by Paul Gilster | May 25, 2007 | Exoplanetary Science |
Brown dwarfs, most of them unobserved, doubtless litter the galaxy. The more we can learn about them and their possible companions, the better for our understanding of how planets form and stars evolve. These minute ‘failed stars,’ far less massive than the Sun, cannot sustain hydrogen fusion, but they’re players in the exoplanet hunt. The brown dwarf 2MASS1207-3932, for example, has a planetary companion of five Jupiter masses, thought to be the first for which an image was obtained.
Now we learn that this young star, perhaps eight million years old and surrounded with a protoplanetary disc, is also producing jets of matter. The results, growing out of work at the European Southern Observatory’s Very Large Telescope, are surprising. The dwarf’s mass is itself only 24 times that of Jupiter, making it the smallest object known to produce such jets.
Image: Using ESO’s VLT, astronomers found jets coming out from a 24 Jupiter-mass brown dwarf, showing that outflows are rather ubiquituous in the universe and leading to the prospect that that young giant planets could also be associated with outflows. (c) ESO.
Jets are not uncommon in young T-Tauri stars, which are stellar infants that have not yet joined the main sequence. But detecting these relatively bright jets is a far cry from observing the faint signature coming from 2MASS1207-3932. Its jets stretch about a billion kilometers from the star, moving away from it at a speed of a few kilometers per second. Emma Whelan (Dublin Institute for Advanced Studies), lead author of the paper reporting on these events, is worth quoting on the matter:
“Discoveries like these are purely reliant on excellent telescopes and instruments, such as the VLT. Our result also highlights the incredible level of quality which is available today to astronomers: the first telescopes built by Galileo were used to observe the moons of Jupiter. Today, the largest ground-based telescopes can be used to observe a Jupiter size object at a distance of 200 light-years and find it has outflows!”
So now we can add brown dwarfs to the vast array of objects that produce outflows, from active galactic nuclei massing tens of millions of solar masses down to objects weighing only a few tens of masses larger than Jupiter. The paper is Whelan et al., “Discovery of a Bipolar Outflow from 2MASSW J1207334-393254 a 24 MJup Brown Dwarf,” The Astrophysical Journal Vol. 659, p. L45 (abstract available). One more quote from Whelan: “This leads us to the tantalizing prospect that young giant planets could also be associated with outflows.”
by Paul Gilster | May 24, 2007 | Exoplanetary Science
Two gas giants discovered around the star HD 155358 raise again the question of planetary formation and the mechanisms behind it. Most planets detected through radial-velocity methods, which measure the effects unseen companions have on a star’s motion, have been found to orbit stars that are high in metal content. ‘Metals’ in this context means elements higher than hydrogen and helium, and of the two primary models for planetary formation, high metal content seems to favor the one known as the core accretion model, about which more in a moment.
What to do about a low metal star whose system is dominated by two massive planets? HD 155358 contains only 20 percent of the metal content of our Sun. Such a finding may favor the rival disk instability model. Here the notion is that the rotating disk of gas and dust in a protoplanetary system becomes unstable not long after it forms, causing it to fragment. As clumps begin to appear, they become large enough to cause their gases to collapse under gravitational forces. And you wind up with a planet forming quite quickly, perhaps in as little as a few centuries.
But the University of Texas team that discovered the two Jupiter-like planets around HD 155358 doesn’t rule out core accretion. That model, which works on much longer timescales, says that a Jupiter-like planet gradually forms by accumulation of solid materials into a core that develops over about a million years, finally reaching the point where its gravity is large enough to pull large amounts of gas along with it. A gas giant forms, though in a few million years rather than a few hundred.
Nonetheless, if the two new planets did form through core accretion, the results are unusual, since the star has little of the material needed for that kind of planet-building. But maybe what’s significant here isn’t just the metallicity of the star in question but the size of the protoplanetary disk, says Michael Endl (University of Texas at Austin):
“The major result of our discovery is that these planets required a very massive disk to form, several times more massive than we think our solar system disk was. This demonstrates that disk masses can vary significantly and might even be the most crucial factor in planet formation.”
The core accretion/disk instability argument has a long run ahead of it, though core accretion seems to have the upper hand in most exoplanetary systems we’ve examined thus far. As for the new planets, one is at least 90 percent the mass of Jupiter and orbits at about 0.6 AU, while the other appears to have half of Jupiter’s mass, orbiting at 1.2 AU. HD 155358 is somewhat hotter than the Sun but also less massive. The Texas team estimates its age at some 10 billion years, though we’ve seen recently how much play there can be in stellar age calculations.
