This above spectacular image was constructed from three mosaics, each taken with UFTI through a different filter. The images, taken in the near-infrared I, J and H-bands, were colour-coded red, green and blue and combined to produce the "colour" image shown here. In reality, a deep colour photograph of this region taken with an optical telescope would only show the bright foreground stars - the trapezium stars - which illuminate the front surface of the OMC1 molecular cloud. Only at near-infrared wavelengths do we see deep inside the cloud, where stars are being formed through the gravitational collapse of gas and dust cores.

Brown Dwarfs and Free-Floating Planets in Orion

A large number of brown dwarfs have been discovered in recent years both in young Galactic Clusters and in the local field population of stars near the sun. These are objects which were too small to become stars, since their centres never became hot enough for the nuclear reactions of hydrogen which make stars shine. The image shown here reveals a large population of very young brown dwarfs in the Trapezium Cluster in Orion. This was one of the first projects undertaken with the infrared camera UFTI, on the United Kingdom Infrared Telescope, in December 1998 and the results of this research have just been accepted by Monthly Notices of the Royal Astronomical Society. It is the most sensitive search yet for low mass objects, observing three different near infrared colours (known as I, J and H) to discover the mass, luminosity and temperature of the objects.

About two thirds of the 600 or so point sources seen in the data (the image accompanying this article has been cropped slightly) are stars, just under one third are brown dwarfs and a few, about 13 in fact, are even smaller objects with masses similar to planets. Unlike ordinary planets however, these do not orbit any star but float by themselves in space and shine by the residual heat left over from when they were born. The smallest object found so far has about 8 times the mass of Jupiter, quite massive by planetary standards, but still below the threshold for nuclear reaction of deuterium, which occurs at 13 Jupiter masses and is the minimum mass of a brown dwarf.

Orion is the nearest of the Giant Molecular Clouds - the places where most stars are thought to be born out of dense clouds of gas and dust like those seen here. Thus it is the best place to study in order to find out about the population of stars, brown dwarfs and free-floating planets that exist in the rest of the galaxy. Brown dwarfs and free floating planets are much easier to find when they are young and still retain some heat from the formation process. The objects in the Trapezium cluster are mostly between three hundred thousand and two million years old - very young compared to the 5 billion year age of the sun. The backdrop of the Orion Molecular Cloud obscures everything that lies behind it, which is very useful because it means that all the objects seen are members of the cluster, except for perhaps a handful which lie in the foreground.

An interesting feature of the study is that no smaller planets were found. This may indicate that there is a limit to how small these free-floating planets can be, although even more sensitive surveys will be needed to confim this. In the meantime, about 20 of the brown dwarfs and planets have been looked at again with UKIRT, examining their spectra to give us more information about them. The results are still being analysed but they show the signature of water that is expected in relatively cool stars and brown dwarfs, at a temperature of a mere 2800 degrees Centigrade.

The data shown above, and the attached article, are courtesy of Dr Phil Lucas (University of Hertfordshire) Dr Pat Roche (University of Oxford).