Research interests

My research focuses on brown dwarfs and giant exoplanets. I develop and use remote sensing techniques to analysis data from ground and space-based telescopes to understand the dynamical, physical and chemical processes at work in their atmospheres.

Brown dwarfs are gaseous substellar objects whose low mass means they never reach sufficient core temperatures to sustain hydrogen fusion as stars do. We study them by observing them at infrared wavelengths where they emit most of their light. Just like stars they are mostly composed of hydrogen that has collapsed from large gas clouds in space. Newly forming stars heat up as they contract and eventually reach high enough temperatures for hydrogen fusion to start releasing energy which halts the collapse. The more massive they are the hotter they get and the quicker this happens. Brown dwarfs do not have enough mass to reach these temperatures. After initially heating up as they contract, their collapse grinds to halt before hydrogen fusion can start, and they start to cool again.
Brown dwarfs start life as warm as the coolest stars, but cool through to full temperature range of known giant exoplanets. When we study their atmospheres we are studying the same processes that take place in the atmospheres of exoplanets as they evolve through the same temperature range. They span a mass range from about 13 times the mass of Jupiter (13 M_Jup) upto about 75 M_Jup. The upper limit is the minimum mass needed to fuse hydrogen to helium. The lower mass limit is minimum mass needed to fuse hydrogen's heavier isotope, deuterium. Below this mass they are known as "planetary mass objects". The science of substellar atmospheres links the stellar regime of ionised gases (plasma) at temperatures of 1000s of degrees, down to the cold planetary regime where neutral molecular gas dominates, ice-clouds can form, and complex processes such as weather can be studied.

Clouds are a common feature of planets in the Solar System, from the familiar water clouds of Earth's atmosphere to the colourful ammonia-ice cloud tops of Jupiter. Clouds have also been identified in a range of atmospheres beyond the solar system. And, as in the solar system, they are made of a variety of condensed grains and droplets depending on the temperature of the atmosphere (see below). My group is actively researching the clouds in brown dwarf atmospheres, measuring their distribution and composition. We were the first to identify quartz clouds in a substellar atmosphere and are seeking to understand how cloud composition relates to the bulk composition of the gas from which they form.

Where there are clouds there is weather. Clouds fundamentally alter the way that energy is transported in atmospheres. Their presence causes localised warming as they block heat from escaping to space, and this in turn can raise temperatures and drive winds. Moreover, the increased temperatures can lead to clouds evaporating, and then releasing the trapped heat.These dynamic processes may create tell tail signatures in the light we see emitted by brown dwarfs in the form of variations in brightness as gaps in the clouds rotate in and out of view. Such variations in brightness have been seen, and we are now able to gather exquisite time-series spectra of variable brown dwarfs. However, there is another possible source of variability that we must disentangle from that caused by weather.
Jupiter displays the most powerful aurorae in the Solar system, visible by space telescopes at UV and infrared wavelengths and detectable by their radio emissions. These aurorae are fed by gas from Jupiter's moons, and powered by its strong magnetic field and rapid 10 hour rotation period. The same kind of radio emissions have been detected in about 1 in 20 brown dwarfs, and all of these objects are also variable at near-infrared wavelengths. It is not yet understood how much of the variability is related to auroral activity, and what impact this activity has on the atmospheres of these brown dwarfs. Their magnetic fields are in some cases 1000x stronger than Jupiter's and rotation periods can be as little as 2 hours, which can drive powerful electric currents. In the Solar System, heating by auroral activity has been suggested as an important factor in explaining the hot stratospheres of the giant planets. We have discovered one brown dwarf with a 300 degree temperature inversion in the upper atmosphere that makes methane gas at the top of its atmosphere glow. We are developing techniques to unpick what is driving the variability in substellar objects, and to understand the impact of auroral process and clouds on energy transport.