by Paul Gilster | Jan 24, 2007 | Astrobiology and SETI, Culture and Society |
From the Paramus Post, a story by Bruce Lieberman looks at contrasting views of SETI:
On both sides of the SETI debate, scientists acknowledge that what’s certain is the limit of what they know.
“I personally think that because the origin of life is an extremely difficult process … even simple life is very rare in the galaxy,” Zuckerman said. “But I have no particular claims other than my gut feeling.”
Shostak has publicly debated Zuckerman on the issue, and he remains confident that future searches will make contact. “I doubt that I would conclude that nobody’s out there,” he said. “To me that seems like a last-resort option. But that’s simply my feeling on the matter. And my feeling on the matter … actually means nothing because what counts is what you can find.
“That’s the difference between science and belief.”
A quick overview of the topic, available here.
by Paul Gilster | Jan 23, 2007 | Exoplanetary Science |
Habitable planets in multiple star systems are one of science fiction’s great tropes. Find a second star somewhere in the daylight sky and you know you’re not in Kansas anymore. It makes slow going today, but as a kid I was struck with William F. Temple’s The Three Suns of Amara (1962), a story whose questionable science and creaky plot was somewhat mitigated by its striking imagery. For Amara managed to weave its orbit through a triple star system in which each star was a different color. Talk about great visual effects!
Image: My battered copy of Temple’s The Three Suns of Amara, rescued from a closet. Not the best novel I’ve ever read, but it did instill a lifelong fascination with habitable planets in multiple star systems.
Yesterday we looked at Elisa Quintana’s work on habitable planets in binary systems, and while reviewing for that story, I found Temple’s Amara again coming to mind. For it turns out we do have five confirmed exoplanets known to orbit one member of a triple star system. Quintana’s paper discusses the planet HD 188753 Ab, for example. It has a minimum mass 1.14 times that of Jupiter and orbits its primary (slightly over one solar mass) in a tight 3.35 day orbit.
Now get this: Around this star/planet duo orbits a short period binary star system (total mass 1.63 that of the Sun) that comes as close as 6 AU to HD 188753 A. Just how this system evolved its planets is a fascinating speculation, one that will receive much study as we learn more about it, especially as to the existence of other planets there. Amara it’s not, but the exoplanet around HD 188753 A has a lot to tell us. The tight passage of the binary pair offers us a unique laboratory for planet formation.
Three other exoplanet systems, though not triple, are intriguing because their stellar separations are well within the dimensions of our own Solar System. GJ 86 is a binary star with stellar separation somewhere around 20 AU, a situation paralleled by Gamma Cephei and HD 41004. So far we only know about giant planets in these systems, but as we saw yesterday, the separation between each of these stars does allow for planets in their habitable zones, making those SF-style speculations about day and night sky scenarios come inevitably to mind.
I should also mention that Gamma Cephei has been the object of recent study by Philippe Thebault and collaborators. Simulations building from small planetesimals up to Mars size worlds show favorable results, which is not to say that there are terrestrial worlds here but that we’re beginning to work out the parameters of how planets form in complex stellar environments involving more than one star. The paper is Thebault et al., “Planetary formation in the Gamma-Cephei system,” Astronomy & Astrophysics 427 (2004) pp. 1097-1104, available online.
by Paul Gilster | Jan 22, 2007 | Exoplanetary Science |
The findings about possible terrestrial worlds around the Alpha Centauri stars have become more encouraging than ever. Key work in this regard has been performed by Elisa Quintana and collaborators, who have shown in their simulations that, depending on initial disk inclinations, 3-5 such planets might form around Centauri A and 2-5 around Centauri B.
We’ve already discussed that research and I don’t want to linger on Quintana’s 2002 paper (reference below) other than to note one interesting comparison. When the same initial disk parameters are placed around a single star like the Sun, the accretion of the planetary disk occurs over a much larger expanse of time. Evidently a stellar companion hastens the process of planetary formation, one billion years in the case of the Sun vs. perhaps 200 million years in the Centauri scenario.