The paper on this discovery is Cochran et al., “A Planetary System Around HD 155358: The Lowest Metallicity Planet Host Star,” in press at The Astrophysical Journal and available online. The intricate orbital resonances of these two worlds — when one planet’s orbit becomes more circular, the other becomes more eccentric — make for interesting reading here.
by Paul Gilster | May 24, 2007 | Culture and Society
The Carnival of Space #4 is now up at Universe Today, and is well worth a look to find out what a wide range of writers are saying about everything from terraforming Mars to the linkages between science and science fiction. Ian Musgrave’s Astroblog offers good background on GJ 436 b, and I particularly like Universe Today‘s own take on a story we’re currently featuring about the expansion of the cosmos. Lots of new reading here, and busy readers will appreciate the selection process that singled these items out. A sharp editor is a godsend for the information-deluged.
by Paul Gilster | May 23, 2007 | Deep Sky Astronomy & Telescopes |
Centauri Dreams‘ recent post on the eventual merging of the Milky Way with the Andromeda galaxy took us to a future some five billion years from now. But it also speculated on something even more distant in time. What happens if the universe’s expansion does not stop accelerating? Eventually the galaxies beyond our own Local Group will exit the visible universe. Astronomers of that era would have no way of knowing those galaxies had ever existed, and would shape their cosmology accordingly.
Meanwhile, our Local Group should still be visible — the merged Andromeda/Milky Way elliptical galaxy and the survivors of the more than thirty galaxies, held together by mutual gravitational attraction, that make up the LG today. These galaxies should remain gravitationally bound despite the effects of the accelerated expansion, according to a paper by Lawrence Krauss (Case Western Reserve) and Richard Scherrer (Vanderbilt) to be published in October.
A starry island in an endless black sea. It’s an odd scenario, to be sure, but one that violates no currently understood laws of physics. The reason the more distant galaxies will seem to disappear is that space will be expanding faster than the speed of light. Einstein, to be sure, told us that nothing could move faster than the speed of light within space, but his theories put no speed limit on the expansion of spacetime itself.
As for those distant galaxies beyond the Local Group, they will still be out there, but their light will be unable to reach us. Unless, of course, our understanding of the universe’s expansion is incomplete (also a reasonable assumption). After all, we don’t understand the dark energy that almost has to exist to explain current observations, nor can we necessarily assume (although it would seem logical to do so) that dark energy will always have the same effects that we observe today.
All we can do is extrapolate from what we know and be aware of the gaps in our knowledge. Those gaps will be profound in the universe of the far future. Listen to Krauss and Scherrer on the question of whether or not physicists of that era will be able to puzzle out the Big Bang:
The answer is no. The inference that the universe must be expanding or contracting is dependent upon the cosmological hypothesis that we live in an isotropic and homogeneous universe. For future observers, this will manifestly not be the case. Outside of our local cluster, the universe will appear to be empty and static. Nothing is inconsistent with the temporary existence of a non-singular isolated self-gravitating object in such a universe, governed by general relativity. Physicists will infer that this system must ultimately collapse into a future singularity, but only as we presently conclude our galaxy must ultimately coalesce into a large black hole. Outside of this region, an empty static universe can prevail.
Which leads to this extraordinary assessment:
Observers when the universe was an order of magnitude younger would not have been able to discern any effects of dark energy on the expansion, and observers when the universe is more than an order of magnitude older will be hard pressed to know that they live in an expanding universe at all, or that the expansion is dominated by dark energy. By the time the longest lived main sequence stars are nearing the end of their lives, for all intents and purposes, the universe will appear static, and all evidence that now forms the basis of our current understanding of cosmology will have disappeared.
How do we know our own era doesn’t suffer similar constraints? Are there events in the early universe of which we are unaware because of the expansion of spacetime, and how does our lack of understanding distort our own view of reality? “It’s very important for all cosmologists to be very humble,” said Fred Adams, co-author (with Greg Laughlin) of The Five Ages of the Universe: Inside the Physics of Eternity, when queried about Krauss and Scherrer’s work in this Discovery News story.
Indeed, cosmology seems to teach humility above all else (The Five Ages of the Universe is a useful primer). The Krauss/Scherrer paper is “The Return of a Static Universe and the End of Cosmology,” slated for the Journal of Relativity and Gravitation and available online.
by Paul Gilster | May 22, 2007 | Breakthrough Propulsion |
The beauty of magnetic sail concepts — magsails — is that they let us leave heavy tanks of propellants behind and use naturally occurring phenomena like the solar wind to push us where we’re going. Solar sails, of course, do the same thing, though they use the momentum imparted by photons rather than the energetic plasma stream of the solar wind. And Cornell University’s Mason Peck is now suggesting another kind of mission that leaves the fuel behind. Instead of using the solar wind, it taps magnetic fields like those around the planets.
