Discovery on miniature solar systems blurs the definition of 'planet'

Forget our traditional ideas of where a planetary system forms — new research led by a University of Toronto astronomer reveals that planetary nurseries can exist not only around stars but also around objects that are themselves not much heftier than Jupiter. It suggests that miniature versions of the solar system may circle objects that are some 100 times less massive than our sun.

That's the dramatic conclusion of two studies being presented today at the American Astronomical Society meeting in Calgary by Professor Ray Jayawardhana and his colleagues. The new findings show that objects only a few times more massive than Jupiter are born with disks of dust and gas, the raw material for planet making. Research done by Jayawardhana's group and others in recent years had shown that disks are common around failed stars known as "brown dwarfs". Now, they report, the same appears to be true for their even punier cousins, sometimes called planetary mass objects or "planemos." These objects, discovered within the past five years, have masses similar to those of extra-solar planets, but they are not in orbit around stars — instead, they float freely through space.

"Now that we know of these planetary mass objects with their own little infant planetary systems, the definition of the word 'planet' has blurred even more," says Jayawardhana, an associate professor of astronomy and astrophysics. "In a way, the new discoveries are not too surprising — after all, Jupiter must have been born with its own disk, out of which its bigger moons formed."

Unlike Jupiter, however, these planemos are not circling stars. In the first study, Jayawardhana and Valentin Ivanov of the European Southern Observatory (ESO) in Chile used two of ESO's telescopes — the 8.2-metre Very Large Telescope and the 3.5-metre New Technology Telescope — to obtain optical spectra of six candidates identified recently by researchers at the University of Texas at Austin. Two of the six turned out to have masses between five to 10 times that of Jupiter while two others are a tad heftier, at 10 to 15 times Jupiter's mass. All four of these objects are just a few million years old and are located in star-forming regions about 450 light-years from Earth. The planemos show infrared emission from dusty disks that may evolve into miniature planetary systems over time.

In the other study, Subhanjoy Mohanty (Harvard-Smithsonian Center for Astrophysics, CfA), Jayawardhana (UofT), Nuria Huelamo (ESO) and Eric Mamajek (CfA) used the Very Large Telescope to obtain infrared images and spectra of a planetary mass companion discovered two years ago around a young brown dwarf that is itself about 25 times the mass of Jupiter. The brown dwarf, dubbed 2M1207 for short and located 170 light-years from Earth, was known to be surrounded by a disk. Now, this team has found evidence for a disk around the eight-Jupiter-mass companion as well. Researchers think the pair probably formed together, just like a binary star system, instead of the companion forming in a disk around the brown dwarf. Moreover, Jayawardhana says, it is quite likely that smaller planets or moons could now form in the disk around each one.

Both sets of discoveries point to objects not much more massive than Jupiter forming the same way as stars like the sun, and perhaps being accompanied by their own retinues of small planets. "The diversity of worlds out there is truly remarkable," Jayawardhana adds. "Nature often seems more prolific than our imagination."

Contact:

Professor Ray Jayawardhana, Office: 416-946-7291, Cell: 647-233-7749, Hotel in Calgary (June 4 to 6): 403-717-1234; e-mail: rayjay@astro.utoronto.ca

Sonnet L'Abbé, U of T Strategic Communications, 416-978-6974; e-mail: sonnet.labbe@utoronto.ca