en.wikipedia.org/wiki/Binary_star#Planets wrote:
<<Science fiction has often featured planets of binary or ternary stars as a setting, for example George Lucas' Tatooine from Star Wars, and one notable story, Nightfall, even takes this to a six-star system. In reality, some orbital ranges are impossible for dynamical reasons (the planet would be expelled from its orbit relatively quickly, being either ejected from the system altogether or transferred to a more inner or outer orbital range), whilst other orbits present serious challenges for eventual biospheres because of likely extreme variations in surface temperature during different parts of the orbit. Planets that orbit just one star in a binary pair are said to have "S-type" orbits, whereas those that orbit around both stars have "P-type" or "circumbinary" orbits. It is estimated that 50–60% of binary stars are capable of supporting habitable terrestrial planets within stable orbital ranges.

Simulations have shown that the presence of a binary companion can actually improve the rate of planet formation within stable orbital zones by "stirring up" the protoplanetary disk, increasing the accretion rate of the protoplanets within.

Detecting planets in multiple star systems introduces additional technical difficulties, which may be why they are only rarely found. Examples include the white dwarf-pulsar binary PSR B1620-26, the subgiant-red dwarf binary Gamma Cephei, and the white dwarf-red dwarf binary NN Serpentis. More planets around binaries are listed in THE PHASES DIFFERENTIAL ASTROMETRY DATA ARCHIVE. V. CANDIDATE SUBSTELLAR COMPANIONS TO BINARY SYSTEMS, Muterspaugh et al.

A study of fourteen previously known planetary systems found three of these systems to be binary systems. All planets were found to be in S-type orbits around the primary star. In these three cases the secondary star was much dimmer than the primary and so was not previously detected. This discovery resulted in a recalculation of parameters for both the planet and the primary star.>>

http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980122c.html wrote:
The Question:

Binary stars are popular in science fiction. Star Wars' Tatooine, for example.

I'm wondering about the orbit of a planet around binary stars. Is it possible?

Would the greater combined mass of two stars be more likely to pull in surrounding material, hence making the formation of planets less likely in a binary system?

Would the combined heat and radiation from binary stars mean that habitable planets would have to have a VERY large orbital radius?

The Answer:
David Palmer and Maggie Masetti
for Ask an Astrophysicist

<<There are stable orbits for planets in binary star systems. There are various stability criteria which say when an orbit is stable. One such criteria (and I don't know the actual numbers) says that if all orbits are circular and the stars are the same size, then the planet must orbit one of them at less than /some fraction/ of the inter-star distance, or must orbit both combined at more than /whatever/ times the inter-star distance. Figure-eight orbits are unstable, and can eject the planet from the system.

If you have two Sun-like stars at the center of the system, a planet would be the same temperature as Earth if it were at sqrt(2) = 1.4 astronomical units away, rather than Earth's 1 AU. This distance is closer than Mars's orbit (1.6 AU). Most stars are dimmer than our Sun, so the orbit could be even smaller.

The high energy astronomers at NASA don't know much about this subject, so we asked an expert: Eric Mamajek of Pennsylvania State University:

The solar-like stars 16 Cygni B and 55 Cancri A have been found to have Jupiter-size extrasolar planets orbiting them. So we do have indirect proof, through Doppler spectroscopy methods (Marcy & Butler, SFSU, Lick Observatory), that planets indeed form in binary systems.

The formation mechanisms for forming stars and planets are very different. Planets require accretion to form, specifically accretion in a protoplanetary disk around a young star. Stars can form from the collapse of a molecular cloud core on their own, however planets can only form in the disk around a star. (Pulsar planets are likely formed "posthumously" around pulsars, and are a different beast all together). The main problem with forming planets in multiple star systems is dynamic ejection... stars can simply toss planetesimals out of the system all together (or even accrete them). An example of this is the Kirkwood gaps in the asteroid belt where Jupiter doesn't allow asteroids to exist in certain orbits, and conversely it "shepherds" asteroids in to certain other orbits. A companion star would have a similar effect, except there would be a lot less "shepherding" orbits. The vast majority of binary stars have eccentric orbits. It is difficult for bodies to exist in a system with two very massive bodies in an eccentric orbit. They can only exist very close to each star, or very far from both stars.