As we’ll see in a moment, we might one day use this method to send a fleet of micro-probes to Proxima Centauri. But let’s examine it first in light of planetary missions, which is what Peck has in mind with his Phase II NIAC study “Lorentz-Actuated Orbits: Electrodynamic Propulsion Without a Tether.” What the researcher is proposing is that a spacecraft can be made to accelerate in a direction perpendicular to a magnetic field. We know from Cassini images how the orbits of dust particles in Saturn’s rings are governed by such forces.
In fact, this ‘Lorentz force’ proves to be tremendously useful in the near-planetary environment. A spacecraft in Earth orbit, for example, creates a charge as it moves through the plasma surrounding the planet. The charge is minute, but it can be boosted either by emitting charged particles from a high-energy beam, or by using a lightweight surface (Peck suggests a thin, cylindrical wire mesh) to house a greater charge. Once charged sufficiently, the spacecraft will be deflected by the planetary magnetic field in a direction perpendicular to the magnetic field lines.
Jupiter’s magnetic field, containing fully 18,000 times the energy of Earth’s magnetosphere, would be ideal for this kind of work, offering plentiful opportunity not just for orbital adjustment but even for ‘hovering’ in place over a particular area to be studied (Robert Forward used to discuss doing something like this with ‘statites,’ satellites that would use solar sails to hover in Earth polar orbit or elsewhere). And imagine the increased payload that could be added to a Galileo-style spacecraft to Jupiter without the 371 kg of propellant that flew aboard that mission!
But the notion really opens up when you begin considering much smaller vehicles. Here I’m going to quote our own Larry Klaes, who wrote Peck’s work up for Ithaca (NY’s)’s Tompkins Weekly:
[Peck] notes that the concept might be ideal for small spacecraft. Cornell graduate student Justin Atchison is developing a satellite that is the size and heft of a single wafer of silicon.
“At this small scale, a spacecraft might be surprisingly susceptible to Lorentz force effects,” explains Peck. “But rather than launching just one of these ‘ChipSats,’ NASA might launch millions of them that would act as a swarm of very small sensors to detect life on another planet, provide communications, or serve as a distributed-aperture telescope many kilometers in diameter.”
As we move into the realm of ChipSats, Peck has my full attention. Take the ChipSat to its logical conclusion and you can envision thousands of tiny spacecraft slung out from the Solar System at ten percent of lightspeed to make the journey to the Centauri stars. “When these small craft arrive,” says Peck (I’m quoting from Larry’s story again), “they might send back a single, simple signal; one bit of information confirming or denying some scientific principle, such as is there a blue-green planet, for example.”
Peck’s completed Phase I study for NIAC is here, and you can read a precis of the Phase II project as well. Compared to solar sails or tether concepts, the Lorenz-Actuated Orbit (LAO) offers singular benefits. Peck writes:
“Electrodynamic tethers and solar sails certainly have their place. Tethers are convenient for deorbiting spacecraft in a passive way (i.e. without applied power). Solar sails work just as well, if not better, outside the geomagnetic field as they do near the earth. However, both suffer from the problem that the very large structures involved can deform under the action of the forces on them, reducing their performance. In the case of a tether, it appears that only gravity-gradient balance or spinning will help align a tether stiffly enough for it can raise an equatorial orbit in a mass-efficient way without buckling, tangling, or becoming redirected into a useless orientation. Solar sails are virtually impossible to reorient in an agile fashion. Our goal is to develop the LAO concept to the point where it is highly compact but offers the same propellantless benefits. The result will be an agile propellantless spacecraft. Even if the LAO spacecraft includes a long wire for capacitance, this wire will result in the same effect regardless of its direction. This significant advantage argues for the continued investigation of the LAO concept and suggests that it may prove more readily adaptable to existing mission architectures than are tethers.
You can read more about the concept at Peck’s site, and the issue of the Tompkins Weekly with Larry Klaes’ article is here. I’m also reminded of Robert Freitas idea of the ‘needle probe,’ an interstellar vehicle the size of a sewing needle but equipped with the nanotechnological tools to create an observing station out of raw materials it finds in the planetary system to which it is sent. Send not one or two but thousands of these for redundancy and you open up the nearby stars to minute examination. Will ChipSats offer a way to put instrumentation into Centauri space and beyond?
Addendum: I had originally referred to “Jupiter’s magnetic field, fully 18,000 times stronger than Earth’s…,” which Paul Dietz points out in the comments below is a mis-statement, as now corrected above.
