Miniature Suns and their Planets

As if we didn’t already have enough trouble defining what a planet is, astronomers have now discovered a brown dwarf only eight times the mass of Jupiter. Surrounded by a dusty disk, the object is actually smaller than a number of planets already known to be orbiting other stars. Any miniature solar system that formed around the brown dwarf would be roughly 100 times smaller than our own. All of which raises the question of what to call objects that might be found around this tiny dwarf: planets or moons?

The question has obvious resonance in an era marked by repeated discoveries in the Kuiper Belt that could be considered of planetary size. And another sign of the ambiguity in definition is that worlds like Titan, Ganymede and Callisto are large enough in their own right to qualify as planets, if we overlook the inconvenient fact that they orbit massive planets of their own. The question may seem insignificant, but how we define things is ultimately a measure of how extensive our knowledge has become, and this issue shows us that we have a long way to go in understanding how and where planets form.

“There are two camps when it comes to defining planets versus brown dwarfs,” said Giovanni Fazio of the Harvard-Smithsonian Center for Astrophysics, a member of the discovery team. “Some go by size, and others go by how the object formed. For instance, this new object would be called a planet based on its size, but a brown dwarf based on how it formed.”

Miniature brown dwarf systemThe new brown dwarf, called Cha 110913-773444, is located some 500 light years from Earth in the constellation Chamaeleon. A number of observing sites contributed to the find, but it took the Spitzer Space Telescope’s Infrared Array Camera to pick out the dim protoplanetary disk. As to the disk itself, the team speculates that it would be sufficient to form a small gas giant and several Earth-sized rocky planets, as depicted in the illustration.

Image (click to enlarge): This artist’s conception compares a hypothetical solar system centered around a tiny “sun” (top) to a known solar system centered around a star, called 55 Cancri, which is about the same size as our Sun. The tiny system consists of an unusually small “failed” star, or brown dwarf called Cha 110913-773444, and a surrounding disk of gas and dust that might one day form planets. At a mass of only eight times that of Jupiter, the brown dwarf is actually smaller than several known extrasolar planets. The largest planet in the 55 Cancri system is about four Jupiter masses. Credit: NASA/JPL-Caltech.

The work on Cha 110913-773444 was performed by a team led by Kevin Luhman of Penn State University. A paper on its findings will appear in the December 10 issue of The Astrophysical Journal Letters. And here is a news release from the Harvard-Smithsonian Center for Astrophysics.

Red Dwarf Stars and SETI

M-class red dwarfs have never figured prominently in the SETI search. The reason for this is apparent: such stars, of which Proxima Centauri, Earth’s nearest stellar neighbor, is one, are flare stars. The intense radiation from solar flares should cleanse a planetary surface of life, especially given the close proximity of such a planet to its star. Remember, the habitable zone around a red dwarf is going to occur well inside the orbit of Mercury.

Gliese 876 planetAnd there’s a second reason. By virtue of having to orbit the host star so tightly, a planet around a red dwarf is going to be tidally locked. One side would be baked, the other frozen, which makes the odds on liquid water look slim. But assumptions are made to be questioned, which is why work at Ames Research Center in the late 1990s remains so interesting. One implication of the Ames work, for example, is that there are conceivable weather patterns that could circulate heat to the dark side of a tidally locked world, keeping it warm enough to prevent its atmosphere from freezing out.

Image: An artist’s conception of the recently discovered planet around the M-class red dwarf Gliese 876. Credit: Trent Schindler, National Science Foundation.

SETI pioneer Jill Tarter is well aware of such work, saying in a recent article in Astrobiology Magazine:

“If you put a little bit of greenhouse gas into an atmosphere, the circulations can keep that atmosphere at a reasonable temperature and you can dissipate the heat from the star-facing side and bring it around to the farside. And, perhaps, end up with a habitable world.”

That’s a big ‘perhaps,’ but it ties in with other recent work that shows that most of a red dwarf’s flare activity occurs early in its life cycle (this is why the recent observation of a major flare from Barnard’s Star is so unusual, given the star’s age). A quieting flare environment might allow life to survive. And because seven out of ten stars in our galaxy are red dwarfs like Proxima, we don’t want to ignore possible interesting targets as we focus on what seem the more likely F, G and K class stars.

As the Allen Telescope Array comes online, its 350 antennae will handle traditional radio astronomy as well as SETI, and with careful adjustment, astronomers will be able to form as many as eight virtual antennae, each pointing to a different star. An efficient SETI search will be one that allows the largest number of target stars in the array’s field of ‘view,’ and that means adding M-class dwarfs to increase the target list enormously. From the article:

“It’s not the star that I’m interested in,” Tarter says. “It’s the techno-signature from the inhabitants on a planet around the star. I don’t ever have to see the star, as long as I know that it’s in that direction. I don’t ever have to see the planet. But if I can see their radio transmitter – bingo! – I’ve gotten there. I’ve found a habitable world.”

Last July’s red dwarf workshop, chaired by Tarter, found no ‘showstoppers’ that would preclude adding red dwarfs to the target list. Findings from the conference will eventually be published. Until then, Ken Croswell’s story “Red, Willing and Able,” in New Scientist 169 (January, 2001, pp. 28-31) delivers a concise summary of the Ames work. See also Manoj Joshi, Robert Haberle, and R. Reynolds, “Simulations of the Atmospheres of Synchronously Rotating Terrestrial Planets Orbiting M Dwarfs: Conditions for Atmospheric Collapse and the Implications for Habitability,” Icarus 129 (1997), pages 450-65.