An excellent example in the lines of the Tatooine example is the nearby solar-like stars Alpha Centauri A and B. They orbit each other at an average distance of 23 AU, however the eccentricities of each orbit bring them to as close to 11 AU and as far as 35 AU. Numerical simulations by Paul Weigert at University of Toronto have shown that each star has a "safe zone" about 3 AU in radius in which planets could safely survive for billions of years. Objects placed further out from each star than about 3 AU are dynamically ejected in a matter of millions of years or less. Alpha Cen A is about 1.5 times as luminous as our Sun, and Alpha Cen B is about .45 times as luminous as our Sun, and if you do the simple physics, one can see that a "habitable zone" exists around BOTH stars within the 3 AU dynamic "safe zone." Indeed, it could be possible that BOTH Alpha Cen A and B have planets conducive to life. Theoretical models age them anywhere from 3-8 Gyr... plenty of time for life to develop if the planets have the right conditions... >>

orin stepanek wrote:
It would be interesting to wonder if there were any planets around Alpha Centauri; would they orbit both A and B or just one of the stars. If orbiting only one; then it would have to pass between the two on occasion creating hot climate problem. On the other hand orbiting both would cause climate changes also! Doesn't sound too hospitable for life as we know it.

neufer wrote:

Beyond wrote:
Get sirius neufer.

IF that's a possible lifeform on Alpha Centauri, Alpha Centauri is in B-I-G trouble

Just kidding.


Actual life form on Alpha Centauri

Beyond wrote:
That's more like it

Notice that there is no bridge in the glasses and that the hands only have 4-digits.

Definitely an alien life form.

neufer wrote:All left-handers are from another solar system, IMHO.

aestival wrote:

neufer wrote:
All left-handers are from another solar system, IMHO.

Does that make Triton a visitor from the Alpha Centauri system?

Beyond wrote:

neufer wrote:

Beyond wrote:
Get sirius neufer.

IF that's a possible lifeform on Alpha Centauri, Alpha Centauri is in B-I-G trouble

Just kidding.


Actual life form on Alpha Centauri

That's more like it
Notice that there is no bridge in the glasses and that the hands only have 4-digits. Definitely an alien life form.

Ann wrote: Beyond,that's priceless!!!

bay area john wrote:A suggestion to those writing the text for APOD items... I'd think that an item about "the closest start system" would say how close that is, and how it compares with some others. Wouldn't you expect a story about the oldest living human to mention how old that is, and the average lifespan?

saturn2 wrote:
Object: Spaceship
Speed 30 Km/ sec

How long does it take from Earth to Proxima Centauri ?

42200 years

http://en.wikipedia.org/wiki/Ross_248 wrote:
<<Ross 248 (HH Andromedae) is a red dwarf star located approximately 10.3 light years from Earth in the constellation Andromeda. This star was first catalogued by Frank Elmore Ross in 1926 with his second list of proper motion stars.

This star has about 12% of the Sun's mass and 16% of the Sun's radius, but only 0.2% of the Sun's luminosity. This is a flare star that occasionally increases in luminosity. With high probability there also appears to be a long-term variability with a period of 4.2 years. This variability ranged in visual magnitude from 12.23 to 12.34. In 1950, this became the first star to have a small variation in magnitude attributed to spots on the photosphere.

Long term observations of this star by the Sproul Observatory show no astrometric perturbations by an unseen companion. The proper motion of this star was examined for a brown dwarf or stellar companion orbiting at a wide separation (between 100–1400 AU) but none was found. A search for a faint companion using the Hubble Space Telescope Wide Field Planetary Camera revealed nothing, nor did a search with near infrared speckle interferometry. However, none of these searches rule out a companion that is smaller than the detection minima.

The space velocity components of this star are U = –32.9 ± 0.7, V = –74.3 ± 1.3 and W = 0.0 ± 1.4 km/s. The trajectory of Ross 248 will bring it closer to the Solar System in the future. In 1993 Matthews suggested that in about 33,000 years Ross 248 would be the closest star to the Sun, approaching within a minimum distance of 3.024 light-years in 36,000 years. However, it will recede thereafter and will again be further from the Sun than Proxima Centauri 42,000 years from now.

