2019
December
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When a black hole is simply too big

One of the distinguishing features of black holes is that they are hard to see. Astronomers looking for them sometimes have luck, but at the cost a star: when a ravenous black hole tears off and devours stellar material from an orbiting star, the resulting accretion disk emits radiation that can be measured. Almost all known black holes have been discovered this way.

But it seems logical that those aren’t the only ones out there. Black holes are formed when heavy stars die. And many of these giant stars die alone, without a companion that a resulting black hole could nibble on. Thus, presumably the known black holes represent just a selection – a selection that might not be representative in all aspects.

Chinese astronomers have now succeeded in finding a black hole the way planets are discovered: by its influence on the motion of a visible star. Here, the researchers bumped into a surprise that doesn’t fit nicely into current theories of star formation.

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How exoplanets develop in multiple-star systems

In the novel, “The Three-Body Problem,” a civilization that developed in a system with three stars plays an important role. This situation has dramatic consequences for the civilization’s planet, which I don’t want to spoil for you. I was reminded of the novel when I read a press release of research work at the University of Jena. Dr. Markus Mugrauer, an astrophysicist there, examined 1300 known star systems with exoplanets to determine how many stars there were within these systems. To do this, he used the most up-to-date version of the data from the ESA Gaia Mission.

A clever approach that also produced some interesting insight into how the existence of multiple stars in a system affected the development of planets. In our Solar System, for example, the existence of a second Sun would have undoubtedly had dire consequences for humankind.

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Water detected on one of Jupiter’s moons, Europa

The hottest candidates for the development of extraterrestrial life are one of Saturn’s moons, Enceladus, and one of Jupiter’s moons, Europa – even though it’s very cold on its surface. Life, in fact, might be hiding in an underground ocean under the 50-100 kilometer thick layer of ice. Its existence is indicated, among other things, by the countless fault lines criss-crossing across its surface.

Proof of this underground sea, however, has not yet been found. But now, with the help of the Keck Telescope on Mauna Kea (Hawaii), astronomers have found more evidence, as they have written in an article.

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A black-hole Sun: planets could also form around black holes

What does a builder of worlds need to form planets? A protoplanetary disk made out of a suitable material in which differences in density can develop, and an object at the center of the system that acts as a common center of gravity and uses its force of attraction to prevent the cloud of material from floating off into infinity. Until now it had been assumed that these conditions were met primarily by stars. But apparently much more exotic planetary systems are also conceivable, as Japanese researchers have now described in a study.

The scientists took a closer look at the conditions around black holes – and not just any black holes, but active galactic nuclei in particular. These are black holes that have masses of millions of solar masses and interact with their surroundings. Just like young stars, dust disks can form around them in gigantic dimensions. Just the dust disk itself can reach a mass of 100,000 Suns, corresponding to about a billion times the mass of a normal protoplanetary disk. Thus, there is plenty of material, and there’s also an energy source at the center.

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