Planet or a Failed Star? . . . Discovery Reveals One of the Smallest Stellar Companions Ever Seen

This is an artist's concept of the red dwarf star CHXR 73 (upper left) and its companion CHXR 73 B in the foreground (lower right) weighing in at 12 Jupiter masses. CHXR 73 B is one of the smallest companion objects ever seen around a normal star beyond our Sun. Credit: NASA, ESA and G. Bacon (STScI)

A research team led by a Penn State University astronomer has used NASA's Hubble Space Telescope to discover and photograph one of the smallest objects ever seen around a normal star beyond our Sun. Weighing in at 12 times the mass of Jupiter, the object is small enough to be a planet. The conundrum is that it's also large enough to be a brown dwarf, a failed star.

"New, more sensitive telescopes are finding smaller and smaller objects of planetary-mass size," said Kevin Luhman, assistant professor of astronomy and astrophysics at Penn State and leader of the team that found the object, called CHXR 73 B. "These discoveries have prompted astronomers to ask the question, are planetary-mass companions always planets?" The team's result will appear in the 20 September 2006 issue of the Astrophysical Journal.

The Hubble observation of the diminutive companion to the low-mass red dwarf star named CHXR 73 is a dramatic reminder that astronomers do not have a consensus in deciding which objects orbiting other stars are truly planets — even though they have at last agreed on how they will apply the definition of "planet" to the smaller bodies inside our own solar system. Luhman is casting his vote that the object his team discovered is a brown dwarf.

This NASA Hubble Space Telescope image shows one of the smallest objects ever seen around a normal star. Astronomers believe the object is a brown dwarf because it is 12 times more massive than Jupiter. The brown dwarf candidate, called CHXR 73 B, is the bright spot at lower right. It orbits a red dwarf star, dubbed CHXR 73, which is a third less massive than the Sun. At 2 million years old, the star is very young when compared with our middle-aged 4.6-billion-year-old Sun. Credit: NASA, ESA, and K. Luhman (Penn State University)

Some astronomers suggest that an extrasolar object's mass determines whether it is a planet. Luhman and others advocate that an object is a planet only if it formed from the disk of gas and dust that commonly encircles a newborn star. Our solar system's planets formed 4.6 billion 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 quite enough mass to ignite hydrogen fusion reactions in their cores, which power stars such as our Sun.

CHXR 73 B is 19.5 billion miles from its red-dwarf sun. That's roughly 200 times farther than Earth is from our Sun. At 2 million years old, the star is very young when compared with our middle-aged 4.6-billion-year-old Sun.

"The object is so far away from its star that it is unlikely to have formed in a circumstellar disk," Luhman explained. "Disks around low-mass stars are about 5 to 10 billion miles in diameter. There isn't enough material at that distance from the red dwarf to create a planet. Theoretical models show that giant planets like Jupiter form no more than about 3-billion miles from their stars."

The object was discovered while Luhman's team was using Hubble's Advanced Camera for Surveys to conduct a survey of free-floating brown dwarfs. Astronomers have found hundreds of brown dwarfs in our galaxy since the first brown dwarfs were spied about a decade ago. Most of them are floating through space and are not orbiting stars. "It is important to study young star systems to understand how small bodies formed because young brown dwarfs are brighter and can be imaged directly even at lower masses," said team member John Wilson of the University of Virginia in Charlottesville.

One way to further settle the uncertainty would be if a disk of dust could be observed around CHXR 73's companion. Like stars, brown dwarfs have circumstellar disks, too. They would be no more than about 2-billion miles in diameter.

Most present-day telescopes, even Hubble, cannot detect brown dwarfs because they are small and faint. NASA's Spitzer Space Telescope has detected disks around several free-floating brown dwarfs. But CHXR 73 B is too close to its star for Spitzer to detect the disk. So astronomers will have to wait for the launch of the James Webb Space Telescope in 2013 to determine if this companion has a disk. The Webb telescope will combine Hubble's high spatial resolution, which is needed for detecting close companions, and Spitzer's infrared sensitivity, which is necessary for seeing cool, dusty disks.

The Hubble Space Telescope is an international cooperative project between NASA and the European Space Agency. The Space Telescope Science Institute is operated for NASA by the Association of Universities for Research in Astronomy, Inc., in Washington.

IMAGES:

High-resolution images are on the web at  http://hubblesite.org/news/2006/31.

CONTACTS:

Kevin Luhman, 814-863-4957, kluhman@astro.psu.edu

Barbara K. Kennedy (PIO), 814-863-4682, science@psu.edu