Quintana, Jack Lissauer (both at NASA Ames) and team went on from that study to look at planet formation around close binaries. And they’ve now turned to the factors influencing terrestrial worlds around individual stars in binary systems. That includes, of course, binaries like Alpha Centauri, but its significance goes well beyond our closest stellar neighbor given that the majority of solar-like stars are found in binary systems. Using numerical simulations to model planet formation and stability in these circumstances, their new paper gives us important information on where we might look to find planets like ours elsewhere in the galaxy.
Much depends on how close the two binary stars are, the most important factor being the periastron value — the distance between the two stars at the closest point in their orbits. The periastron value for Centauri A and B is 11.2 AU, a perfectly acceptable figure for terrestrial planet formation. For as these studies show, a periastron greater than 10 AU allows planets to develop unperturbed. From the paper:
When the periastron of the binary is larger than about qB = 10 AU, even for the case of equal mass stars, terrestrial planets can form over essentially the entire range of orbits allowed for single stars (out to the edge of the initial planetessimal disk at 2 AU). When periastron qB < 10 AU, however, the distributions of planetary orbital parameters are strongly affected by the presence of the binary companion."
So the good news for terrestrial planet hunters is that 40 to 50 percent of binary stars are wide enough to allow Earth-like planets to form and remain stable in orbits circling one of the two stars. And interestingly enough, Quintana’s work also shows that about 10 percent of main sequence binaries are close enough to allow the formation and stability of such planets in orbits that circle both stars.
What a finish: “Given that the galaxy contains more than 100 billion star systems, and that roughly half remain viable for the formation and maintenance of Earth-like planets, a large number of systems remain habitable based on the dynamic considerations of this research.”
The paper is Quintana et al., “Terrestrial Planet Formation Around Individual Stars Within Binary Star Systems,” accepted by The Astrophysical Journal and available as a preprint. The 2002 paper, available here, is “Terrestrial Planet Formation in the α Centauri System,” The Astrophysical Journal 576:982-996 (September 10 2002).
by Paul Gilster | Jan 20, 2007 | Deep Sky Astronomy & Telescopes |
The mammoth black hole Sagittarius A* isn’t the only interesting thing near the center of our galaxy. The European Space Agency’s Integral observatory, working with gamma rays, tracks about eighty high-energy sources in the area. About ten of those closest to the galaxy’s center had faded when Integral performed a series of observations last April. A mysterious force? Hardly. “All the sources are variable and it was just by accident or sheer luck that they had turned off during that observation,” says Erik Kuulkers of ESA’s Integral Science Operations Center.
Fair enough, and useful for astronomers, who were able to use the sudden quiet to look for still fainter sources, and to set limits on the brightness of the x-ray binaries involved. These consist of two stars orbiting each other, one a normal star, the other a collapsed object — a white dwarf, neutron star or black hole. The compressed star pulls off gaseous material from its companion, heating it to a million degrees Centigrade and causing it to emit x-rays and gamma rays. The brightness of the emission seems to depend on the amount of gas transferred.
Image: The center of the galaxy, as seen in a mosaic of exposures from Integral. The black hole Sagittarius A* is close to the source marked ‘1’ in the image. Due to the variability these sources possess on all time scales, the region never looks exactly the same. Surprisingly, the sources were ‘off’ around the time of the observation (including the normally bright well-known black-hole candidate and micro-quasar 1E 1740.7-2942), displaying an unusually ‘quiet’ galactic centre. Credit: ESA/ISDC.
What’s intriguing to Centauri Dreams is that dimming of sources near Sagittarius A* could lead to the detection of faint high-energy radiation associated with its immediate neighborhood, telling us more about this enigmatic object. A paper on recent Integral findings is Kuulkers et al., “The INTEGRAL Galactic bulge monitoring program: the first 1.5 years,” accepted by Astronomy & Astrophysics and available online.
by Paul Gilster | Jan 20, 2007 | Culture and Society |
A relativistic trip to Barnard’s Star? Those who read French will want to check out the log of such a journey as Philippe Guglielmetti sees it. Traveling at a constant 1g for acceleration and braking, the mission reaches 0.99999 c, travel time twelve years but only three as experienced by the crew. The fictionalized journey plays fast and loose with the star itself, as Adam Crowl notes in a comment below, but the trip is fun even with my rusty French. Have a look.
