Once upon a time a little satellite gazed at a nearby star and seemed to see a solar system in the act of being born. There was no picture of the blessed event, just an unusually large amount of infrared energy coming from the star. This is exactly what you would expect if the star were surrounded by dust, which absorbs starlight and re-emits it as heat. And dust is what you would expect to find where planets are forming.
The satellite was IRAS (Infrared Astronomy Satellite) and the star was Fomalhaut, a well-known neighbor to our Sun located just 25 light years away. I still remember the excitement that surrounded the discovery; it was one of the first direct hints that our solar system is not unique.
How times have changed! Astronomers don’t have to speculate about the dust around Fomalhaut anymore. Thanks to the Hubble Space Telescope they can see it directly, in the shape of an elegant ring circling around the star. They don’t have to speculate about planets either. The Hubble has spotted one of those too, orbiting Fomalhaut just inside the ring.
Now it appears we have something new to consider: Could we be witnessing a system of moons forming there too?
It’s a fitting question for the International Year of Astronomy, which marks the 400th anniversary of Galileo first use of the telescope and his discovery of Jupiter’s four big moons. These moons probably formed by condensing out of a disk of matter that was circling around the infant Jupiter—just as Jupiter formed from a disk of matter surrounding the infant Sun.
The idea of disks within disks brings to mind a quip by Jonathan Swift, best known in its rephrasing by Augustus De Morgan : “Great fleas have little fleas upon their backs to bite ‘em, and little fleas have lesser fleas, and so ad infinitum.”
The idea applies to Fomalhaut because of the peculiar properties of Fomalhaut b, the planet detected there. Even for the Hubble, the planet appears as nothing more than a dot, first imaged in 2004. (The confirmation came when a second observation taken in 2006 later showed that it had moved some distance along its orbital path. This ruled out the possibility that the object was just a background star.)
The dot that Hubble sees is mainly infrared light, assumed to be from a Jupiter-type planet that is radiating the heat of its own formation. A dot is not a lot to go on, but at least its brightness can tell you about the planet’s size: a bigger planet would correspond to a brighter dot. At the same time, the gap between the planet and the inner edge of the ring can give you a limit on the planet’s mass, since a heavier planet at the same location would have a more disruptive effect on the dust particles in the ring.
Oddly, in the case of Fomalhaut, these two pieces of information do not seem to agree. The planet’s position with respect to the ring suggests an object that is only about half the mass of Jupiter, yet the dot is much brighter than you would expect from such a pint-sized gas giant. An appealing possibility is that a disk of debris around the planet is responsible for its elevated brightness. In fact, measurements through different filters suggest the dot is giving off reflected starlight as well as emitted planet light. A disk would do this.
A disk around the planet could mean that moons are forming there. However, the planet’s reflected light grew dimmer between the two years it was observed. Could the disk be disappearing that fast? Or is it the product not of moon formation but of something more radical and temporary, like dusty debris from a giant comet collision with an existing moon, for example.
James Graham is an astronomer at the University of California, Berekeley, and part of the team that discovered Fomalhaut b. The only way to know what’s really going on, he say, it to take more images, which can’t happen until the Hubble is repaired. This was supposed to be last year but is now scheduled to happen next month. For Graham and his colleagues every day matters. If the planet is changing that fast, then potentially important information is now being lost. “That’s why it will be important to get another image as soon as possible after Hubble’s back,” says Graham.
When it comes to fleas, it's hard to wait.
Ivan Semeniuk is host of The Universe in Mind podcast and a science journalist in residence at the Dunlap Institute for Astronomy and Astrophysics, University of Toronto.