Planet-making disks of gas, dust, and ice are known to form around stars and brown dwarfs. But now, disks with the potential to form planets, or at least moons, have been found outside the solar system orbiting objects that themselves are no heftier than planets.
A study reported this week at a meeting of the American Astronomical Society in Calgary, Alberta, follows up on evidence that several objects only a few times the mass of Jupiter have such disks surrounding them.
The original observations, using the infrared Spitzer Telescope, identified several intriguing, low-mass objects in star-forming regions of the Milky Way. The findings indicated that the objects aren’t only lightweight but also have higher-than-expected infrared emissions. This is a sign that each object is surrounded by a disk of infrared-emitting dust, Katelyn N. Allers of the University of Hawaii in Honolulu and her colleagues report in the June 10 Astrophysical Journal.
Using two visible-light telescopes at the European Southern Observatory in Paranal, Chile, Ray Jayawardhana of the University of Toronto and his colleagues further characterized the objects. They reported at the Calgary meeting that two of the bodies are 10 to 15 times as massive as Jupiter, while two others are just 5 to 10 times as heavy as the giant planet. Over time, the disks surrounding the objects might form miniature solar systems or moons, the researchers say.
In one sense, that’s not a surprise, notes Jayawardhana. Theorists have for decades proposed that Jupiter—as it emerged from the planet-making disk that swaddled the young sun—had its own tiny disk from which the planet’s vast retinue of moons coalesced.
It’s unclear what to call the low-mass objects observed by Jayawardhana, Allers, and their colleagues. Bodies this size that orbit a star, like the denizens of our solar system, qualify as bona fide planets. However, none of the four newly observed bodies orbits a star. That’s a clue that, despite their low mass, they might have formed as stars do—that is, from the gravitational collapse of a gas cloud.
Failed stars, also known as brown dwarfs, form in the same way as stars, but they don’t shine. Brown dwarfs have masses between 12 and 80 times that of Jupiter. Objects smaller than 12 Jupiters have traditionally been classified as planets.
However, the International Astronomical Union refers to small, loner objects—such as the newly reported bodies—as sub-brown dwarfs, notes Alan Boss of the Carnegie Institution of Washington (D.C.).
Jayawardhana calls them planemos, short for planetary-mass objects.
The most important aspect of the findings regards how these low-mass objects form. Jayawardhana says that “nature is able to make objects with quite a wide range of masses—from stars 10 times more massive than the sun to planemos some 100 times less massive than the sun—in the same way. So, any successful theory of star formation has to be able to account for that simple but fundamental fact.”