Over the last few decades, astronomers have refined a cosy picture of star-formation involving the gravitational collapse of a large molecular cloud complex, which subsequently fragments into several sub-clouds, each developing into new stellar systems in only a few million years. While this tidy picture seems largely correct, astronomers have amassed new evidence that some stars -- so-called TW Hydrae Associations -- might be spawned in loosely-bound physical associations.

Warped disk in Beta Pictoris. Photo: C. Burrows and J. Krist (ST ScI) and NASA

Ever since the first object of this kind was brought to the attention of the astronomical community back in 1978 by George Herbig, the number of TW Hydrae members have increased to several dozen. Over the last few years, observations taken at visible and near-infrared wavelengths in the solar neighbourhood have identified several isolated objects that appear to be very young stars -- yet the giant molecular cloud from which they are thought to have formed is no where to be seen.

Thus far, all of the members of the TW Hydrae Association have very low masses, typically between 10 percent and 30 percent of the mass of the Sun, and are located in a patch of sky spanning only 20 angular degrees across. "We really don't know whether these stars originated from a single giant molecular cloud complex and which subsequently escaped, or whether they were formed in isolation," says Ray Jayawardhana, an astronomer at the Harvard Smithsonian Center for Astrophysics, Massachusetts. What's more, because they are exceedingly rich in lithium, an element that is quickly depleted in the cores of young main-sequence stars, most astronomers believe that TW Hydrae stars must be less than a million years old.

Further scrutiny of TW Hydrae stars has indicated that they may be an ideal hunting ground to look for very young planetary systems. Many of these stars have already been found to possess dusty discs with the same dimensions as our own Solar System, and which appear to be in various stages of evolution. What's more, because any planets formed around these stars should still be very hot, they might provide the best opportunity yet to spot them directly using large ground-based telescopes.