Binaries, Gas Giants and Habitable Worlds

Alpha Centauri A and B have a mean separation of 23 AU. In Solar System terms, that gives you a spacing a bit further from the Sun than the orbit of Uranus. But with the two stars moving around a common center of mass, the distance between them varies over time. Centauri B is sometimes as far from Centauri A as Pluto is from Sol, while at other times it closes as close as Saturn. From a planet around Centauri B, Centauri A would sometimes shine with the light of 5000 full moons, creating day and night sky scenarios that would be, to say the least, striking.

Recent research is making it clear that planets can form in such systems, but binaries are tricky, and we still have much to learn about how such planets would form and where, and under what conditions certain kinds of objects are more likely to occur. Twenty percent of the exoplanet systems thus far found are binary, with the majority of these being wide binaries (separated by 250 to 6500 AU, a far cry from our Centauri stars). But three exoplanetary binary systems — GL 86, Gamma Cephei and HD 41004) — with much closer separations are known to harbor gas giants.

And look at these separations. The Gamma Cephei stars are 18.5 AU apart, while GL 86 shows a separation of 21 AU and HD 41004 a separation of 23 AU. Given the presence of gas giants in these circumstances, can binary systems with separations of 50 AU or less also form terrestrial worlds in the habitable zone? And how would the presence of the secondary star and the gas giant afffect the delivery of volatiles to the inner planets of the binary system in this scenario?

A new paper by Nader Haghighipour (NASA Astrobiology Institute, University of Hawaii-Manoa) looks at the question by simulating the late stage of habitable planet formation, using the collision and growth of a little more than a hundred objects of Moon to Mars-size. Those ’embryos’ inside 2 AU were assumed to be dry, while those between 2 and 2.5 AU were considered to have 1 percent water, and those beyond 2.5 AU to have a water to mass ratio of five percent.

Running the simulations over a 100 million year period and varying the distance, orbital eccentricity and mass of the secondary star, the team found that terrestrial class planets with substantial water reserves can form in the habitable zones of the primary star. From the paper:

Since at the beginning of each simulation, the orbit of the giant planet was considered to be circular, a non-zero eccentricity is indicative of the interaction of this body with the secondary star. As shown here, Earth-like objects are formed in systems where the average eccentricity of the giant planet is small. That is, in systems where the interaction between the giant planet and the secondary star has been weak. That implies, habitable planet formation is more favorable in binaries with moderate to large perihelia, and with giant planets on low eccentricity orbits.

Thus we firm up the picture on binary systems that may prove of astrobiological interest. The paper is “Habitability of Planets in Binaries,” slated to appear in Extreme Solar Systems, ASP Conference Series, ed. Debra Fischer, Fred Rasio, Steve Thorsett and Alex Wolszczan, and now available online. Finding gas giants in systems like the three mentioned above is another indication that planets tend to form wherever they can, even where binary separations are relatively small. But the gas giants around GL 86, Gamma Cephei and HD 41004 make the formation of habitable planets in their systems unlikely.

Best Glimpse Yet of Nix and Hydra

Nix and Hydra

Thirty years ago it was all but impossible to tease the presence of Charon out of the Pluto images available to astronomers. Today we’re using ground-based telescopes like the twin Keck instruments on Mauna Kea (Hawaii) to see the far tinier Nix and Hydra, the minute satellites discovered by the Hubble Space Telescope in 2005. Making Nix and Hydra visible results in Pluto and Charon, far brighter objects, appearing as a bright blob in the image at left. The streaks are the result of Pluto’s motion against background stars during the exposure.

Image (above): Nix and Hydra. This image combines all 16 exposures taken at Keck, with the contrast adjusted to show Pluto’s new satellites Nix (left) and Hydra (right) as the small dots in the upper right. Both Nix and Hydra are about 5000 times fainter than Pluto, thus both Pluto and Charon are washed out in the image. The Pluto system moved with respect to the background stars during the one hour of observations, leaving the stars trailed. Credit: David Tholen.

This is precision work indeed — Nix and Hydra are each less than 100 kilometers in diameter. At magnitude 23.5, they are far less bright than Pluto itself (14th magnitude). Moreover, Keck’s adaptive optics (which compensates for the atmospheric turbulence that can blur the light of distant objects) is going to be used for a continuing set of observations. David Tholen (University of Hawaii) has this to say:

“It is our intent to obtain several more images of the Pluto system, hopefully with this same level of quality, so that we can track Nix and Hydra completely around Pluto several times. By making extremely precise measurements of the satellites’ positions, we will determine their masses by detecting the tiny displacements caused by their mutual gravitational attraction. Once the masses are in hand, we’ll be able to say something more definitive about how big these new satellites are.”

Pluto and Charon

Which is useful in itself, but even more valuable in terms of the New Horizons mission, now making its way to the Pluto/Charon system for a 2015 encounter. The New Horizons team needs all the information it can get about the motion of these satellites well in advance of the flyby, and thus far Keck has delivered. The Keck image at left shows Pluto and Charon themselves, the contrast now optimized in ways that wash out Nix and Hydra. We’ll be seeing far better images from the spacecraft, of course, but the improvement in Earth-bound viewing in the past thirty years is still amazing.

Carnival of Space #25

Sam Wise offers the latest Carnival of Space (#25) on his Sorting Out Science blog, which if you haven’t seen (I hadn’t) you must. We’re obviously dealing with a voracious reader, for what we have here is a collection of noteworthy topics — the Allen Telescope Array, asteroid deflection and Cassini findings are particularly germane to Centauri Dreams readers — that Sam has researched over a range of weblogs, giving us perspectives on all these matters. All done with a wit and intelligence that prompts me to add Sorting Out Science to our list of links.

