[BLOG] Some Tuesday links

• At Beyond the Beyond, Bruce Sterling points towards the first step of the exact role that the famed underground tunnels of Gaza have on the political economy of that territory.


• Crooked Timber's John Holbo argues that the legacies of coded racism used by many Republicans in the United States continues to make the party not credible among non-whites.


• At The Dragon's Tales, Will Baird points to a new study arguing that stars richer in heavy elements than our own (elements like uranium) are likely to have planets that have more heavy elements than our Earth, meaning more geologically active planets on account of the additional energy.


• Eastern Approaches notes the ongoing deterioration of Serbian-Croatian relations.


• At False Steps, Paul Drye profiles the nearly successful Hermes spaceplane planned by the European Space Agency for the 1990s, undermined by technical challenges and the costs of German reunification.


• Far Outliers quotes J.H. Elliott on the Catalonial rebellion of 1640, coinciding at the time with rebellion against Spanish rule in Portugal.


• At Normblog, Norman Geras links to a tribunal set up by Iranian exiles to gather evidence about crimes committed by the Islamic Republic.


• Registan's Casey Michel wonders if claims that Kazakhstan in 1992 turned down a proposal by Libya's Gaddafi to keep its nuclear weapons are being publicized to distract from Kazakhstan's authoritarian government.


• Steve Munro gives a positive review of a TTC-themed play.


• The Volokh Conspiracy notes Pat Robertson's statement that young-earth creationism is not biblical. Robertson knows, I suspect, that linking any belief system to something incredible undermines the belief system.

[BRIEF NOTE] On the habitability of brown dwarf worlds

Brown dwarfs are, as Wikipedia puts it, "sub-stellar objects too low in mass to sustain hydrogen-1 fusion reactions in their cores, unlike main sequence stars, which can", massing between 13 Jupiter masses at the low end and 75 to 80 Jupiter masses at the high end. Brown dwarfs seem to be fairly common though perhaps not as common as theoreticians expected, whether orbiting actual stars (Epsilon Indi, with a binary pair of brown dwarfs orbiting the man star distantly, is the nearest Sun-like star known to host brown dwarfs) or existing independently in deep space. Brown dwarfs, despite their low mass, seem capable of producing planets, as noted by a recent Universe Today article by Nancy Atkinson which dealt with a recent study of brown dwarf ISO-Oph 102.

[A]stronomers have found a brown dwarf that has a dusty disc encircling it, just like the discs encircling regular, young stars. It contains millimeter-sized solid grains, and around other newborn stars, these discs of cosmic dust are where planets form. Astronomers say this surprising find challenges theories of how rocky, Earth-scale planets form, and suggests that rocky planets may be even more common in the Universe than expected.

Rocky planets are thought to form through the random collision and sticking together of what are initially microscopic particles in the disc of material around a star. These tiny grains are similar to very fine soot or sand. However, in the outer regions around a brown dwarf, astronomers expected that grains could not grow because the discs were too sparse, and particles would be moving too fast to stick together after colliding. Also, prevailing theories say that any grains that manage to form should move quickly towards the central brown dwarf, disappearing from the outer parts of the disc where they could be detected.

“We were completely surprised to find millimeter-sized grains in this thin little disc,” said Luca Ricci of the California Institute of Technology, USA, who led a team of astronomers based in the United States, Europe and Chile. “Solid grains of that size shouldn’t be able to form in the cold outer regions of a disc around a brown dwarf, but it appears that they do. We can’t be sure if a whole rocky planet could develop there, or already has, but we’re seeing the first steps, so we’re going to have to change our assumptions about conditions required for solids to grow,” he said.

This is the fulfillment of earlier studies that I noted back in 2005.

What about potentially Earth-like planets, though? As noted briefly in a press release from the University of Washington and at length over at Centauri Dreams, the steadily contracting habitable zones of slowly dimming brown dwarfs--and white dwarfs, too, post-sequence degenerate stars--means that worlds which might be in the habitable zone at a certain point would have been superheated before.

