Randy McDonald (rfmcdpei) wrote,
Randy McDonald
rfmcdpei

[LINK] Two Centauri Dreams links

  • First, the possibility of diamond oceans on Uranus and Neptune is broached.


  • Researchers at the University of California at Berkeley demonstrated about ten years ago that the high temperatures and pressures found inside planets like these can turn methane into diamond, and in fact that the diamonds settling into Neptune’s core could account for the excess heat radiated by the planet. Now a team led by Jon Eggert (Lawrence Livermore National Laboratory) has explored the conditions under which diamonds melt, and has shown that, like water, liquid diamond when freezing and melting produces solid forms atop liquid ones.

    To melt diamond requires high pressures, which has made measuring its melting point extremely difficult. But the pressures found inside gas giants, and the high temperatures that go with them, should be enough to do the job. This Discovery News story notes that Eggert’s team subjected a small diamond (about a tenth of a carat) to a laser beam, liquifying the diamond at pressures some 40 million times greater than at sea level on Earth. As they then reduced the temperature and pressure, solid chunks of diamond began to appear at about 11 million times sea level pressure with a temperature of some 50,000 Kelvin.

    The diamond chunks, like tiny icebergs, did not sink but floated. Eggert believes an ocean of liquid diamond could explain the fact that, unlike the relatively close match we find on Earth, the magnetic and geographical poles on Uranus and Neptune do not align, and can be offset by as much as 60 degrees from the north-south axis. Put an ocean of liquid diamond in just the right place and the magnetic field displacement makes more sense.


  • Second, the prospects for planet formation around Alpha Centauri B, the dimmer of the two Sun-like stars in the Alpha Centauri cluster, seem quite good.


  • Astronomers Ji-Wei Xie, Jian Ge (Nanjing University, China) and Ji-Lin Zhou (University of Florida) have created a series of simulations to estimate how long it takes planetesimals embedded in a protoplanetary disk to accrete into planetary embryos. This is rapidly becoming the key issue. What we know so far is that the mass upper limit of a planet in this system is 2.5 Jupiter masses around Centauri A and 3.5 Jupiter masses around Centauri B. That leaves us with the possibility of lower-mass planets if this system will allow planetary embryos to form. The problem is that most simulations have assumed a disk of such embryos as the starting point and have followed planet development from that point.

    [. . .]

    Ji-Wei Xie and team reopen the case by extending Thébault’s work to include the effect of binary inclinations. They develop a new model tested through simulation to study whether the zone from 0.5 to 2.5 AU around Centauri B may allow planets to form. The simulations vary the gas-disk density as well as the binary inclination and work with variables like planetesimal mass distribution, impact rate and the fraction of accreting collisions to come up with a conclusion: Planetesimal accretion into planetary embryos takes significantly longer in this binary environment than around single stars, which does not favor the formation of gas giant planets, but the formation of smaller, terrestrial worlds is possible.


    I love modern astronomy. In case you didn't already know.
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