An Antimatter Molecule?

With Hayabusa apparently stabilized and ready to begin its return journey to Earth, and with the Falcon-1 launch delayed until mid-December, it’s time to return to research. But not before congratulating the Japanese space agency (JAXA) for the probe’s apparent success in landing on the asteroid Itokawa, collecting surface samples, and lifting off again. These would be the first asteroid materials ever returned to Earth, and if their landing in 2007 proceeds as planned, they will be the capstone of a remarkable mission.

On the research front, what catches the eye this foggy North Carolina morning is the report in Nature that scientists may have created positronium molecules made out of two positronium atoms. If so, it would be a singular accomplishment. Positronium replaces the hydrogen proton with a positron (the antimatter equivalent of an electron). So instead of normal hydrogen’s single electron moving around a proton, you get an electron moving around a positron which, like the proton, is positively charged, though far less massive. A team led by Allen Mills (University of California at Riverside) believes it may have seen double positronium molecules, each made up of two electrons and two positrons.

Needless to say, we are talking about an extremely unstable molecule since matter and antimatter annihilate when they meet. If the work stands up to scrutiny — and there are other explanations for these results — it would be the first time positronium molecules were observed to have a brief existence before their destruction and, as Nature opines, “…the first evidence for a new kind of chemistry, resulting from reactions between ‘explosive’ atoms that have a completely different physical make-up from those in nature.”

The experiment worked this way: the researchers targeted a surface made of porous silica with positrons. The positrons combined with electrons to form a concentration of unstable positronium atoms which moved into the pores of the silica and, colliding with each other, produced energetic gamma radiation. More about the experimental method can be found in a UCR press release, from which this:

“This is the first time anyone has been able to observe a collection of positronium atoms that collide with one another,” Mills said. “We knew we had a dense collection of these atoms because, being so close to one another, they were annihilating faster than when they were just by themselves.”

But Mills notes that the evidence for double positronium molecules in this experiment is only a ‘suggestion.’ Up next is the confirmation of their existence and the measurement of their actual properties. A paper by Mills’ team will appear next month in Physical Review Letters.

Antimatter and Its Dangers

“It is quite possible to build atmospheric vehicles using an antimatter drive. After all, a tenth of a gram of the stuff could power a family flivver to orbit and back. But no machine is perfect, and even that tiny smidgin of antimatter would devastate the countryside if anything went wrong. When antimatter drives first become practical, we can expect treaties banning its use for propulsion within Earth’s atmosphere. There are other potential uses for it on Earth; for example, as an ultimate compact source of energy to power an MHD [magnetohydrodynamic] electric plant. The exhaust product is a high-temperature plasma… MHD power does not have to be used to propel vehicles; it could also take care of those demand surges on a nation’s electrical power grid. Will the treaties ban this use, too? We will risk a guess: yes. We will have other sources of energy from space by that time, and they do not involve the potential destruction of even a milligram of antimatter gone astray. So far as we know, antimatter drives are the ultimate in propulsion efficiency. They may have to wait, however, for those orbital factories.

“As a footnote of interest, Jon Post reminds us that if we ever find so much as a single antimatter molecule of heavier stuff that we did not make — anti-uranium, for an extreme example — it would prove the existence of an antistar in which antihydrogen was cooked into heavier nuclei. This in turn would prove the existence of an antigalaxy. We could communicate with its citizens from a mutually safe distance, but handshakes between us would be a mite hazardous to our health.”

Dean Ing and Leik Myrabo, The Future of Flight (New York: Baen, 1985), pp. 158-159.

Centauri Dreams‘ take: Ponder how far we have to go before we can start talking about practical antimatter propulsion. The challenge, of course, is in making the stuff. Yes, a tenth of a gram would devastate the nearby landscape if a craft carrying it were to crash. But today’s best antimatter producer, the huge CERN particle accelerator in Switzerland, can produce about enough in a year to power a 100-watt bulb for 15 minutes. At present, making antiprotons requires 10 billion times more energy than it produces. None of this should be construed as an argument against antimatter propulsion, but rather a reminder that the real breakthrough will come not from engine design but a host of new techniques in antimatter production, storage and transport.

Hayabusa Attempts Second Landing

The Japanese spacecraft Hayabusa evidently managed to land on asteroid Itokawa several days ago after all, according to this from the Japan Aerospace Exploration Agency:

“At the timepoint of Nov. 21, Hayabusa was judged not to have landed on the surface. According to the replayed data, however, it was confirmed that Hayabusa stayed on Itokawa by keeping contact with the surface for about 30 minutes after having softly bounced twice before settling. This can be verified by the data history of LRF and also by attitude control record…”

Hayabusa shadowFor more, you can read the complete JAXA statement here. The spacecraft is now being maneuvered for a second landing (and surface sampling) attempt. Note the shadow in this photograph, much more clearly visible than in the previous images of Itokawa from Hayabusa. There are people who shrug at this sort of thing, but to Centauri Dreams images like these are breathtaking. They remind us that a human presence has now encountered objects hitherto unexplored, the shape of technology defined on a surface untouched since the formation of the Solar System.

Image: Target marker with signatures (image taken at altitude of 32 meters). Credit: JAXA.