Protoplanetary disks around three young stars in Ophiuchus have large central holes, astronomers have found, which were presumably cleared by still-growing Jupiter-mass planets. But there’s a problem: the stars are too young. How would planets have formed in just a couple million years?
This is not the first observation suggesting central gaps in the disks of gas and dust around newborn stars (S&T: November 2008, page 32). In the present case, infrared spectra from NASA’s Spitzer Space Telescope had already provided hints, says lead author Sean Andrews (Harvard-Smithsonian Center for Astrophysics). “One of the disks, SR 21, we knew had a hole from the Spitzer observations.” But the evidence was indirect; Spitzer’s 0.85-meter aperture could not resolve the hole, only detect the absence of short-wavelength infrared radiation that would be emitted by warm dust closer to the star.
In their new paper, Andrews and collaborators provide an actual image of a hole. “This is hot stuff! Clearly we are on the verge of catching planet formation in action,” comments Alan Boss (Carnegie Institution, Washington), who did not take part in the study.
Andrews and collaborators used the Sub-Millimeter Array interferometer on Mauna Kea, Hawaii, to form images with a resolution of 0.3 arcseconds — enough to show holes with a radius of 40 astronomical units at the distance where the stars are, about 400 light-years away in Ophiuchus. This is the nearest active star-forming region to Earth.
The researchers studied nine disks and found that three harbor large central cavities. But these numbers do not imply that every third disk will form planets, because the sample was biased. “We’ve picked the most massive disks to look at,” explains co-author David Wilner.
The disks are larger than our solar system, and the central holes are at least the size of Neptune’s orbit. “You need something with Jupiter’s mass to clear that,” Andrews explains.
In fact the mass clearing out the hole could be one giant planet, or several, or a swarm of rocks and rubble, or perhaps an undetected binary companion to the star. “They still haven’t shown that there isn’t a brown dwarf,” says Carol Grady (NASA).
Finding a fully-formed planet in a disk a few million years old would be big news. Theorists expect the slow process of collisions and accretion to take about 10 million years for dust and rocks to collect into planets and clear away a protoplanetary disk. On the other hand, gravitational instabilities in the disk might act more quickly, forming a stellar-like companion such as a brown dwarf or a lower-mass object mimicking a conventional giant planet. The line between Jupiter-like planets and brown dwarfs is difficult to draw, and scientists would like to know which mechanism drives the formation of each or both.
Interestingly, of the nine disks, the three with holes seem to be the oldest — possibly older than a few million years.
Jack Lissauer (NASA) is impatient to see the analysis techniques of this paper applied to the higher-resolution data that will come from the upcoming ESO Atacama Large Millimeter/submillimeter Array (ALMA), now being built on a high plain in the Andes. “The fact that they can do this with the SMA is exciting, because ALMA is coming up soon.” ALMA, along with other instruments, should enable detailed observations of the structure of the disks and maybe even reveal what lurks in the central clearing.