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Newfound Gas Cloud Points to Possible Planets Near the Milky Way's Black Hole

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An x-ray image of the region around Sgr A*. Credit: NASA/CXC/MIT/F.K.Baganoff et al.

Times are tough on planet Earth right now, but at least we don't have a supermassive black hole lurking just over the horizon.

A new study suggests that stars near the Milky Way's central black hole may well form planets. The researchers based their analysis on a very recent discovery of a gas cloud making its way toward the galactic center.

On December 14 an international team of astronomers led by Stefan Gillessen of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, announced that they had spotted something heading toward a close encounter with the central black hole, known as Sagittarius A* (Sgr A*), which has as much mass as four million suns. Gillessen and his colleagues interpreted the object to be a dusty gas cloud about three times as massive as Earth, possibly belched out as stellar winds (plasma streaming outward from stars) from the young stars that orbit the black hole.

A few days later, Ruth Murray-Clay and Abraham Loeb of the Harvard–Smithsonian Center for Astrophysics suggested that the newfound object may be much more closely connected to those stars. In a preprint posted to the Web site arXiv.org on December 20, Murray-Clay and Loeb say that the cloud Gillessen and his colleagues discovered could be the disrupted remnants of a planet-forming disk surrounding a star that used to orbit Sagittarius A* at a safe distance but is now plunging toward the black hole. A protoplanetary disk is the swirling pancake of gas and dust surrounding a young star, which can coalesce into planets, asteroids and comets. "This cloud of gas naturally originates from a proto-planetary disk surrounding a low-mass star, which was scattered a century ago from the observed ring of young stars orbiting Sgr A*," they write.

The star itself would be too faint to see. But as Sagittarius A* has distorted and fried the disk with the black hole's gravitational pull and the radiation of its environs, it has generated a debris stream around the star that telescopes can detect.

If the young stars orbiting Sagittarius A* host protoplanetary disks, that "implies that planets form in the Galactic centre," the researchers write. But you wouldn't want to live on one of those worlds. The galactic center is awash in intense radiation emitted by material swirling around the outside of the black hole, which gets compressed and heated as it falls inward. (It is that radiation from outside the event horizon that allows astronomers to "see" a black hole, which itself holds tight to all matter and photons and hence emits no light.)

Plus there's always the chance that your host star will get knocked onto an orbit heading right for the black hole, as Murray-Clay and Loeb suspect has happened here. In some cases, the researchers suggest, planets might be torn apart by the black hole's gravitational pull and produce bright flares as the pieces fall in.

We should soon find out whose explanation for the gas cloud is correct. The cloud, whatever its origin, is on track to swing past Sagittarius A* in mid-2013. If it's a simple gas cloud, it will get torn apart and partly consumed by the black hole, temporarily brightening the radiation from around Sagittarius A*. If the cloud instead comprises debris from a protoplanetary disk, the star itself should cruise past Sagittarius A* largely unscathed. But the cloud will grow denser as more and more mass from the disk is dragged away from the star. Either way, it ought to be quite a show.

The views expressed are those of the author and are not necessarily those of Scientific American.

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