14 DAY TRIAL // One of the smallest objects ever seen around a normal star beyond our Sun, the low-mass red dwarf CHRX 73, has been photographed using the NASA/ESA Hubble Space Telescope. Having 12 times the mass of Jupiter, the object is on the line between the largest planets and the smallest stars. It is small enough to be a planet but it is also large enough to be a brown dwarf, a failed star. The object was called CHRX 73 'B'.
Hubble's Advanced Camera for Surveys imaged the pair in infrared light in 2005. While most extrasolar "planets" are difficult to image directly because they are so faint compared to their host stars, Hubble was able to capture this system because the larger star is relatively faint itself. The objects are also far away from each other, "so the companion is outside the glare of the larger star", explains Kevin Luhman of Penn University, USA.
This observation reopen the debate about how to define planets and stars outside our Solar System. The issue comes two weeks after International Astronomical Union approved the first official definition of "planet" for our solar system and downgraded Pluto at the status of a dwarf planet.
The new discovery is a reminder that objects in nature do not always fit into the neat categories created by scientists. Recently, astronomers spied even stranger planetary-mass objects that drift freely through space, far away from any star. Some scientists are calling the bizarre objects "planemos," but Luhman says the name is unnecessary and that the objects are really just brown dwarfs.
"These discoveries have prompted astronomers to ask the question, are planetary-mass companions always planets?" ? Some astronomers suggest that an extra-solar object's mass determines whether it is a planet. Others, including Luhman, advocate that an object is a planet only if it has formed from the disk of gas and dust that commonly encircles a newborn star. Our Solar System planets formed 4.6 thousand million years ago out of a dust disk around our Sun. ?Brown dwarfs, by contrast, form just like stars: from the gravitational collapse of large, diffuse clouds of hydrogen gas. Unlike stars, brown dwarfs do not have enough mass to initiate nuclear fusion reactions in their cores, which power stars such as the Sun.
CHXR 73 B is 31.2 thousand million km from its sun. This is roughly 200 times farther than Earth is from Sun. This distance is so great that even though CHXR 73 B has about the right mass to be a planet, it probably didn't form in the same way that planets in our solar system did. "The object is so far away from its star that it is unlikely to have formed in a disk of gas and dust" Luhman explained. Theoretical models show that giant planets like Jupiter form no more than about 5 thousand million km from their stars. Disks around low-mass stars are about 8 to 16 thousand million km in diameter. There isn't enough material at that distance from the red dwarf star to create a planet.
More likely, scientists say, CHXR 73 B formed in the manner of stars: from the gravitational collapse of large, diffuse clouds of hydrogen gas. This star is very young, about two million years old, compared with our middle-aged 4.6-thousand-million-year-old Sun.
Hubble discovered the object while conducting a survey of free-floating brown dwarfs. Astronomers have found hundreds of brown dwarfs since a decade ago, when first brown dwarfs were seen. Most of them are floating through space and not orbiting stars.
Being stars, brown dwarfs have circumstellar disks, too. Disks around several free-floating brown dwarfs have been discovered so far. The problem is CHRX 73 B is too close to its star. Astronomers will have to wait till 2013 to determine if this companion has a disk, when a combined technology will be used on this purpose.