A globular cluster is a roughly spherical ensemble of stars, as many as several million of them, gravitationally bound together in groups whose diameters can be as small as only tens of light-years. To sense the dramatic implications of this dense packing, consider that the nearest star to the Sun, Proxima Centauri, is about four light-years away. Messier 4 (M4) is the closest globular cluster to Earth at a distance of about six thousand light-years, and a puzzle to astronomers. Normal gravitational effects should, over time, redistribute the stars in a globular cluster until they are more numerous towards the center, but while M4 shows a central concentration of stars it does not show evidence for a steep central cusp even though astronomers think enough time has passed.
To understand what is going on in this globular cluster, and to help understand how these clusters evolve in general, CfA astronomer Maureen van den Berg and her collaborators have undertaken a large and unprecedented set of deep images of M4 with the Hubble Space Telescope to look for binary stars, that is stars with companions. The dynamical interactions between the densely crowded stars in a globular cluster should disrupt many such binaries, but for reasons that are not understood about fifteen percent of the stars in M4 are binaries, at least based on monitoring brightness variations (a more typical number is two percent). Whether or not this unusual abundance is connected to the lack of a central cusp in stellar density is also not understood.
The astronomers set out to use Hubble to study the binary star population in M4 looking at both brightness variations and stellar wobble (astrometric) variations, in particular due to binaries with a massive, faint, and evolved companion like a white dwarf or neutron star. The team was able to find and characterize a much more complete set of binaries, including thirty-six new variables. They note in passing that, as part of the search process, any stars with massive "hot Jupiter" exoplanet companions would probably also have been detected, but that none were. The extensive results are still being analyzed, but the improved statistics will make the conclusions much more reliable.
"The M 4 Core Project with HST – III. Search for Variable Stars in the Primary Field," V. Nascimbeni, L. R. Bedin, D. C. Heggie, M. van den Berg, M. Giersz, G. Piotto, K. Brogaard, A. Bellini, A. P. Milone, R. M. Rich, D. Pooley, J. Anderson,, L. Ubeda, S. Ortolani, L. Malavolta, A. Cunial1, and A. Pietrinferni, MNRAS 442, 2381, 2014