The spacecraft Voyager 2 is traveling on a path headed roughly in the direction of Ross 248, and is expected to come within 1.76 light-years of the star in 40,176 years. A spacecraft that escaped the Solar System with a velocity of 25.4 km/s would reach this star 37,000 years from now when the star is just past its nearest approach. By comparison, the Voyager 1 has an escape velocity of 16.6 km/s.>>

http://en.wikipedia.org/wiki/Proxima_Centauri wrote:
<<Proxima Centauri's mass is about an eighth of the Sun's, and its average density is about 40 times that of the Sun. Although it has a very low average luminosity, Proxima is a flare star that undergoes random dramatic increases in brightness because of magnetic activity. The star's magnetic field is created by convection throughout the stellar body, and the resulting flare activity generates a total X-ray emission similar to that produced by the Sun. The mixing of the fuel at Proxima Centauri's core through convection and the star's relatively low energy production rate suggest that it will be a main-sequence star for another four trillion years, or nearly 300 times the current age of the universe.

Searches for companions orbiting Proxima Centauri have been unsuccessful, ruling out the presence of brown dwarfs and supermassive planets. Precision radial velocity surveys have also ruled out the presence of super-Earths within the star's habitable zone. The detection of smaller objects will require the use of new instruments, such as the proposed James Webb Space Telescope. Since Proxima Centauri is a red dwarf and a flare star, whether a planet orbiting this star could support life is disputed. Nevertheless, because of the star's proximity to Earth, it has been proposed as a destination for interstellar travel.

Based on the parallax of 768.7 ± 0.3 milliarcseconds, measured using the Fine Guidance Sensors on the Hubble Space Telescope, Proxima Centauri is roughly 4.2 light years from Earth, or 270,000 times more distant than the Sun. From Earth's vantage point, Proxima is separated by 2.18° from Alpha Centauri, or four times the angular diameter of the full Moon. Proxima also has a relatively large proper motion—moving 3.85 arcseconds per year across the sky. It has a radial velocity toward the Sun of 21.7 km/s.

Distances of the nearest stars from 20,000 years ago until 80,000 years in the future. Proxima Centauri is in yellow

---

Among the known stars, Proxima Centauri has been the closest star to the Sun for about 32,000 years and will be so for about another 33,000 years, after which the closest star to the Sun will be Ross 248. In 2001, J. García-Sánchez et al. predicted that Proxima will make its closest approach to the Sun, coming within 3.11 light years of the latter, in approximately 26,700 years. A 2010 study by V. V. Bobylev predicted a closest approach distance of 2.90 ly in about 27,400 years. Proxima Centauri is orbiting through the Milky Way at a distance from the galactic core that varies from 8.3 to 9.5 kpc, and with an orbital eccentricity of 0.07.

From the time of the discovery of Proxima, it was suspected to be a true companion of the Alpha Centauri binary star system. At a distance to Alpha Centauri of just 0.21 ly (15,000 ± 700 astronomical units [AU]), Proxima Centauri may be in orbit around Alpha Centauri, with an orbital period of the order of 500,000 years or more. For this reason, Proxima is sometimes referred to as Alpha Centauri C. Modern estimates, taking into account the small separation between and relative velocity of the stars, suggest that the chance of the observed alignment being a coincidence is roughly one in a million. Data from the Hipparcos satellite, combined with ground-based observations, is consistent with the hypothesis that the three stars are truly a bound system. If so, Proxima would currently be near apastron, the farthest point in its orbit from the Alpha Centauri system. More accurate measurement of the radial velocity is needed to confirm this hypothesis.

If Proxima was bound to the Alpha Centauri system during its formation, the stars would be likely to share the same elemental composition. The gravitational influence of Proxima may also have stirred up the Alpha Centauri protoplanetary disks. This would have increased the delivery of volatiles such as water to the dry inner regions. Any terrestrial planets in the system may have been enriched by this material.

Six single stars, two binary star systems, and a triple star share a common motion through space with Proxima Centauri and the Alpha Centauri system. The space velocities of these stars are all within 10 km/s of Alpha Centauri's peculiar motion. Thus, they may form a moving group of stars, which would indicate a common point of origin, such as in a star cluster. If it is determined that Proxima Centauri is not gravitationally bound to Alpha Centauri, then such a moving group would help explain their relatively close proximity.>>