M87’s Jet (and Memories of Clarke)

The massive galaxy M87, the central object of the Virgo cluster, has drawn our attention for a long time. It was in 1918 that Heber Curtis discovered a jet pushing at least 5000 light years away from the center of the galaxy. In 1949, the radio source Virgo A was identified with M87, and by the 1960s it was believed that the jet was actually two sided, its one-sided appearance due to relativistic Doppler beaming, which increased the luminosity of the jet in the direction of the observer.

That latter point was confirmed by recent observations using the Very Long Baseline Array (VLBA), with a resulting image showing detail down to a resolution of one milli-arcsecond. Some fifty times better than what Hubble can manage at optical wavelengths, the radio image (seen below in false color) shows the faint counter-jet structure that had been posited by the Russian astrophysicist Iosif Shklovsky. The latter noted that the jet liberated as much energy as the explosion of ten million supernovae.

M87 jet in false color

Image: The Inner Jet of the Radio Galaxy M87 located in the Virgo cluster. The angular resolution of this false-color radio image observed by the VLBA at 2 cm wavelength is approximately one milli-arcsecond, fifty times better than that of the Hubble Space Telescope at optical wavelengths. The image shows a limb brightened jet and a faint counter-jet. The central gap is consistent with the presence of a fast inner jet which is beamed away from the observer surrounded by a slower moving outer plasma seen by the VLBA. Credit: Y.Y. Kovalev, MPIfR Bonn.

The apparent mechanism in all this is a supermassive black hole in the center of the galaxy. The rapidly rotating accretion disk surrounding it feeds the hole, while matter is ejected from the nucleus in the observed jets. Of course, a playful look for macro-engineering might suggest alternative explanations. M87 fascinated Arthur C. Clarke, who wrote about it in a book called The Scientist Speculates: An Anthology of Partly-Baked Ideas (New York: Capricorn Books, 1962), likening it to a scenario he had painted in his 1954 novel The City and the Stars:

Man was about to leave his Universe, as long ago he had left his world. And not only Man, but the thousand other races that had worked with him to make the Empire. They were gathered together, here at the edge of the Galaxy, with its whole thickness between them and the goal they would not reach for ages.

They had assembled a fleet before which imagination quailed. Its flagships were suns, its smallest vessels planets. An entire globular cluster, with all its solar systems and all their teeming worlds, was about to be launched across infinity.

The long line of fire smashed through the heart of the Universe, leaping from star to star. In a moment of time a thousand suns had died, feeding their energies to the monstrous shape that had torn along the axis of the Galaxy and was now receding into the abyss . . .

It’s that last line that seizes the attention, a galactic jet’s inconceivable energies created by artificial means. Clarke later noted in a postscript to the reprint of this essay in Cosmic Search magazine that he had written it before the realization that explosive events tied to such active galactic nuclei (AGN) seem to be common in the universe. He also gave a nod both to Freeman Dyson and to Stanley Schmidt’s chilling line in The Sins of the Fathers (1976) that “Seyfert galaxies are industrial accidents.”

Now we have a better look at the central region of M87 than ever before, one corresponding to a linear resolution of three light months. Adding the Effelsberg 100m radio telescope to provide a trans-Atlantic baseline should provide an even more detailed image of the galaxy’s jet. An industrial accident it’s doubtless not, nor is it evidence of extraterrestrial engineering, but what a magnificent natural phenomenon grows out of the tortured interactions at this galaxy’s heart.

Amateur Discovers GRB Afterglow

What we know about gamma-ray bursts is dwarfed by what we don’t, but chipping away at the problem is getting us places, particularly with the help of amateur astronomers. Thus the news that Finnish amateur Arto Oksanen had found the optical afterglow of GRB 071010B, a gamma-ray burst detected by NASA’s Swift satellite. Oksanen did his work with a 40-centimeter telescope at the Hankasalmi Observatory in Finland.

Afterglow of a GRB

This is the kind of discovery that would have been all but impossible until recently, relying as it does not only on the Swift satellite’s detection capabilities but also on immediate notification of Earth-based observers over the Internet. Remember: Gamma-ray bursts last anywhere from a few milliseconds to a few hundred seconds, and even though they seem to occur once a day, aligning the Swift data with an optical afterglow means looking just as soon as the notification comes in.

Is luck involved? You would think so, and Oksanen agrees:

…you have to be very lucky (and others have to be unlucky) to discover a GRB afterglow nowadays. The sky location of this GRB was a big advantage for me as I was able to observe this one right from the alert being able to ‘look over the North pole’. And this was a bright afterglow and also the weather was good (just after two weeks of cloudy nights).

Hard work pays off, I have been hoping and trying to do this since 1997 when I got interested about GRBs. Indeed “at last!”.

Image: The afterglow of GRB 071010B. Credit: A. Oksanen/AAVSO.

As far as what GRBs are, the question remains unsettled, although GRB 071010B looks to have been a collapsing, supermassive star whose supernova explosion has resulted in a black hole. With Oksanen’s data in hand, the Gemini and Keck telescopes in Hawaii were able to measure the object’s properties, finding a redshift measurement consistent with a distance of at least seven billion light years. Thus the Finn becomes the first amateur to bag a GRB afterglow since 2003, when South African observer ‘Berto’ Monard pulled it off. More information can be found in this news release from the American Association of Variable Star Observers, which coordinates work between amateurs and professionals.

Meanwhile, I’m thinking about Timothy Ferris’ Seeing In the Dark special on PBS (with excellent accompanying site). The book was terrific, but it was fascinating to actually see the assembly of a remote amateur instrument at the New Mexico Skies site, and to watch Ferris handling the incoming image over the Net. When I think about my own tree-obstructed skies, the idea of using the Net to manipulate a remote telescope gets more enticing with every passing day.