2020
August
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The last of its kind?

Stellar streams consist of groups of stars moving in orbit together. They are usually remnants of small galaxies that were absorbed by larger galaxies or former star clusters. The Phoenix stream discovered four years ago is the latter. It was, as researchers show in an article in Nature, once a globular cluster, and a very special one at that.

Globular clusters are special objects in themselves. Imagine the night sky full of gleaming stars shining much brighter than the brightest planets in our Solar System. The average distance between two stars of a globular cluster is only 0.1 light-years, while the closest star to the Sun is 4.5 light-years away. Every cubic parsec holds between 1000 to 10,000 stars (the stellar density in the vicinity of the Sun is around 0.14 stars per cubic parsec).

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How many planets fit into a star’s habitable zone?

The habitable zone of our Solar System is relatively narrow. Mars is at the very outer edge of it, while Venus, which orbits closer to the Sun than Earth, is not quite inside it. Of eight planets, only the Earth is at just the right distance from its host star. A ratio like this would naturally lower the chances of finding inhabitable worlds in the universe. But is the Solar System an exception or the rule?

Astronomers have, in fact, found other star systems that give a rosier outlook. For instance, three planets are in the habitable zone of the red dwarf, Trappist-1. In a study in the Astronomical Journal, the astrobiologist Stephen Kane from the University of Colorado has determined what the maximum possible number of inhabitable planets could be. With his team, he tested models of a wide range of different planetary systems to find out how the members of these systems interacted with each other over billions of years.

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This star system will never be the Solar System

TYC 8998-760-1 might someday become something like our Sun. Right now, however, the young star is still a few billion years away from that. It’s been around for only about 17 million years. If it were the Sun, there would still be a long time before it would even be able to watch the dinosaurs. Nevertheless, the whippersnapper is still something special: astronomers using the European Southern Observatory’s (ESO) Very Large Telescope (VLT) photographed it and found two planets in its orbit.

“Even though astronomers have indirectly detected thousands of planets in our galaxy, only a tiny fraction of these exoplanets have been directly imaged,” says co-author Matthew Kenworthy, an associate professor at the University of Leiden, adding that “direct observations are important in the search for environments that might support life.” The direct imaging of two or more exoplanets around the star is even rarer; only two such systems had been directly observed before, both around stars that differ significantly from our Sun.

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Centaurs: they’ve been with us for a long time

We’ve been waiting for extraterrestrial visitors our whole lives – but in reality, they’re already here and have been with us for a long time. No, I don’t mean “Men in Black.” But it’s also not science fiction, it’s the truth. When astronomers discovered 2017 1I/ʻOumuamua, their surprise was enormous: we’d never seen an interstellar object inside our Solar System before. Or had we?

We had. For some time, astronomers have known about asteroids that don’t orbit the Sun in the same plane as the planets (the ecliptic), but instead on orbits that are at a greater or smaller angle to this plane. We know that the whole Solar System was made from a flat disk of dust and gas. Therefore, every object formed from that disk should also move within the plane of that flat disk. Unless, of course, some collision or gravitational interaction changed an object’s orbit at some point during the history of the Solar System.

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