[B]oth brown and white dwarfs could support a habitable zone, but what sets them apart from red dwarfs is that they cool slowly and continuously, meaning their habitable zones shrink inward toward the star. Imagine, Barnes and Heller say, a planet that starts out as a Venus-like world beset with a runaway greenhouse effect. Eventually the habitable zone contracts enough to create the needed temperatures for liquid water to exist, but by now the planet’s surface water is gone and so is the chance for life.

Two possibilities come to my mind.

1. Young worlds orbiting young, bright brown dwarfs could still be habitable. Life would have to become present at a very early stage, however, and unless these brown dwarfs themselves orbited in the habitable zone of an actual stars these young worlds would be doomed to freeze over.

2. Desert worlds, worlds lacking water oceans, are apparently less likely to overheat Venus-style than worlds with oceans. Perhaps worlds located in closer orbits to brown dwarfs, initially inside the habitable zones of bright young brown dwarfs, might, if they don't host much water, avoid being sterilized Venus-style.

Thoughts?

[LINK] "Galaxy Grande: Milky Way May Be More Massive Than Thought"

Scientific American's Ken Croswell reports on yet another amazing feat of modern astronomy: measuring the mass of our Milky Way Galaxy through painstakingly charting the motion of one of its dwarf satellites.

Although scientists know the masses of the sun and Earth, it's a different story for the galaxy. Mass estimates range widely: At the low end, some studies find that the galaxy is several hundred billion times as massive as the sun whereas the largest values exceed two trillion solar masses. Astronomers would have an easier task if the galaxy consisted solely of stars. But a huge halo of dark matter engulfs its starry disk and vastly outweighs it. Now remarkable observations of a small galaxy orbiting our own have led to a new number.

In studies of the Milky Way's mass one little galaxy plays an outsize role: Leo I. "The value of Leo I is twofold," says Michael Boylan-Kolchin of the University of California, Irvine. "It's both very distant and moving quite quickly." Discovered in 1950 and located 850,000 light-years from the Milky Way's center, Leo I is a dwarf spheroidal galaxy and the farthest of the many galaxies that are thought to orbit our own. Most of the Milky Way's dark matter halo should fit inside Leo I's orbit—that is, if the dwarf galaxy is actually in orbit and not just passing by.

Astronomers know from Leo I's Doppler shift that it is racing away from us. If the Milky Way has enough mass, its gravity will hold it in orbit. Moreover, astronomers would be able to observe the motion of Leo I and use it to deduce the Milky Way's total mass—including its dark matter halo—out to the dwarf galaxy's great distance. But if the Milky Way does not have enough mass, Leo I will fly away, its high speed revealing little of consequence.

To deduce Leo I's path through space, astronomers have to determine the small galaxy's precise motion. The Doppler shift reveals Leo I's velocity along our line of sight, but no one knew how fast the little galaxy was moving across it. Determining that requires measuring its proper motion—the change in the galaxy's position from one year to the next. Proper motion is easy to gauge for a nearby star but difficult to measure for another galaxy, because far-off objects have tiny proper motions.

Sangmo Tony Sohn of the Space Telescope Science Institute and his colleagues therefore used the Hubble Space Telescope to compare Leo I's position in 2006 and 2011 with more than a hundred background galaxies. In work submitted to The Astrophysical Journal, Sohn's team reports success: the first proper motion measurement of Leo I.

[. . .]

Combined with the Doppler shift, the proper motion reveals that Leo I orbits the Milky Way at 200 kilometers per second. By comparison, that's nearly as fast as the sun orbits the Milky Way's center, even though the dwarf galaxy is much farther away. Says Boylan-Kolchin, "To sustain a similar velocity at that far a distance requires a lot of extra mass."

How much mass? In a companion study Boylan-Kolchin and his colleagues simulate how giant galaxies such as the Milky Way grow by swallowing lesser galaxies, finding that dwarf galaxies moving as fast as Leo I are almost always bound to the giants, which means Leo I is a true satellite. His team then derives a mass for the Milky Way of 1.6 trillion suns.