Astrophysical calculations show that any star that is smaller than about 1/10th of the mass of the sun cannot sustain hydrogen fusion reactions at its core. These failed stars never light up. Instead they wander the galaxy as warm, dark balls of hydrogen known as brown dwarfs.
Brown dwarfs probably form through the same process that lead to ordinary stars but merely on a smaller scale. If that’s correct, planets should also form in the protoplanetary disks of gas and dust around brown dwarfs. Indeed, astronomers have seen a number of protoplanetary disks of this type.
Until now, however, they’ve never seen a planet orbiting a brown dwarf. That’s not really surprising.
The standard methods for detecting planets look for the way a star wobbles as a planet orbits or at how its magnitude changes as a planet passes in front. But given that brown dwarfs are dim and difficult to see, these methods have yet to produce fruit.
All that changes today with the announcement by an international team of astronomers that they’ve discovered a planet orbiting a brown dwarf the first time. These guys have made their discovery using an entirely different method of detection called gravitational lensing. This occurs when one body passes in front of another and its gravity focuses light from the more distant object towards Earth. That works regardless of the brightnesses involved.
The brown dwarf in question is almost 6000 light years from Earth in the Fish Hook constellation. Astronomers first noticed an unusual change in its brightness in April 2012. Further investigation showed that this was indeed a lensing event.
These guys conclude that the brown dwarf is being orbited by a planet about twice the mass of Jupiter at a distance of just under one astronomical unit. The brown dwarf itself is about 10 times larger than its companion.
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